SW2 Review

Newton’s

Laws

What is Force?

This Photo by Unknown Author is licensed under CC BY-NC-ND

Mass is…

■ The amount of matter in an

object.

■ Measured in kilograms.

■ NOT a force.

■ The same at any location,

even on another planet.
Not influenced by gravity.

Mass is…

We can think of Mass as

a measure of inertia.

The more Mass

something has, the more
inertia it has.

A Force is…

■ Measured in Newtons (N) in the

metric (SI) system and pounds
(lbs) in the English system

■ A vector quantity requiring

magnitude and direction to
describe it

■ Represented by drawing

arrows on a diagram

Types of Forces

(that we will study now – there are many

more)

■ Weight - force of gravity

■ Friction - resistance force that opposes

motion

■ Applied force - force you exert, push or

pull

■ Net force – total vector sum of all forces

■ Balanced forces – equal and opposite

forces

■ Unbalanced forces – not equal and

opposite

Newton’s 1stLaw

(Law of Inertia)

An object at rest will stay at rest,
and an object in motion will stay in
motion at a constant velocity unless
acted on by an unbalanced or net
force.

This statement contradicted Aristotle’s teaching but
supported Galileo’s idea of inertia. Newton proposed that
there was an unrecognized force of resistance between
objects that was causing them to stop in the absence of an
applied force to keep them moving. This new unseen
resistance force became known as “friction”.

Multiple Choice

Suppose that an astronaut throws a baseball in outer space at a location far from significant influences of gravity and air resistance. One would expect that the baseball would ____.

• eventually stop since all objects ultimately "lose their steam"

• eventually stop as its inertia slowly becomes used up

• continue in motion with the same speed and direction

• either a, b, or c -- depending on whether the astronaut continues to push it

Multiple Choice

Inertia depends on ______

• Velocity

• Speed

• Distance

• Mass

Multiple Choice

Inertia is the tendency of an object to resist a change in its ___________.

• temperature

• shape

• size

• motion

Force

Symbol

Definition

Direction

Applied

Force

Fapp
The force being

applied to an object,
either a push or a pull

Parallel to the
surface and in
the direction of

movement.

Friction

Ff
The contact force that
acts to oppose sliding

motion between

surfaces

Parallel to
surface &
opposite

direction of

sliding

Normal

FN
The contact force

exerted by a surface

on an object

Perpendicular to
& away from the

surface

Weight

Fg
A long-range force
due to gravitational
attraction between

two objects, generally

Straight down
toward center

of Earth

Normal Force (FN)

Defined as the force of a

surface pushing back on an
object.

Always directed

perpendicular to the
surface.

This is a contact force. No

contact…no normal force.

NOT always equal to weight.

Examples:

FN

Table

W
a
l l

FN

Newton’s 2ndLaw
Fnet = ma

If an unbalanced force acts on a
mass, that mass will accelerate in
the direction of the force.

Newton’s 1stLaw says that without an unbalanced
force objects will remain at constant velocity
(a=0)…so it seems logical to say that if we apply a
force we will see an acceleration.

2 N

8 N

a

Since 8N is greater than 2N,
the unbalanced force (6N) is to
the right so the acceleration is
to the right.

Math Response

A bat hits a 0.5 kilogram ball with a force of 500 Newtons. What is the acceleration of the ball?

Type answer here

Correct Answer: 1000

Deg^°

Rad

Math Response

A 6800 kilogram school bus hits the brakes and slows down at an accleration of $$-2.5\ \frac{m}{s^2}$$ ​. What was the net force on the school bus from the brakes?

Type answer here

Correct Answer: -17000

Deg^°

Rad

Multiple Choice

What is the NET FORCE of this diagram?

• 17 N, right

• 17 N, left

• 23 N, right

• 12 N, up

Multiple Choice

What is the net force?

• 10 N
• 0 N
• 25 N
• 1 N

Multiple Choice

Name the missing force...

• normal force
• applied force
• frictional force
• tension force

Newton’s 3rdLaw
Action - Reaction

For every action force there is
an equal and opposite
reaction force.

Example: If you punch a wall with your fist in anger,

the wall hits your fist with the same
force. That’s why it hurts!

Action-reaction forces cannot balance each
other out because they are acting on
different objects. The forces acting on an
object determine their motion.

Projectile Motion

Assumptions we make…

 We assume NO AIR RESISTANCE! The path of a

projectile is a parabola.

 Horizontal motion is constant velocity.

 Vertical motion is in “free-fall”.

 Vertical velocity at the top of the path is zero

 Time is the same for both horizontal and

vertical motions.

