Unit 3 Lesson 1: Newton's Laws

NEWTON'S LAWS

Chapter 5

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FORCE

 A force is a push or a pull.

 A force is an action that has the ability to change

motion.

 Forces do not always create a change of motion,

but they could.

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INERTIA: NEWTON’S FIRST LAW

 Newton’s First Law: An object will move at a

constant velocity until a force acts upon it.

 In other words, an object will keep doing what it’s

doing until forced to change.

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INERTIA: WHATCAUSESIT?

 Inertia is an object’s ability to RESIST a change

of motion.

 The word “inertia” comes from the Latin word for

lazy.

 How easy/difficult is it to change what an object

is doing?

 Easy ➔ Low inertia

 Difficult ➔ High inertia

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INERTIAVS. MASS

 Inertia is a physical property of MASS.

 What is MASS? Mass is the amount of matter in

an object. It does not reflect the amount of space
it takes up.

 Mass doesn’t change from position or speed or

acceleration*

*at non-relativistic speeds, that is

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EVERYDAYEXAMPLEOF INERTIA

 If you are in a car without a seatbelt that is

traveling down the road, and the car suddenly
brakes, you will fly forward.

 Since you are a separate object from the car and

you are traveling at a velocity, you will want to
continue traveling at that velocity. When the car
stops, you will keep going until forced to stop by
the road or the windshield.

 Seat belts make us a “part of the car,” so when

the car stops, so do we.

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MAGICOR PHYSICS?

 A magician pull a tablecloth out from a perfelcty

set table. The dishes remain in place.

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The dishes have inertia, which mean
that they won’t want to move
sideways. When the tablecloth is
pulled very rapidly and doesn’t have
time to react with the dishes, they
stay put.

Multiple Choice

When an object does not want to change motion, it is called ________________.

• inertia

• motion

Multiple Choice

Kicking a ball is an example of ________________.

• force

• speed

Multiple Choice

A force is a _________________.

• push or pull

• speed

Multiple Choice

Newton's first law states that an object in motion stays in motion and an object at rest stays at rest unless acted on by ______________________________.

• a balanced force

• an unbalanced force

Multiple Choice

Which has more Inertia?

• a skateboard

• a truck

Multiple Choice

What kind of objects have inertia?

• all objects with mass

• only objects in motion

Multiple Choice

Which type of force will cause an object to change motion?

• balanced force

• unbalanced force

Multiple Choice

What can a force do?

• start a motion

• stop a motion

• change the direction of the motion

• all of the above

Multiple Choice

The amount of matter in an object is called ________________.

• weight

• mass

Multiple Choice

Objects with greater ________________ have more inertia.

• mass

• speed

Multiple Choice

Newton's first law is also called the law of __________________

• motion

• inertia

Multiple Choice

According to Newton's first law of motion, if you were riding in a car and hit another car in front of you without wearing a seatbelt, you would _______________.

• fall forward because your body keeps moving in that direction

• fall back because of force

Multiple Choice

A book will not move by itself because it has ________________.

• gravity

• inertia

Multiple Choice

When a car stops suddenly, your body tends to keep moving ______________________.

• backwards

• forward

Multiple Choice

Balanced forces cause ________________________.

• no motion

• motion

NEWTON’S SECOND LAW: F=MA

 Newton’s second law states that the acceleration

of an object is directly proportional to the applied
force and indirectly proportional to its mass.

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ma

F =ma

UNITSOF FORCE

 The equation symbol for force is F.

 The SI unit of force is the Newton (N).

 Definition: 1 Newton  the amount of force

required to accelerate 1kg by 1m/s2.

 Therefore, the Newton is a derived unit and

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​1 N =

​1 kg m

​s²

QUICK EXAMPLE

 Caleb pushes Chris with a force of 56N. If Chris’

mass is 82kg, what is his acceleration?

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SOLUTION: SET UP

 Knowns and Unknowns:
F = 56N; m = 82kg; a = ???

 Equation:

 Isolate the variable.

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ma

F =

a =

​F

​m

Math Response

Caleb pushes Chris with a force of 56N.  If Chris’ mass is 82kg, what is his acceleration in m/s²?

Type answer here

Correct Answer: 0.68

Deg^°

Rad

SOLUTION

 Plug in numbers

12
2

68

.0

82
56

=

=

=

ANOTHER EXAMPLE

 Eric pushes Ariel with a force of 97N and she

accelerates at 2.1m/s². What is her mass?

