Physics Motion- Yr 10

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ACKNOWLEDGEMENTS

Unicorn Academy

FORCES AND MOTION

ENERGY AND CHANGE

INTRODUCTION

CHANGING MOTION

DESCRIBING MOTION

CONCEPT BANK

INTRODUCTION

CHANGING MOTION

DESCRIBING MOTION

CONCEPT BANK

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The Digital Science Learning Resources series of CD-ROMs is a joint project between
the Chamber of Minerals and Energy of Western Australia, and PLC, Perth, Western
Australia. It consists of a series of ten CDs covering aspects of Biology, Chemistry,
Physics and Earth Science for years 8 to 10.

The Chamber of Minerals and Energy of Western Australia, and PLC, Perth, Western
Australia gratefully acknowledge the contribution of the following staff at PLC.

Original concept and design

Keith Anderson and Kim Edwards
Atoms Elements Periodic Table

Kim Edwards
Ions Formulae Equations Reactions

Keith Anderson
Metals in our World

Geoff Quinton and Jessica Stinton
Introduction to Chemical Calculations Geoff Quinton and Jessica Stinton
Introduction to Bonding

Geoff Quinton and Jessica Stinton
The Road to Rocks

Lana Salfinger and Nicole Dorrington
Forces and Motion

Greg Moran and Steve Zander
Graphing and Calculating

Greg Moran and Steve Zander
Basic Biological Concepts

Guinevere Hodges
Genetics

Guinevere Hodges

Layout and CD Design

Glydepath Consulting

ACKNOWLEDGMENTS

P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

ACCELERATION

CIRCULAR MOTION

DESCRIBING MOTION

CHANGING MOTION

SPEED & VELOCITY

NEXT

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4

CONCEPT BANK

Describing Motion - Introduction
Every one of us has had experience with motion from the things that we do everyday such as
walking, running, riding a bike, or riding in a vehicle like a bus, train or car. In this unit of science
we will be investigating motion and the physics concepts associated with movement. In this
section we will start with the concepts that a physicist would use to describe movement.

Use the video link to view some examples of objects moving. While viewing each example
attempt to write a sentence, or list a few words that describe the motion you are looking at.

VIDEO: MOTION EXAMPLES

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

ACCELERATION

CIRCULAR MOTION

DESCRIBING MOTION

CHANGING MOTION

SPEED & VELOCITY

NEXT

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5

CONCEPT BANK

Look at the images on this page. Write a sentence or use a few words that describe the type
of motion you would expect to find in each one.

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

ACCELERATION

CIRCULAR MOTION

DESCRIBING MOTION

CHANGING MOTION

SPEED & VELOCITY

NEXT

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6

CONCEPT BANK

Place your cursor on each picture to see some words that we used to describe the motion.
How do they compare to yours?

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

ACCELERATION

CIRCULAR MOTION

DESCRIBING MOTION

CHANGING MOTION

SPEED & VELOCITY

NEXT

BACK

7

CONCEPT BANK

Describing Distance
When we want to describe motion we will need to be able to use words that indicate how
objects change their position from one place to another.

One means of doing this is to describe the distance an object moves. In order to compare
different examples of motion we need to use a unit of measurement.

You may know examples of units that can be used to measure distance. These might include
imperial units such as the mile, yard, feet and inches, or metric units like kilometre, metre and
centimetre to name just a few. Scientists have adopted a standard unit of measurement. For
length (or distance) scientists all over the world use the same measurement known as the
metre.

When describing motion we measure the distance an object has travelled.

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

ACCELERATION

CIRCULAR MOTION

DESCRIBING MOTION

CHANGING MOTION

SPEED & VELOCITY

NEXT

BACK

8

CONCEPT BANK

Measuring Distance
When measuring distance we need to be clear about what it is we are measuring. Consider
your journey to school this morning. Imagine if you could trail string out behind you as you
travelled from your home to school. The total length of string that you let out as you made
your way to school is the distance you have travelled.

Use the first video link below to view an example of measuring the total distance that a person
or object has travelled.

When measuring the total distance travelled you must add up the individual distances that
make up the journey. The second video link examines the journey of a person from their
home to their school. In this example a map with a distance scale is used to measure the
total distance travelled.

When you are ready try the Challenge below, then attempt the problems to check your
understanding of distance.

VIDEO: DISTANCE

VIDEO: DISTANCE WALKING

CHALLENGE

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

ACCELERATION

CIRCULAR MOTION

DESCRIBING MOTION

CHANGING MOTION

SPEED & VELOCITY

NEXT

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9

CONCEPT BANK

Challenge: Use a piece of string to measure the distance along the path from Enrico’s house
to Marie’s Laboratory. Compare your string length to the scale.

50 metres

PROBLEMS

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ANSWER

P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

ACCELERATION

CIRCULAR MOTION

DESCRIBING MOTION

CHANGING MOTION

SPEED & VELOCITY

NEXT

BACK

10

CONCEPT BANK

Describing Distance Problems

1. An ant crawls along a metre rule from the 10 cm mark to the 85 cm mark. How far has the ant crawled?

2. You walk from your house to the local shop. You walk 10 m along your driveway, turn left and walk 200 m to a corner

where you turn left again and walk a further 250 m. You then turn right and walk 20 m, before turning left and walking

20 m to the shop. How far have you walked?

3. Using the odometer (the distance meter) of a car. You note that before going on a camping trip with your parents that

it reads 27653 km. After driving for 2 hours you and your family arrive at your favourite camping spot. The odometer

now reads 27838 km. Your Father switches the odometer to a “trip meter” which measure distance. On your return

from camping you detour from the normal way of going home to drop off a tent at a friend’s place. When you get home

you note that the trip meter is reading 211 km.

a) What is the distance from your home to the camping spot?

b) What would the odometer reading be on your arrival home?

c) What distance did the detour to drop off the tent add to the total journey home?

ANSWER

ANSWER

ANSWER

ANSWER

ANSWER

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

ACCELERATION

CIRCULAR MOTION

DESCRIBING MOTION

CHANGING MOTION

SPEED & VELOCITY

NEXT

BACK

11

CONCEPT BANK

Describing Displacement
In many situations we may not want to measure the total distance that some person or object
has travelled. We may be more interested in the change in position of the object. We can
do this by comparing the change that took place from an objects starting position to where
it finishes up.

To describe this type of change of position we would need to measure how far it is between
the two points and the direction from the starting position to the finishing position.

This is a measure of an object’s displacement.