0

constant

=

=

x

x
a

v

down

ay
,

m/s

8.9

2

=

0
top=
yv

𝑡𝑥 = 𝑡𝑦

How are the formulas different for
projectiles? They must be applied
along only one axis at a time.

horizontal or “x” – direction
vertical or “y” – direction

𝑣𝑦𝑓 = 𝑣𝑜𝑦 + 𝑔𝑡

𝑑𝑦 = 𝑣𝑜𝑦 + 𝑣𝑦𝑓

2
𝑡

𝑑𝑦 = 𝑣𝑜𝑦𝑡 + ൗ
1 2 𝑔𝑡2

𝑣𝑦𝑓2= 𝑣𝑜𝑦2+ 2𝑔𝑑𝑦

Remember that for projectiles, the horizontal (x) and vertical (y) motions must
be separated and analyzed independently.

𝑣𝑓𝑥 = 𝑣𝑜𝑥 + 𝑎𝑥𝑡

𝑑𝑥 = 𝑣𝑜𝑥 + 𝑣𝑓𝑥

2
𝑡

𝑑𝑥 = 𝑣𝑜𝑥𝑡 + 1

2 𝑎𝑥𝑡2

𝑣𝑓𝑥2= 𝑣𝑜𝑥2+ 2𝑎𝑥𝑑𝑥

0

0

0

Multiple Select

A ball rolls off a table and follows a parabolic path (projectile motion). Which quantities are zero?

• initial vertical velocity

• horizontal acceleration

• vertical acceleration

• horizontal displacement

• initial horizontal velocity

Match

Match the following angles to their situations

• 90°

• 45°

• 0°

Maximum vertical displacement

Maximum horizontal and vertical displace

Maximum horizontal displacement

• Maximum vertical displacement

• Maximum horizontal and vertical displace

• Maximum horizontal displacement

Circular Motion

The Circle

One of the first

things you need
to know is how to
calculate the
circumference of
a circle.

𝐶 = 2𝜋𝑟 = 𝜋𝑑

This Photo by Unknown Author is licensed under CC BY-NC-ND

Centripetal Velocity

 Imagine there is a ball spinning around on a string over

your head. We know the length of the string and the
time it takes for the ball to make one turn. We can find
how fast the ball is moving using some simple math.

𝑉𝑡 =

2𝜋𝑟

𝑡=

𝜋𝑑
𝑡

 Note, this is tangential velocity. Angular velocity is related, but

we’re not going to dive into that.

This Photo by Unknown Author is licensed under CC BY-SA

Centripetal Force

 Picture that spinning ball again. What is the

string providing for the ball?

 The string is “pulling” the ball back towards

the center, providing what is called a
Centripetal Force.

𝐹𝐶 =

𝑚∗𝑉2

𝑟

Centripetal Acceleration

 Now that we know how fast something is moving, and

the forces involved, we can figure out the acceleration.

 How do we know there is an acceleration though?

 Because the ball is changing direction

 The string is “pulling” the ball and not allowing it to go flying off

𝑎𝐶 =

𝑉2

𝑟

Math Response

A ball is swung in a horizontal circle at a constant speed. Each circle takes 0.85 seconds to complete and the rope is 0.40 m long. What is the centripetal acceleration?

Type answer here

Correct Answer: 21.86

Deg^°

Rad

Fill in the Blank

What direction does the centripetal acceleration point?

Universal
Gravitation

Newton’s Law of Universal
Gravitation

Every object attracts every other

object with a force that is directly
proportional to the product of the
masses of the objects and
inversely proportional to the
square of the distance between
them.

r
m1
m2

Fg
Fg

Mathematically…

Fg = gravitational force or weight
or force of attraction, N

G = universal constant of
gravitation (= 6.67 x 10 -11Nm2/kg2)

m1, m2 = masses of objects, kg

r = distance from center of one
object to center of other object,
m

F𝑔= G

𝑚1 𝑚2

𝑟2

What this means Physically…

 I have gravity

 You have gravity

 The Earth, Sun, Moon, and

planets have gravity

 EVERYTHING HAS

GRAVITY!!!

 Gravity is an attractive

force, so…

Change of
Gravitational
Force with
Distance

■ Law of universal

gravitation is
known as an
inverse square
law.

Multiple Choice

If I doubled the mass of two objects, what is the new Force of Gravity between them?

• Double
• Remain the same
• Halve
• Quadruple

Multiple Choice

Doubling the distance between two objects does what to the Force of Gravity between them?

• Increases by a factor of four
• Remains the same
• Decreases by a factor of two
• Decreases by a factor of four

Multiple Choice

How does the Force of Gravity change if I triple the mass of one object and triple the distance between them?

• increases by a factor of 9
• increase by a factor of 3

• decreases by a factor of 9

• decreases by a factor of 1/9

Fill in the Blank

How does the Force of Gravity relate to the distance between two objects?

Math Response

Determine the gravitational force of attraction between two 3.5 kg bowling balls whose centers are exactly 2.0 meters from each other. Write your answer in scientific notation.

Type answer here

Correct Answer: 2.04\times10^{-10}

Deg^°

Rad