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Math Response

Eric pushes Ariel with a force of 97N and she accelerates at 2.1m/s2.  What is her mass in kg?

Type answer here

Correct Answer: 46.2

Deg^°

Rad

Multiple Choice

How much force is needed to accelerate a 3kg skateboard at 5m/s²?

• 8N
• 1.6N
• 0.6N
• 15N

Multiple Choice

According to __________, the acceleration of an object is equal to the net force acting on the object divided by the object's mass. 

• Newton's Second Law
• Newton's Third Law
• Newton's First Law

Multiple Choice

If a 10 kilogram object accelerates suddenly at 15 m/s^2, how much force was applied to the object?

• 150 Newtons
• 15 Newtons
• 150 kilograms
• 100 kilograms

Multiple Choice

A car is 2000kg and accelerating at 20m/s², what is the net force applied to it?

• 40000 N
• 100 N
• 5 N
• 200 N

Multiple Choice

A force of 90N is applied to each cart below, which one will accelerate the fastest?

• Cart 1
• Cart 2
• Cart 3 
• Cart 4

Multiple Choice

A ball has an acceleration of 8 m/s². If the force is tripled, the ball's new acceleration will be:

• 24 m/s²

• 11 m/s²

• unchanged

• 2.67 m/s²

Multiple Choice

An object is moving with an acceleration of 6 m/s². If the force is doubled and the mass is cut by one-third, the new acceleration will be:

• 4 m/s²

• 1 m/s²

• 36 m/s²

• unchanged

Multiple Choice

These two force arrows are acting on an object. The net force (in N) acting on this object is:

• 16

• 12

• 28

• 4.0

Multiple Choice

How much force is needed to accelerate a 2 kg physics book to an acceleration of 6 m/s/s?

• 36 N
• 12 N
• 3 N
• 0.33 N

Multiple Choice

A 20 kg bike accelerates at 10 m/s². what was the force?

F = m x a

• 25 Newtons

• 20 Newtons

• 200 Newtons

• 10 Newtons

Multiple Choice

I the mass of an object is 10 kg. And the acceleration is 10 m/s², what is the force on the object?

• 1000 N

• 20 N

• 10 N

• 100 N

Multiple Choice

If the mass of an object is 5 kg, and the acceleration is 2 m/s². What is the force?

• 10 N

• 7 N

• 3 N

• 12 N

NEWTON’S THIRD LAW

Action and Reaction

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THIRD LAW

 Newton’s third law states that for every action

there is an equal and opposite reaction.

 If you push it, it pushes back!!

 The action/reaction forces are acting on different

objects.

 You push on the wall, the wall pushes on you.

 You stand on the floor, the floor holds you up.

 The hammer hits the nail, the nail pushes back on

the hammer.
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Multiple Choice

A baseball player hits a ball with 400 N of force. How much force does the ball exert on the bat?

• 300

• 400

• 500

• Not enough info to tell

Multiple Choice

Your friend pushes you for making fun of him/her. What is the reaction force?

• You pushing back on your friend

• You pushing on the ground

• There is no reaction force

• Your friend pushing on the ground

Multiple Choice

When you jump up, you push the earth downwards. Why can't you tell that the earth has moved?

• The force of you jumping up doesn't actually push the earth down.

• The earth has much more mass so it doesn't move at all

• The earth is firmly rooted

• The earth has much more mass so you can't tell that it moved

Multiple Choice

When you hit a tennis ball with a racket...

• The ball exerts more force on the racket

• The ball and racket exert the same force on each other

• The racket exerts more force on the ball

• It depends on how hard you hit it

NORMAL FORCE

 The normal force, FN, is the force of the surface

pushing back on the object.

 It is the reaction to the action.

 For example, if you push on a wall, the force with

which the wall pushes back on you is the normal
force.

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Fapp

FN

Normal force is ALWAYS
perpendicular to the
surface

Normal force only
occurs with
contact forces

USING NEWTON’S THIRD LAW

 When you walk forward, you push against the

ground. You push the ground backward slightly,
while the ground pushes you forward.

 A jet engine pushes massive amounts of air

behind it, which propels the plane forward

 Space shuttle launch

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CONSIDERTHEFOLLOWING:

 If everyone on Earth gathered in a straight line

around the equator and began walking in the
same direction, could we affect the rotation of the
Earth?

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