Lesson Proper (MOTION)

Ch. 3 & 4

Motion & Forces

II. Describing Motion

◆ Motion

◆ Speed & Velocity

◆ Acceleration

Newton’s First Law

⚫Newton’s First Law of Motion

▪An object at rest will remain at

rest and an object in motion
will continue moving at a
constant velocity unless acted
upon by a net force.

motion

constant velocity

net force

A. Motion

⚫Problem:

▪Is your desk moving?

⚫We need a reference point...

▪nonmoving point from which

motion is measured

A. Motion

⚫Motion

▪Change in position in relation to

a reference point.

Reference point

Motion

A. Motion

Problem:

⚫ You are a passenger in a car

stopped at a stop sign. Out of the
corner of your eye, you notice a
tree on the side of the road begin
to move forward.

⚫ You have mistakenly set yourself

as the reference point.

B. Speed & Velocity

⚫Speed

▪rate of motion
▪distance traveled per unit time

time

distance
speed =

v

d

t

B. Speed & Velocity

⚫Instantaneous Speed

▪speed at a given instant

⚫Average Speed

time

total

distance

total
speed

avg.

=

B. Speed & Velocity

⚫Problem:

▪A storm is 10 km away and is

moving at a speed of 60 km/h.
Should you be worried?

▪It depends

on the
storm’s
direction!

B. Speed & Velocity

⚫Velocity

▪speed in a given direction
▪can change even when the

speed is constant!

C. Acceleration

⚫Acceleration

▪the rate of change of velocity
▪change in speed or direction

t

v

v
a

i

f−
=

a: acceleration
vf: final velocity
vi: initial velocity
t: time

a

vf - vi

t

C. Acceleration

⚫Positive acceleration

▪“speeding up”

⚫Negative acceleration

▪“slowing down”

D. Calculations

⚫ Your neighbor skates at a speed of 4 m/s.

You can skate 100 m in 20 s. Who skates
faster?

GIVEN:
d = 100 m
t = 20 s
v = ?

WORK:
v = d ÷ t

v = (100 m) ÷ (20 s)

v = 5 m/s

You skate faster!

v

d

t

D. Calculations

⚫ A roller coaster starts down a hill at 10 m/s.

Three seconds later, its speed is 32 m/s.
What is the roller coaster’s acceleration?

GIVEN:
vi = 10 m/s
t = 3 s
vf = 32 m/s
a = ?

WORK:
a = (vf - vi) ÷ t

a = (32m/s - 10m/s) ÷ (3s)

a = 22 m/s ÷ 3 s

a = 7.3 m/s2
a

vf - vi

t

D. Calculations

⚫ Sound travels 330 m/s. If a lightning bolt

strikes the ground 1 km away from you,
how long will it take for you to hear it?

GIVEN:
v = 330 m/s
d = 1km = 1000m
t = ?

WORK:
t = d ÷ v

t = (1000 m) ÷ (330 m/s)

t = 3.03 s

v

d

t

D. Calculations

⚫ How long will it take a car traveling 30 m/s

to come to a stop if its acceleration is
-3 m/s2?

GIVEN:
t = ?
vi = 30 m/s
vf = 0 m/s
a = -3 m/s2

WORK:
t = (vf - vi) ÷ a

t = (0m/s-30m/s)÷(-3m/s2)

t = -30 m/s ÷ -3m/s2

t = 10 s
a

vf - vi

t

E. Graphing Motion

⚫ slope =

⚫ steeper slope =

⚫ straight line =

⚫ flat line =

Distance-Time Graph

A

B

faster speed

constant speed

no motion

speed

E. Graphing Motion

⚫ Who started out faster?

▪ A (steeper slope)

⚫ Who had a constant speed?

▪ A

⚫ Describe B from 10-20 min.

▪ B stopped moving

⚫ Find their average speeds.

▪ A = (2400m) ÷ (30min)

A = 80 m/min

▪ B = (1200m) ÷ (30min)

B = 40 m/min

Distance-Time Graph

A

B

0

100

200

300

400

0

5

10

15

20

Time (s)

Distance (m)

Distance-Time Graph

E. Graphing Motion

⚫ Acceleration is

indicated by a
curve on a
Distance-Time
graph.

⚫ Changing slope =

changing velocity

E. Graphing Motion

0

1

2

3

0

2

4

6

8

10

Time (s)

Speed (m/s)

Speed-Time Graph

⚫ slope =

⚫ straight line =

⚫ flat line =

acceleration

▪ +ve = speeds up
▪ -ve = slows down

constant accel.

no accel.

(constant velocity)

E. Graphing Motion

0

1

2

3

0

2

4

6

8

10

Time (s)

Speed (m/s)

Speed-Time Graph

Specify the time period

when the object was...

⚫ slowing down

▪ 5 to 10 seconds

⚫ speeding up

▪ 0 to 3 seconds

⚫ moving at a constant

speed
▪ 3 to 5 seconds

⚫ not moving

▪ 0 & 10 seconds