The displacement is the shortest distance between the starting and finishing positions.

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

ACCELERATION

CIRCULAR MOTION

DESCRIBING MOTION

CHANGING MOTION

SPEED & VELOCITY

NEXT

BACK

12

CONCEPT BANK

As displacement is a straight line from the start to the finish a single direction can describe
the way the displacement is aimed.

Physicist use the cardinal compass directions (north, south, east and west) and angles to
describe a displacement’s direction from a cardinal point.

N

45o

W

E

S

south or S

east or E

east 45o south

or E 45o S

The last displacement is measured from the easterly direction, 45o towards the southern
direction ( it could also be called S 45o E ).

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

ACCELERATION

CIRCULAR MOTION

DESCRIBING MOTION

CHANGING MOTION

SPEED & VELOCITY

NEXT

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13

CONCEPT BANK

In the first video link below you can view an example of measuring the
displacement that a person or object has travelled. Notice the difference between how the
total distance travelled is found and how the displacement is found.

When measuring the displacement from your original position you must find the length of the
path straight from your starting point to your finishing point and include the direction.

The second video link below revisits the situation described previously when a person goes
from their home to school. In this example a map with a scale is used to measure the
displacement travelled. A compass rose is used to find the direction.

When ready check your understanding of the concepts by doing the Challenge below and by
attempting the problems.

VIDEO: DISPLACEMENT

VIDEO: DISPLACEMENT WALK

CHALLENGE

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

ACCELERATION

CIRCULAR MOTION

DESCRIBING MOTION

CHANGING MOTION

SPEED & VELOCITY

NEXT

BACK

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CONCEPT BANK

Challenge: Use a piece of string to measure the displacement from Enrico’s house to
Marie’s Laboratory. Compare your string length to the scale.

50 metres

PROBLEMS

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ANSWER

P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

ACCELERATION

CIRCULAR MOTION

DESCRIBING MOTION

CHANGING MOTION

SPEED & VELOCITY

NEXT

BACK

15

CONCEPT BANK

Describing Displacement Problems

1. A plane flies 200 km North and then 200 km East. How far is the plane from
its original position?

2. An ant crawls along a metre rule from the 10 cm mark to the 85 cm mark then turns
and returns to the 50 cm mark.

a) How far has the ant crawled?

b) What is the ant’s displacement?

3. You walk from your house to the local shop. You walk 10 m along your driveway, turn
left and walk 200 m to a corner where you turn left again and walk a further 250 m.
You then turn right and walk 20 m, before turning left and walking 20 m to the shop.
Using graph paper, determine the magnitude of your displacement.

4. Attempt to determine your displacement from question 3, if from your starting position
you walked west along your driveway.

5. You travel north from your home in Perth to New Norcia and then return home. What is
your displacement at the end of your journey?
ANSWER

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

ACCELERATION

CIRCULAR MOTION

DESCRIBING MOTION

CHANGING MOTION

SPEED & VELOCITY

NEXT

BACK

16

CONCEPT BANK

Displacement Web Links
These web links provide more information to further explain the concept or to graphically
illustrate the concept. These links were found by searching the following string of key
words:

distance displacement java

http://www.control.co.kr/java1/displacement/DDis.html

distance displacement flash

http://faraday.physics.utoronto.ca/GeneralInterest/Harrison/Flash/ClassMechanics/
DisplaceDistance/DisplaceDistance.swf

You might want to look for other web pages that are similar to these by doing your own
search. Looking at alternate explanations can help you to increase your understanding of
the concepts of distance and displacement.

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

CIRCULAR MOTION

ACCELERATION

DISTANCE & DISPLACEMENT

SPEED & VELOCITY

DESCRIBING MOTION

CHANGING MOTION

NEXT

BACK

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CONCEPT BANK

Describing Speed
How quickly did you travel to school this morning?

The answer to this simple question is a description of motion. We have already looked at
describing motion by using the change in position by measuring distance. The next level in
describing motion is in terms of the rate at which the distance is changing, that is the time
over which the change in distance has taken place. This is the concept of speed.

What units would you use in answering the question posed above? Your answer will
depend on how far you had to travel and the method used to travel to school. Did you
walk? Did you ride a bike? Did you travel in a vehicle such as a bus or car? Or did you
use a combination of these methods?

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

CIRCULAR MOTION

ACCELERATION

DISTANCE & DISPLACEMENT

SPEED & VELOCITY

DESCRIBING MOTION

CHANGING MOTION

NEXT

BACK

18

CONCEPT BANK

Describing Speed
We determine the speed of an object by measuring the total distance travelled and dividing
by the total time it took to move that distance.

There are many different units we can use when describing motion; kilometres
per hour, miles per hour, metres per second, centimetres per minute and
Mach 1 are a few examples. The unit adopted by scientists as the standard unit of speed is
metres per second (m/s or m s-1).

Use the video link to view an example of measuring speed. Listen for the clicks from the
trundle wheel, each click is 1 metre. Notice the time it takes him to travel this distance. Can
you calculate his speed?

v = s
t

VIDEO: TIMED WALK

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ANSWER

P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

CIRCULAR MOTION

ACCELERATION

DISTANCE & DISPLACEMENT

SPEED & VELOCITY

DESCRIBING MOTION

CHANGING MOTION

NEXT

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19

CONCEPT BANK

Describing Speed
We sometimes need to compare speeds. We must also identify the type of speed we are
measuring.

For example, at what speed do you walk? What speed do you travel at when riding a bicycle?
What is the speed of a car, bus, train or plane? How fast does a snail move? What is the
fastest animal on the planet? What is the land speed record? At what speed has the fastest
human run? Sometime in the future you might also like to know about speeding tickets.
What speed is recorded on the speeding ticket?

Answers to these questions need us to be clear about the type of speed being measured.
Is it an instantaneous speed, an average speed, a constant speed, an initial speed or a final
speed? View the video of a car speedometer. This may help you understand the difference
in the types of speed.

VIDEO: SPEEDO 2

VIDEO: CAR WITH SPEEDO 1

VIDEO: SPEEDO 1

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

CIRCULAR MOTION

ACCELERATION

DISTANCE & DISPLACEMENT

SPEED & VELOCITY

DESCRIBING MOTION

CHANGING MOTION

NEXT

BACK

20

CONCEPT BANK

Describing Speed
The common unit of speed is usually given in metres per second (m s-1). However, there are
situations in which speed is given in kilometres per hour (km h-1), and it will be necessary to
covert one unit of speed into another. To make the conversion of km h-1 to m s-1 we use the
following information.

