Newton's Third Law of Motion

Slide 131 / 159

Newton's 3rd Law of Motion

& Momentum

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Slide 132 / 159

Newton's Third Law of Motion

Newton's third law of motion,
unlike the first and second,
pertains to forces between two
objects.

When you kick a soccer ball,
do you feel the force of the
ball against your foot?

Does the ball "feel" the force
of your foot?

Newton's third law explains this occurrence.

Slide 133 / 159

Have you ever jumped off a skateboard? What happens to the
board when you jump off? Draw an arrow below to show what
direction the board will go.

Newton's Third Law of Motion

This is Newton's third law of motion at work! You applied a force to
the board and the board applied an equal (in magnitude) and
opposite (in direction) force on you.

We call these Action-Reaction Forces.

Multiple Choice

When you sit on a chair, the seat of the chair
pushes up on you with more force than your
weight.

• True

• False

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44 When you sit on a chair, the seat of the chair
pushes up on you with more force than your
weight.

True

False
*equal and opposite

Multiple Choice

Action-Reaction forces are always found
in pairs that are equal and opposite.

• True

• False

Slide 136 / 159

Newton's Third Law of Motion

Forces always exist in pairs! Newton's third law defines these
action and reaction forces.

Click here to see how Newton's Third Law

applies to the physics of a rocket.

Slide 137 / 159

As you kick the soccer ball, you apply an action force to the ball, and
the ball applies a reaction force on your foot.

These forces are equal in strength and opposite in direction. Why
does the ball accelerate more quickly than you and your foot?

Newton's Third Law of Motion

Because even though both apply an equal and opposite force on each other,
the ball has LESS MASS so therefore, it will accelerate much more. Think

back to Newton's 1st and 2nd Law (less mass = more acceleration). All 3
laws are connected.

Slide 138 / 159

Newton's Third Law of Motion

The third law states that for every action force, there is an equal,
but opposite reaction force.

action

reaction

This means that if one object applies a force to another object, then the
other object exerts an equal and opposite force on the first object.

Multiple Choice

When you jump off a skateboard, which of the
following is true?

• You accelerate more than the skateboard.

• You and the skateboard accelerate equal but
  opposite amounts.

• The skateboard accelerates more than you.

Slide 139 / 159

46 When you jump off a skateboard, which of the
following is true?

A You accelerate more than the skateboard.

B You and the skateboard accelerate equal but
opposite amounts.

C The skateboard accelerates more than you.

*because it has less mass

Slide 140 / 159

Newton's Third Law of Motion

According to Newton's 3rd law, the cart pulls on the man just as
hard as the man pulls on the cart. Do these forces cancel each
other out preventing the cart and man from moving?

Slide 141 / 159

Newton's Third Law of Motion

Action: The man applies force to the cart that moves the cart forward.

Reaction: The cart applies an equal and opposite force on the man.

Each force in the pair acts on a different object!

Slide 142 / 159

The Truth about Action Reaction Forces

If a force occurs, there are action reaction forces!

Action reaction forces:

can cause
changes in
motion

are equal in
strength but
opposite in
direction

act on different
objects

Multiple Choice

Which of the following statements pertains to the
third law of motion?

• Action and reaction pairs always act on the
  same object.

• Mass is indirectly proportional to
  acceleration.

• An object at rest will remain at rest unless
  acted on by an unbalanced force.

• If a force occurs, action reaction forces are
  present.

Slide 143 / 159

47 Which of the following statements pertains to the
third law of motion?

A Action and reaction pairs always act on the
same object.

B Mass is indirectly proportional to
acceleration.

C An object at rest will remain at rest unless
acted on by an unbalanced force.

D If a force occurs, action reaction forces are
present.

*not always the SAME object

*Law 2

*Law 1

Multiple Choice

A textbook is resting on a table. Tommy pushes
the book rightwards. What is an action reaction
pair in this scenario?

• The book exerts a downward force on the
  table. The table exerts an upward on Tommy.

• Tommy exerts a rightward force on the book. The
  book exerts a downward force on the table.

• The book exerts a leftward force on Tommy.
  Tommy exerts a rightward force on the book.

• Tommy exerts a downward force on the table.
  The table exerts a rightward force on Tommy.

Slide 144 / 159

48 A textbook is resting on a table. Tommy pushes
the book rightwards. What is an action reaction
pair in this scenario?

A The book exerts a downward force on the
table. The table exerts an upward on Tommy.