1 kilometre = 1000 metres
1 hour = 3600 seconds

So 1 kilometre per hour = 1000 metres per hour

In 1 second the object will travel 1/3600 of the distance it travels in 1 hour so
1 kilometre per hour =

1000 metres per second =

1 metres per second

3600

3.6

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

CIRCULAR MOTION

ACCELERATION

DISTANCE & DISPLACEMENT

SPEED & VELOCITY

DESCRIBING MOTION

CHANGING MOTION

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CONCEPT BANK

Describing Speed
So to convert kilometres per hour to metres per second you must divide by 3.6

Example: 72 km h-1 to m s-1 = 72 = 20 m s-1

3.6

To convert m s-1 to km h-1 we multiply by 3.6

Example: 30 m s-1 to km h-1 = 30 x 3.6 = 108 km h-1.

If you forget whether to divide or multiply then try either one and then think if the answer
makes sense. Is 72 km h-1 more likely to be 259.2 m s-1 or 20 m s-1?

The video provides a step-by-step method using another example. When ready attempt the
questions on the next two pages to check your understanding of speed.

PROBLEMS

CHALLENGE

EXTRA EXAMPLES

VIDEO EXPLANATION

VIDEO: SHORT m s-1 to km h-1

VIDEO: LONG m s-1 to km h-1

VIDEO: SHORT km h-1 to m s-1

VIDEO: LONG km h-1 to m s-1

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

CIRCULAR MOTION

ACCELERATION

DISTANCE & DISPLACEMENT

SPEED & VELOCITY

DESCRIBING MOTION

CHANGING MOTION

NEXT

BACK

22

CONCEPT BANK

Describing Speed Problems

1. Below is a timetable for an electric train that travels between Cottesloe and Fremantle. Use this to answer the
questions below. When calculating speed find it in metres per second (m s-1).

Station

Distance from Cottesloe

Time of Arrival

Cottesloe

0.0 km

10.14 am

Mosman Park

1.1 km

10.18 am

Victoria Street

2.0 km

10.20 am

North Fremantle

4.0 km

10.23 am

Fremantle

7.2 km

10.30 am

a) Which stations are furthest apart?

b) Which section of the journey took the longest time?

c) Find the average speed of the train between Cottesloe and Mosman Park.

d) What was the average speed between Mosman Park and Victoria Street?

e) Calculate the average speed between Cottesloe and Victoria Street.

f) Determine the average speed for the journey from Cottesloe to Fremantle.

ANSWER

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North Fremantle

& Fremantle

P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

CIRCULAR MOTION

ACCELERATION

DISTANCE & DISPLACEMENT

SPEED & VELOCITY

DESCRIBING MOTION

CHANGING MOTION

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23

CONCEPT BANK

Describing Speed Problems

2. What is the speed (in metres per second) of a bicycle rider that travels
4 km in 10 minutes?

3. What is the speed of a plane that travels 1000 km in 2 hours

a) in km h-1?

b) m s-1?

4. How long will it take a person to walk 7200 m if they travel with a constant
speed of 6 km h-1?

5. What distance will a bird travel if it can fly at 5 m s-1 for 6 hours?

ANSWER

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

CIRCULAR MOTION

ACCELERATION

DISTANCE & DISPLACEMENT

SPEED & VELOCITY

DESCRIBING MOTION

CHANGING MOTION

NEXT

BACK

24

CONCEPT BANK

Describing Velocity
We have already looked at describing motion by using the change in position of an object by
measuring the total distance travelled.

We then extended our ability to describe motion by looking at the change in position of an
object with regard to where the object started from and where it finished. We called this the
vector quantity displacement.

Similarly, we have described motion by comparing the rate of change of an object by using
the concept speed, that is the total distance travelled divided by the time it took to travel that
distance.

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

CIRCULAR MOTION

ACCELERATION

DISTANCE & DISPLACEMENT

SPEED & VELOCITY

DESCRIBING MOTION

CHANGING MOTION

NEXT

BACK

25

CONCEPT BANK

Describing Velocity
If we now think about the rate of change of an objects position with regard to where it started
and where it finished, the objects displacement, we have another way of describing motion.
The rate of change in displacement is defined as average velocity.

average velocity

=

displacement

vave = s

time taken

t

The unit adopted by scientists as the standard unit of velocity is the same as for speed, that
is metres per second (m/s or m s-1). However, as velocity is a vector quantity it must also
have a direction. For example a car travelling 200 m north in a time of 10 seconds would
have a speed of 20 m s-1 but the same car has a velocity of 20 m s-1 north.
Use the video links below to view two examples of measuring velocity.

VIDEO: TIMED WALK

VIDEO: JOGGING

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

CIRCULAR MOTION

ACCELERATION

DISTANCE & DISPLACEMENT

SPEED & VELOCITY

DESCRIBING MOTION

CHANGING MOTION

NEXT

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26

CONCEPT BANK

Describing Velocity
In the previous videos the length of the measuring tape on the floor was 25 metres. Notice
the time for both of the people. Calculate the velocity of the two people?

Does it matter that they both went different ways? They both started at the same point and
ended up at the same point too so they both have the same displacement.

They covered that displacement over different times so their velocity must be different.

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ANSWER

ANSWER

P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

CIRCULAR MOTION

ACCELERATION

DISTANCE & DISPLACEMENT

SPEED & VELOCITY

DESCRIBING MOTION

CHANGING MOTION

NEXT

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27

CONCEPT BANK

Describing Velocity
We sometimes need to compare velocities. Like speed, we must also identify the type of
velocity we are measuring.

The velocity used in describing motion could be an instantaneous velocity, an average
velocity, a constant or uniform velocity, or an initial velocity and a final velocity.

Use the video link to view examples of different velocities.

When ready test your understanding of the concept of velocity by attempting the problems.

PROBLEMS

VIDEO: VELOCITIES

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

CIRCULAR MOTION

ACCELERATION

DISTANCE & DISPLACEMENT

SPEED & VELOCITY

DESCRIBING MOTION

CHANGING MOTION

NEXT

BACK

28

CONCEPT BANK

Describing Velocity Problems

1. A plane flies 200 km North and then 200 km East in a time of 40 minutes.

a) What is the plane’s average speed?

b) What is the plane’s average velocity?

2. You walk from your house to the local shop. You walk 10 m West along your
driveway, turn left and walk 200 m to a corner where you turn left again and walk
a further 250 m. You then turn right and walk 20 m, before turning left and
walking 20 m to the shop.

a) What is your average speed if you took 400 seconds to walk to the shop?

b) What is your average velocity for the same trip?

3. Kylie can run at 6.00 m s-1, whilst Marian can only manage 5.00 m s-1.

a) If they are in a 1500 m event how long would it take each to run the race?

b) By what distance does Kylie beat Marian to the finishing line?

Continued on the next page.

ANSWER

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

CIRCULAR MOTION

ACCELERATION

DISTANCE & DISPLACEMENT

SPEED & VELOCITY

DESCRIBING MOTION

CHANGING MOTION

NEXT

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29

CONCEPT BANK

Describing Velocity Problems

4. A car travelling along a straight stretch of road has a velocity of 100 km h-1 East.
What will be its displacement after a time of 1 minute?

5. A plane flies north with an average speed of 200 km h-1 for 2 hours before
turning east and travelling at the same speed for a further 3 hours.

a) What is the distance travelled by the plane?

b) What is the plane’s displacement?

c) What is the average velocity of the plane?

ANSWER

ANSWER

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

CIRCULAR MOTION

ACCELERATION

DISTANCE & DISPLACEMENT

SPEED & VELOCITY

DESCRIBING MOTION

CHANGING MOTION

NEXT

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30

CONCEPT BANK

Describing Velocity Web Links
These links were found by searching the following string of key words:

velocity speed java

http://faraday.physics.utoronto.ca/PVB/Harrison/Flash/ClassMechanics/
MotionDiagram/MotionDiagram.html

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

CIRCULAR MOTION

ACCELERATION

DESCRIBING MOTION

CHANGING MOTION

SPEED & VELOCITY

NEXT

BACK

31

CONCEPT BANK

Describing Acceleration
On the previous pages we have encountered a few ways of describing motion using terms
such as distance travelled, displacement, speed and velocity. Displacement and velocity are
both vectors whereas distance travelled and speed are scalar quantities. The motion we
have been describing has mostly involved objects moving at a uniform or constant rate. We
also need a way of describing when objects speed up or slow down. To do this we use the
term acceleration.

An object accelerates when it increases its speed, for example when a car is stationary and
moves off from a set of traffic lights it is accelerating. Acceleration is defined as the rate of
change of velocity and is a vector quantity.

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

CIRCULAR MOTION

ACCELERATION

DESCRIBING MOTION

CHANGING MOTION

SPEED & VELOCITY

NEXT

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32

CONCEPT BANK

Describing Acceleration
If an object slows down the acceleration is in the direction opposite to the direction in which
the object is travelling. Scientists also use the term deceleration to describe this type of
motion. We use the symbol ‘a’ to represent acceleration.
Acceleration is the rate of change of velocity

acceleration = change in velocity

time taken

We can use an algebraic equation to represent acceleration. We describe the final velocity as
v and the initial velocity u. The change in velocity would then become v - u.

So a = v - u which can be rearranged to v = u + at.

t

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

CIRCULAR MOTION

ACCELERATION

DESCRIBING MOTION

CHANGING MOTION

SPEED & VELOCITY

NEXT

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CONCEPT BANK

Describing Acceleration
The units of acceleration (and deceleration) would be m s-1 = m s-2.

s

View the first video link to watch an example of acceleration. The second video shows the
car’s speedometer as the car accelerates.

When you have finished watching the video, describe the motion of the car.

There are many different examples of objects undergoing acceleration. List a few examples
of acceleration or deceleration that you have experienced or know about.

When you have done this view the third video and describe the motion of the cart on the
ramp.

VIDEO: CART ON A RAMP

VIDEO: CAR SPEEDOMETER

VIDEO: CAR ACCELERATING

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

CIRCULAR MOTION

ACCELERATION

DESCRIBING MOTION

CHANGING MOTION

SPEED & VELOCITY

NEXT

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CONCEPT BANK

Describing Acceleration
The video of the car showed it accelerating in a straight line from a stationary position.
The car’s initial velocity was 0 m s-1. As the car accelerated its velocity was changing. In
this particular example the car accelerated at a uniform rate until it achieved a velocity of
30 km h-1 south. This was shown by the steady increase of the speedometer. The car then
stopped accelerating and continued to move with a constant velocity of 30 km h-1 south
(8.33 m s-1 south). In this example the car was using the power generated by the car’s engine
to move.

In the second example a cart was moving down a ramp. In this example the cart is accelerating
down the slope of the ramp at a constant rate. The car accelerates because of the action of
the Earth’s gravity. We will investigate the cart on the slope in more detail when we look at
gravity.

Complete the questions to check your understanding of acceleration.

PROBLEMS

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

CIRCULAR MOTION

ACCELERATION

DESCRIBING MOTION

CHANGING MOTION

SPEED & VELOCITY

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CONCEPT BANK

Describing Acceleration Problems

1. A car at rest at a set of traffic lights accelerates uniformly to a velocity of 10 m s-1

East in a time of 5 seconds. What is the acceleration of the car?

2. A train decelerates at a constant rate from a velocity of 23 m s-1 West to a

stationary position in 5.75 seconds. What is the deceleration of the train?

3. A bike speeds up from 5 m s-1 West to 15 m s-1 West in a time of 20 seconds.

What is the acceleration of the bike?

4. A truck initially at rest is accelerated at 2 m s-2 South for 5 seconds. What is the

trucks final velocity?

5. A motor bike travelling at 30 m s-1 East decelerates at a constant 6 m s-2 West.

How long will it take for the motor bike to stop?

ANSWER

ANSWER

ANSWER

ANSWER

ANSWER

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

ACCELERATION

SPEED & VELOCITY

CIRCULAR MOTION

DESCRIBING MOTION

CHANGING MOTION

NEXT

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36

CONCEPT BANK

Describing Circular Motion
In the previous examples of motion we have been using situations involving objects moving
in straight lines. This is called linear motion. But we know that not everything travels in
straight lines. Objects can move in various paths including objects that trace out a circular
path. Think about examples where you have experienced motion in a circle or examples of
motion where you know that objects move in a circular path. Write down a few examples of
this circular motion.

The three video links below show examples of circular motion.