B Tommy exerts a rightward force on the book. The
book exerts a downward force on the table.

C The book exerts a leftward force on Tommy.
Tommy exerts a rightward force on the book.

D Tommy exerts a downward force on the table.
The table exerts a rightward force on Tommy.

Multiple Choice

Action reaction pairs cancel each other out since
they are equal and opposite to each other.

• True

• False

Slide 145 / 159

49 Action reaction pairs cancel each other out since
they are equal and opposite to each other.

True

False

*that would only happen if the mass of both were exactly
the same, otherwise the smaller mass will always accelerate

more

Slide 146 / 159

Momentum

Newton's third law tells us that action reaction forces are equal and
opposite, but that does not mean that the effects of those forces are
equal.

After watching the video, discuss the following:

· what was the action reaction pair?
· did both carts accelerate the same? Why or why not?
· how is this related to Newton's 1st and 2nd law of motion?

Click here to see how equal action

reaction pairs cause different motions.

Slide 147 / 159

Momentum

Remember Newton's law of inertia. Objects with more mass (more
inertia) have more reluctance to change their motion and vice versa.

Also recall Newton's 2nd law. Objects with more mass accelerate less
than objects with less mass under the same force (and vice versa).

Both carts experience the same strength in force (Newton's 3rd
law). But the cart on the right experiences a greater change in
motion because it has less mass. Why?

Slide 148 / 159

Momentum

If we understand Newton's third law and momentum, we can predict

how the motion of colliding objects will change.

Momentum is the result of the mass of the object times the object's

velocity.

momentum (kg-m/s) = mass (kg) x velocity (m/s)

*write the formula in the

bottom margin of this page on
your skeleton notes

*also momentum units will be

ALL UNITS COMBINTES

Slide 149 / 159

Momentum

Find the momentum of a skateboarder with a mass of 50 kg traveling
at a velocity of 4 m/s west.

p = mv

Multiple Choice

How can a small insect have the same momentum
as a large car?

• insect has large speed

• both car and insect are at rest

• insect has no mass

• Both Insect has large speed and both car and insect are at rest

Slide 150 / 159

50 How can a small insect have the same momentum
as a large car?

A

insect has large speed

B

both car and insect are at rest

C

insect has no mass

D

A & B

Multiple Choice

If a 2 kg toy truck is moving at 4 m/s, what is the
toy's momentum?

• 3 m/s

• 2 kg m/s

• 8 kg m/s

• 8 m/s

Slide 151 / 159

51 If a 2 kg toy truck is moving at 4 m/s, what is the
toy's momentum?

A

3 m/s

B

2 kg m/s

C

8 kg m/s

D

8 m/s

Slide 152 / 159

Law of Conservation of Momentum

Momentum is conserved (remains the same) during an

interaction as long as the objects are not affected by outside

forces.

This means that the total momentum of objects prior to hitting
each other will equal the total momentum of the objects after

the interaction.

Any momentum lost by one object is gained by the other!

Slide 154 / 159

Law of Conservation of Momentum

Fill in the missing values. Remember that the law of conservation
tells us that the total momentum before a collision is equal to the
total momentum after a collision.

CarMass
(kg)

Velocity
(m/s)

Momentum
(kg m/s)

1

1000

6

2

2000

0

CarMass
(kg)

Velocity
(m/s)

Momentum
(kg m/s)

1

1000

-12

2

2000

Before Collision Data

After Collision Data

Total Momentum: ___________

Total Momentum: ___________

HINT: p=mv can be rearranged to v=p/m and a negative momentum
means the object moves left!

Multiple Choice

What two variables does momentum depend on?

• Mass and volume

• mass and acceleration

• mass and velocity

• mass and force

Slide 155 / 159

52 What two variables does momentum depend on?

A

mass and volume

B

mass and acceleration

C

mass and velocity

D

mass and force

Multiple Choice

The total amount of momentum before and after a
collision may vary.

• True

• False

Slide 156 / 159

53 The total amount of momentum before and after a
collision may vary.

True

False

*will always be the SAME

Slide 159 / 159

Images Cited

A7N8X 2012, 4-Stroke Engine, gif, viewed 29 June 2015,

<https://commons.wikimedia.org/wiki/File:4-Stroke-Engine.gif>.

Luke Ma 2012, Pull Carts Kyoto, Japan, jpg, viewed 7 July 2015,

<https://commons.wikimedia.org/wiki/File:Pull_carts,_Kyoto,_Japan_(8587844087).jpg>