VIDEO: STOPPER & STRING

VIDEO: CIRCULAR MOTION

VIDEO: CEMENT MIXER

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P H Y S I C S
DESCRIBING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

ACCELERATION

SPEED & VELOCITY

CIRCULAR MOTION

DESCRIBING MOTION

CHANGING MOTION

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37

CONCEPT BANK

Describing Circular Motion
When moving in a circular path an object is constantly changing its direction of travel where
the speed may remain constant. As there is a constant change in direction there is a change
in velocity. This implies an acceleration. In these instances the acceleration is always
directed to the centre of the objects circular path and is know as centripetal (centre seeking)
acceleration.

Some examples of circular motion include the objects moving in a horizontal circular path
such as a merry-go-round, riding the Gravitron or a car going around a bend or an athlete
throwing the hammer. Another example includes satellites orbiting the Earth.

Another type of circular motion is vertical circular motion. Examples of this type of motion
include riding a Ferris Wheel or driving a car over a hump or into a dip in the road. You will
investigate circular motion in more detail if you continue onto studies of physics at year 11
and 12.

1•2

P H Y S I C S
CHANGING MOTION

INTRODUCTION

MOMENTUM & FORCES

CIRCULAR MOTION

CHANGE SPEED & VELOCITY

NEWTONʼS 2nd & 3rd LAWS

DESCRIBING MOTION

CHANGING MOTION

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CONCEPT BANK

Changing Motion - Introduction
In the first section you were introduced to a number of concepts and units that are used
to describe motion. These included distance travelled, displacement, speed, velocity,
acceleration and centripetal acceleration. Most of these concepts are related to changing
the motion of objects.

We will use these concepts in this section and will begin to understand in more depth the
relationship between them, as well as being introduced to some other ideas and laws that
are related to changing motion.

Use the link below to view some examples of various objects changing their motion. In each
case use the concepts to which you have been introduced to describe the changes in the
motion taking place.

VIDEO: OBJECTS CHANGING

1

P H Y S I C S
CHANGING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

DESCRIBING MOTION

CHANGING MOTION

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39

CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Changing Speed
In the previous section there were a number of examples of motion that were described
using terms such as uniform or constant motion.

There were other examples where the description involved objects either increasing or
decreasing their motion. In these examples you were introduced to the terms acceleration
and deceleration.

Use the video link to view further examples where the speed of objects is changing by either
going faster or slowing down.

VIDEO: CHANGING MOTION

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P H Y S I C S
CHANGING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

DESCRIBING MOTION

CHANGING MOTION

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40

CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Changing Speed
In these examples the speed of the objects are changing because of acceleration. Note that
as it is the speed of the object that is changing, then we are only interested in the magnitude
of the change and not the direction in which the object is travelling. However, for objects to
change speed the objects are experiencing some form of acceleration.

The video link below shows examples of where objects are not changing speed, that is, the
magnitude of the motion is an example of constant or uniform motion.

In these examples the objects continue to move with the same speed with or without a
change in direction, or remain stationary. For example a car that turns around a corner without
slowing down has a constant speed even though the direction of its travel has changed.

VIDEO: UNIFORM MOTION

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P H Y S I C S
CHANGING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

DESCRIBING MOTION

CHANGING MOTION

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41

CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Changing Velocity
Velocity is a vector quantity so when we discuss objects experiencing a change in velocity
we are now concerned with both the magnitude and the direction that results in the change
in velocity.

If an object accelerates in the same direction as it is travelling initially then the object will get
faster. If an object decelerates in the same direction as it is travelling it will slow down. For
example if you are riding a bike along a straight stretch of road and start pedalling faster your
velocity will increase. If you then stop pedalling the bike and apply the brake, your velocity
will then decrease.

View the video link to see another example of where a variety of objects are changing just the
magnitude of their velocity.

VIDEO: CHANGE MAGNITUDE

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P H Y S I C S
CHANGING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

DESCRIBING MOTION

CHANGING MOTION

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42

CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Changing Velocity
Objects that travel with a constant speed that experience a change in direction without
altering the magnitude of speed are changing velocity.

Using our previous example of the car that turns around a corner without changing speed,
the car is experiencing a change in velocity because there is a change in the direction of
travel. The change in velocity resulting in the altered motion must have both a magnitude and
direction. A change in the velocity of an object involves an acceleration.

When solving problems involving vectors you may need to draw a vector diagram. To solve
problems may require either the addition or the subtraction of vectors. To find changes in
velocity you must perform a vector subtraction.

View the video links to see explanations of how to add and subtract vectors.

VIDEO: ADD VECTORS

VIDEO: SUBTRACT VECTORS

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P H Y S I C S
CHANGING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

DESCRIBING MOTION

CHANGING MOTION

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43

CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Changing Velocity
For example a car travelling at 10 m s-1 west (u) turns a corner without a loss of speed to
travel with a velocity of 10 m s-1 south (v). The change in velocity requires a vector subtraction
[v + (-u)].

N

45o

W

E

v

R

S

-u

The result (R) of the vector subtraction [v + (-u)] is a change in velocity of
14.1 m s-1 east 45o south or E 45o S. If we know the time over which this change took
place we can measure the acceleration, which will be in the same direction as the change
in velocity, south-east.

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P H Y S I C S
CHANGING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

DESCRIBING MOTION

CHANGING MOTION

NEXT

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44

CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Changing Velocity
View the video link below to see examples of objects changing velocity because the object is
experiencing a change in the direction of its travel without changing the speed of travel.

An object that is moving with a constant speed in a circular pattern is constantly changing its
velocity because its direction of travel is constantly changing. We call this constant change
centripetal acceleration.

When you are ready check your understanding of changes of speed and velocity by attempting
the problems.

VIDEO: CHANGING VELOCITY

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PROBLEMS

P H Y S I C S
CHANGING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

DESCRIBING MOTION

CHANGING MOTION

NEXT

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45

CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Changing Velocity Problems

1. A car travelling north at 10.0 m s-1 increases its velocity to 20.0 m s-1. What is
the change in velocity of the car?

2. A bicycle travelling west along a road at 6.0 m s-1 slows down to 4.0 m s-1
when the brakes are applied. What is the bicycle’s change in velocity?

3. A plane flies north with a velocity of 300 km h-1 and then turns west without
changing its speed. What is the change in velocity of the plane?

ANSWER

ANSWER

ANSWER

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P H Y S I C S
CHANGING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

DESCRIBING MOTION

CHANGING MOTION

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46

CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Inertia and Mass
Mass can be defined simply as the amount of matter that is contained within an object or
material. In science we use the units such as kilogram, gram, tonne etc. to describe the mass
of an object. The standard unit of mass is the kilogram.

Because objects have mass this will affect the way that the object moves. Objects that have
mass also have a property called inertia, the property of the object that resists changes to its
motion. All objects have inertia whether they are stationary or moving.

View the video link that shows examples of objects with different mass.

VIDEO: DIFFERENT MASSES

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P H Y S I C S
CHANGING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

DESCRIBING MOTION

CHANGING MOTION

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CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Inertia and Mass
We often talk or describe objects as being “massive”. These are objects that contain more
mass that other objects to which they are being compared. Massive objects have greater
inertia than less massive objects. Consequently it is much harder to change the motion of
more massive objects whether these objects are stationary or moving.

To cause a stationary object to move requires overcoming this property of inertia. Similarly,
to change the speed or velocity of an object also requires overcoming the object’s inertia. To
overcome inertia requires a force.

The video below demonstrates examples of the push or pull required to alter the motion of
objects with different masses.

VIDEO: PUSH AND PULL

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P H Y S I C S
CHANGING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

DESCRIBING MOTION

CHANGING MOTION

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48

CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Newton’s First Law
To change the motion of an object requires a force to overcome the objects inertia. Sir Isaac
Newton described this observation in his book titled “Principia“ in 1687, in a law now known
as Newton’s First Law of Motion.

“Every body perseveres in its state of rest, or uniform motion in a straight line, except in so
far as it is compelled to change that state by forces impressed upon it.”

We have modified this statement to read: An object at rest or moving with a constant speed
in a straight line will remain at rest or continue moving with a constant speed unless acted
upon by an externally unbalanced force.

The video link below illustrates this law by looking at the forces required to change the motion
of objects with different masses.

VIDEO: FORCES AND MASS

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P H Y S I C S
CHANGING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

DESCRIBING MOTION

CHANGING MOTION

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49

CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Momentum
Momentum is a property of an object that is the product of the object’s mass and velocity.

Physicists use the symbol ρ to represent momentum. It is the product of a vector quantity
(velocity) so momentum is also a vector quantity where the direction of the momentum will
be in the same direction as the velocity. The standard international (SI) unit for momentum is
kg m s-1.

A fast moving car has more momentum than a car of the same mass moving more slowly.
Similarly, a car will have less momentum than a truck moving with the same speed.

The video links below show some examples of objects with differing momenta. When ready
check your understanding by attempting the problems.

ρ = m x v

VIDEO: MOMENTUM

PROBLEMS

VIDEO: COMPARING ρ

VIDEO: COMPARING ρ

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INTRODUCTION

DISTANCE & DISPLACEMENT

DESCRIBING MOTION

CHANGING MOTION

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50

CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Momentum Problems

1. What is the momentum of a 50.0 g bullet travelling at 155 m s-1 west
towards a target?

2. What is the mass of an object if it has a momentum of 32.0 kg m s-1 and
travels at 0.500 m s-1?

3. What is the momentum of a 30.0 tonne locomotive travelling east towards
Eucla at a speed of 100.0 km h-1?

4. What is the mass of an object travelling at 25.0 m s-1 north if its momentum
is 675 kg m s-1?

5. Which has the greater momentum a 75.0 kg netballer running at 2.00 m s-1
or a 92.0 kg footballer running at 2.20 m s-1?

ANSWER

ANSWER

ANSWER

ANSWER

ANSWER

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CHANGING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

DESCRIBING MOTION

CHANGING MOTION

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CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Changing Momentum
As the momentum of an object is the product of mass and velocity, a change in momentum
would mean that the object is experiencing a change in velocity by either getting faster or
slower, or changing the direction of its motion. To cause a change in velocity, from Newton’s
First Law of Motion an object must experience some form of acceleration resulting from the
application of an externally unbalanced force.

There are several things that could cause a change in momentum.

An object that is initially at rest could experience a pushing or pulling force to overcome its
inertia to give it a velocity. The object’s initial velocity was zero therefore its initial momentum
was also zero. The final momentum is calculated by the product of its mass and final velocity.
An example could be a runner in a 100 metre sprint race.

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INTRODUCTION

DISTANCE & DISPLACEMENT

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CHANGING MOTION

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CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Changing Momentum
An object that is moving with a constant velocity could experience a force that causes it to
increase or decrease its speed, or to stop it moving completely.

Examples could include a plane that experiences a strong tail wind such that it increases its
speed or the same plane could encounter a strong headwind that slows it down. A speeding
car that experiences an inelastic collision with a tree or telegraph pole could be a tragic
example of a large change in momentum.

Alternatively an object might experience a force that could change the direction in which the
object is moving such as a cricket bat or hockey stick hitting a ball.

See if you can suggest some other examples where objects are experiencing a change in
momentum.

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P H Y S I C S
CHANGING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

DESCRIBING MOTION

CHANGING MOTION

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CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Changing Momentum
The change in momentum (∆ρ) is equal to the final momentum minus the initial momentum,
and has the units kg m s-1.

View the video that demonstrates a number of examples where an object is experiencing
a change in momentum. For each example describe the change in momentum that is
occurring.

When ready attempt the questions to check your understanding of changing momentum.

Δρ = (m x v)-(m x u)

or
Δρ = m (v - u)

VIDEO: CHANGE MOMENTUM 1

PROBLEMS

VIDEO: CHANGE MOMENTUM 2

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P H Y S I C S
CHANGING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

DESCRIBING MOTION

CHANGING MOTION

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CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Changing Momentum Problems

1. What is the change in momentum of a 50 g bullet that starts from rest and is

accelerated to a speed of 100 m s-1 west towards a target?

2. A 30 tonne locomotive travelling at 2.0 m s-1 north is struck from behind by another

similar locomotive to double its velocity to 4.0 m s-1 north. What is the locomotives

change in momentum?

3. A 150 g cricket ball is bowled at a batsman with a velocity at the point of impact of 80 km h-1 travelling west and

level with the ground. The ball is hit directly back along the ground towards the bowler with no loss in speed. What

is the ball’s change in momentum?

4. A 1000 kg car travelling west at 60 km h-1 stops completely on impacting a brick wall.

What is the car’s change in momentum?

5. A 0.1 kg billiard ball travelling at 0.5 m s-1 strikes the cushion at an angle of 45 ̊ and

rebounds at the same angle with no loss of speed. Calculate the change in

momentum for the ball.

6. The same ball as in problem 5 strikes and rebounds the cushion at right angles without

any loss of speed. Calculate the change in momentum of the ball in this instance.

ANSWER

ANSWER

ANSWER

ANSWER

ANSWER

ANSWER

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CHANGING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

DESCRIBING MOTION

CHANGING MOTION

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CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Forces and Momentum
As indicated in Newton’s First Law of Motion and described by examples in the previous
section, an object that experiences a change in momentum does so because it experiences
an externally unbalanced force.

The change in momentum ∆p = mv - mu

or ∆p = m (v-u).

If we know the time in which the change in momentum has taken place then we can find the
rate of change of momentum. This is equal to the force applied.

F = m x (v-u)

t

or F = m a

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DISTANCE & DISPLACEMENT

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CHANGING MOTION

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CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Forces and Momentum
We can rearrange the force equation:

F = m (v-u)

t

so

F x t = m (v-u)

F x t = Δρ

We call the change in momentum (Δρ) the impulse.

View the video and identify and describe the externally unbalanced forces that cause the
change in momentum in each specific example.

VIDEO: IMPULSE 1

VIDEO: IMPULSE 2

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CHANGING MOTION

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CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Forces and Momentum
The change of momentum in the first example is caused by the large air-track slider impacting
on the small slider. The force during the impact changed the small slider’s momentum, as it’s
velocity changes. Notice that the large slider’s momentum also changed due to the velocity
of the large slider decreasing during the impact.

In the second video the larger slider had it’s momentum changed three times during each
impact with the small slider. The first impact force decreased the momentum of the large
slider by half. The second impact force completely stops the large slider, so it’s momentum
is zero. The third impact force changes the large slider’s momentum once again. Notice
once again that the smaller slider had it’s momentum changed during each impact too.

When you are ready check your understanding of forces and momentum by attempting the
problems.

PROBLEMS

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DISTANCE & DISPLACEMENT

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CHANGING MOTION

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CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Forces and Momentum Problems

1. Calculate the impulse required to change a 30 tonne locomotive travelling at
2.0 m s-1 north to velocity to 4.0 m s-1 north.

2. If the time for the change in momentum in problem 1 above was 5 seconds,
what force was required to change the velocity of the locomotive?

3. A force of 10 N west is applied to a 10 kg toy truck originally at rest for
2 seconds. What is the toy truck’s change in momentum?

4. A 150 g cricket ball is bowled at a batsman with a velocity at the point of impact
of 80 km h-1 travelling west and level with the ground. The ball is hit directly back
along the ground towards the bowler with no loss in speed. The bat is in contact
with the ball for 0.01 seconds to cause the change in momentum. What force does
the bat exert on the ball?

ANSWER

ANSWER

ANSWER

ANSWER

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P H Y S I C S
CHANGING MOTION

INTRODUCTION

DISTANCE & DISPLACEMENT

DESCRIBING MOTION

CHANGING MOTION

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CONCEPT BANK

CHANGE SPEED & VELOCITY

CIRCULAR MOTION

MOMENTUM & FORCES

NEWTONʼS 2nd & 3rd LAWS

Acceleration
Any object that speeds up, slows down or changes its direction of travel is undergoing an
acceleration. Acceleration was defined in an earlier section as the rate of change of velocity.
In the same section the units of acceleration were described as m s-2 and as acceleration is
a vector quantity, we know we must also have a specified direction.

The change in velocity is the difference between the final velocity (v) and the initial velocity (u).
We can also use an algebraic equation to represent acceleration.

The first video shows some examples of objects that are accelerating. The second and third
video shows examples of acceleration calculations. When ready attempt the questions to
check your understanding.

a = (v - u)

t

VIDEO: ACCELERATION

PROBLEMS

VIDEO: EXAMPLE 1

VIDEO: EXAMPLE 2

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Acceleration Problems

1. A pebble is dropped down a well. If it took 3.0 seconds to strike the water below
with a velocity of 29.4 m s-1, what is the acceleration of the pebble?

2. What is the average velocity of the pebble from question 1 above?

3. A shuttle is lifted off into space with an acceleration of 22.0 m s-1 for 6 seconds.
What is the change in velocity of the shuttle?

ANSWER

ANSWER

ANSWER

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Newton’s Second Law
Newton discovered many interesting physical principles and laws associated with motion that
are fundamental to a branch of physics known as Newtonian mechanics. He also discovered
many physical principles associated with optics and fluids.

In an earlier section Newton’s First law of Motion was described. This is often described
as Newton’s Law of Inertia. In other sections you have been introduced to the concept of
momentum and change in momentum. In his book Pricipia Newton described these concepts
in what is known as his Second law of Motion.

“The rate of change of linear momentum is proportional to the force applied, and takes place
in the straight line in which that force acts.”

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Newton’s Second Law
So the force is equal to the momentum divided by time.

By slightly rearranging the equation we get:

But a= (v - u)/t , so:

F = m (v - u)

t

F = m x (v - u)

t

F = m x a

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Newton’s Second Law
We have modified Newton’s original statement to read:

“The acceleration of an object is directly proportional to the force applied and inversely
proportional to the mass of the object. The direction of the acceleration is in the direction of
the applied force”.

In other words Force is equal to the mass times acceleration. This is often referred to as
Newton’s Law of momentum.

F = m x a

PROBLEMS

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Newton’s Second Law Problems

1. What force is required to accelerate a 10 kg mass at 2.0 m s-2 east?

2. A stationary 1000 kg car is subjected to a force of 800 N west? What will be the car’s
acceleration?

3. A mass is accelerated at 10.0 m s-2 north-west whilst a force of 5 N north-west
is applied. What is the mass of the object?

4. Compare the acceleration of a 5000 kg truck and a 100 kg scooter if both are
subjected to the same force of 75 N south.

ANSWER

ANSWER

ANSWER

ANSWER

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Forces Cause Acceleration
A force is a push, pull or twist that changes the position, speed, direction of motion or shape
of an object. Forces cause masses (whether large or small) to undergo an acceleration or a
deceleration. The size of the resultant acceleration depends on the size of the force being
exerted and the mass of the object experiencing the force. Forces are measured using the
unit known as the newton (N) named after Sir Isaac Newton. A force of 1 newton is defined
as the force that would accelerate a 1.00 kg mass at 1.00 m s-2.

The video link demonstrates some examples of objects that are being accelerated by different
forces. Identify the force causing the acceleration in each of the examples.

When ready attempt the questions to test your understanding of forces and acceleration.

VIDEO: ACCELERATION

PROBLEMS

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Force Causing Acceleration Problems

1. If the shuttle and launcher have an initial combined mass of 200 000 kg, what
force is required to give the shuttle an acceleration of 30.0 m s-2?

2. What acceleration does a 40.0 kg passenger experience when a car exerts a
force of 400 N on her?

3. A force of 700 N accelerates a ski lift chair at 2.5 m s-2. What is the mass of
the chair?

4. The same ski lift chair now has two 60.0 kg passengers, with the same force
applied what will be the acceleration of the chair and passengers?

ANSWER

ANSWER

ANSWER

ANSWER

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Newton’s Third Law
We have seen two of Newton’s Laws of Motion; his Law of Inertia and his Law of
Momentum.

Newton identified that all forces are caused by the interaction of pairs of bodies. His Third
Law is described as

“The force exerted by A upon B and the force exerted by B upon A are: simultaneous;
equal in magnitude; opposite in direction; in the same straight line; caused by the same
mechanism.”

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Newton’s Third Law
This is often simplified to read:

“For every action (force) there is an equal and opposite reaction (force)”

Please note that this interpretation can result in a misconception that implies that one “action”
force subsequently causes a “reaction force”. This is incorrect, as object A exerts a force on
object B, object B exerts an equal force simultaneously on object A. For example, if you stand
on your feet, the Earth attracts your mass to it whilst your mass attracts the Earth equally. It is
the interaction between your mass and the mass of the Earth that causes the paired forces.

View the video that demonstrates various interactions between pairs of bodies. When ready
attempt the questions to test your understanding of forces and acceleration.

VIDEO: INTERACTIONS

PROBLEMS

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Newton’s Third Law Problems

1. Calculate the force your mass exerts on the ground. This is your weight. What force
does the ground exert back on you?

2 A 100 000 kg wide bodied jet can exert 40 000 N thrust from each of its 4 engines.

a) What will be the acceleration of the plane down the runway?

b) To take off the plane must achieve a speed of 300 km h-1. How long will it take
the plane to achieve the necessary speed to take off?

c) What distance will the plane travel in achieving take off speed?

3. What force will exist in a wire cable of a crane, if it is suspending a 1000 kg iron girder
5 metres above a building site?

4. What is the frictional force required to give a 50 kg bike an acceleration of
5.0 m s-2 east?

ANSWER

ANSWER

ANSWER

ANSWER

ANSWER

ANSWER

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Forces Cause Circular Motion
We have seen that forces cause a change in velocity. So far we have only considered
changes in line with the objects initial velocity, either in the same direction or in the opposite
direction.

Forces that act at right angles to the objects velocity cannot increase or decrease the magnitude
of the object’s velocity, as the force is not pushing or pulling it forwards or backwards. The
force is pushing or pulling it sideways so the direction of the velocity will change.

velocity

force

car

path of the car

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Forces Cause Circular Motion
If the direction of the force is always at right angles to the velocity then the direction will
always change too. This results in the object moving in a circular path.

velocity

force

circular path

velocity

force

velocity

force

velocity

force

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Forces Cause Circular Motion
When you turn the steering wheel in a car the tyres push outwards on the road, you can
sometimes see stones and dirt being forced outwards when rally cars turn a corner.

The road applies an equal and opposite force on the car. So the car experiences a force
inwards, towards the centre of the curve it is going around. The video below shows a car
experiencing an inwards force causing circular motion.

force of car on the road

force of road on the car

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VIDEO: CIRCULAR

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Introduction
The following pages contain definitions for the concepts associated with motion and a link to
a video that shows the concept in a real situation.

Use the links below to go to the definitions of the concepts.

Vector & Scalar

Gravity
Distance & Displacement

Mass & Weight
Speed and Velocity

Centre of Gravity
Acceleration & Deceleration

Energy & Work
Force, Impulse & Momentum

Gravitational Potential Energy
Friction

Kinetic Energy

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Vector and Scalar
Vector - this is a physical quantity that needs to be described using both a magnitude (size)
and direction.

Scalar - this is a physical quantity that needs to be described only using a magnitude (size).

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Distance and Displacement
Distance - this is a measure of length. It could be the total distance travelled by an object or
the space between objects. Scientists measure distance using the unit metre. Distance is a
scalar quantity

Displacement - this is defined as the change in position that an object has experienced. It is
calculated by measuring the straight-line distance between the initial or starting position of
an object and the final or finishing position of the object and describing the direction from the
starting position to the finishing position. Displacement is a vector quantity.

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Speed and Velocity
Speed - this is the rate of change of distance. Speed is a scalar quantity measured in metres
per second, kilometres per hour.

Velocity - this is defined as the rate of change of displacement. There are several forms of
velocity used in describing motion such as the initial, final, instantaneous, uniform or constant
velocity. These are all vector quantities usually measured in metres per second or kilometres
per hour.

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Acceleration and Deceleration
Acceleration - this is defined as the rate of change of velocity (or speed) where an object
is getting faster. It is a vector quantity that is usually measured in metres per second per
second.

Deceleration - this is the rate of change of velocity (or speed) where an object is slowing
down. It is a vector quantity that is usually measured in metres per second per second.

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Force, Impulse and Momentum
Force - a force is a push, pull or twist that changes the position, speed, direction of motion
or shape of an object. Forces are vector quantities measured in newtons (N).

Impulse - is defined as the change in momentum, that is calculated by the product of the
force acting and the time over which that force acts. Its units are kg m s-1, or N s.

Momentum - linear momentum is defined as the product of the mass and the velocity of that
mass. Its units are kg m s-1. It is a vector.

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Friction
Friction - a force that opposes the sliding of one surface over another. Friction acts between
any two surfaces in contact with one another that creates heat as one surface retards the
motion of the other.

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Gravity
Gravity - this is a non-contact force that causes mutual attraction between objects such as
the force that pulls together a mass and the earth. This force is also caused gravitation.

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Mass, Weight & Inertia
Mass - this is the amount of matter that makes up an object or substance. It is usually
measured in grams, kilograms or tonnes.

Weight - this is a downward force acting on a mass due to gravity. It is measured in newtons
(N).

Inertia - This is the property of a mass that resists changes to its motion.

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Centre of Gravity
Centre of Gravity - this is a single point within an object at which it appears as if all the mass
of the object is acting.

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Energy
Energy - this is the ability to do work. Energy is measured in joules (J).

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Gravitational Potential Energy
Gravitational potential energy- this is the energy an object has because of its position relative
to the centre of the earth, or the objects height above the Earth’s surface.

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Kinetic Energy
Kinetic energy - this is the energy involved with motion of objects.

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