Name Key Period Date

NEWTON’S LAWS OF MOTION

If I am anything, which I highly
doubt, I have made myself so by hard
work.
– Isaac Newton

Goals:
1. Students will use conceptual and mathematical models to predict
and understand patterns in motion. (3.1.10.B-C)
2. Students will be able to apply Newton’s laws of motion to solve
problems related to forces and mass. (3.4.10.C, 3.1.10.E)
3. Students will integrate new information into existing theories
and explain implied results. (3.2.10.A)
Information
A Force is defined most simply as any push or pull.

Newton’s Three
Law’s of Motion

Newton’s Newton’s
First Law Third Law
Newton’s
Second Law
Know as the Know as
Law of Inertia Action-Reaction

Expressed as
F = ma
Moving objects Objects at
rest stay at rest If an object The 2 exertsndan
keep moving
exerts a force on another
equal
object
& opposite force on the 1st

More force meansMore mass means

more accelerationless acceleration
Unless acted
on by an unbalanced force.

Critical Thinking Questions – Part I

1. Which law is associated with inertia? 1st

2. If you increase the force on an object what happens to the acceleration? increases

3. If you use the same force on a less massive object what happens to the acceleration? increases

4. Which law states force is dependent on the mass and acceleration of an object? 2nd

5. What causes an object to slowdown or speed-up? Unbalanced force

6. What law is known as the law of action-reaction? 3rd

7. Which law explains why when you bump into something you fall backwards? 3rd

8. If you double the force of an object what happens to the acceleration? doubles

9. If you double the mass of an object what happens to the acceleration? 1/2

10. Force is measured in newtons (N). A newton is based on base units in the metric system.
What is a newton equal to in terms of units of mass and acceleration? kg x m/s2

Exercises
1. When Jane drives to work, she always places her purse on the passenger’s seat. By the time
she gets to work, her purse has fallen on the floor in front of the passenger seat. One day, she
asks you to explain why this happens in terms of physics. What do you say?

Inertia

2. You are waiting in line to use the diving board at your local pool. While watching people dive
into the pool from the board, you realize that using a diving board to spring into the air before a
dive is a good example of Newton’s third law of motion. Explain how a diving board illustrates
Newton’s third law of motion.

You bounce. The board bounces and pushes back with equal and opposite force.

3. You know the mass of an object and the force applied to the object to make it move. Which of
Newton’s laws of motion will help you calculate the acceleration of the object?

2nd Law. F = ma
2
4. How many newtons of force are represented by the following amount: 3 kg·m/sec ? Justify
your answer.

1 N = kg x m/s2, therefore, 3 kg x m/s2 = 3 N

5. Your shopping cart has a mass of 65 kilograms. In order to accelerate the shopping cart down
2
an aisle at 0.3 m/sec , what force would you need to use or apply to the cart?

F = 65 x 0.3 = 19.5 N
6. A small child has a wagon with a mass of 10 kilograms. The child pulls on the wagon with a
force of 2 newtons. What is the acceleration of the wagon?

2 = 10 x a = 0.2 m/s2 = a

7. You dribble a basketball while walking on a basketball court. List and describe at least 3 pairs
of action-reaction forces in this situation.
Information
Newton’s First Law Forces are Balanced
Newton's first law of motion is often stated as an
object at rest tends to stay at rest and an object in
Objects at Rest Objects in Motion
motion tends to stay in motion with the same speed V = 0m/s V ≠ 0m/s
and in the same direction unless acted upon by an
unbalanced force.
In fact, it is the natural tendency of objects to resist a = 0 m/s2 a = 0 m/s2
changes in their state of motion. This tendency is
described as inertia. A more massive object has a
greater tendency to resist Stays at Rest Stays in Motion
changes in its state of V = 0m/s Same Velocity

motion.
Inertia: the resistance an object has to a change in its state of motion.
Forces are said to be balanced when the net force on the object are zero.
That is when all the forces are added up the result is zero. For example
when you are standing the force of gravity is balanced by the force of the
floor holding you up.
Forces are unbalanced when the net force is not zero. In a tug-of-war both
teams apply forces. For a team to win the forces must be unequal or
unbalanced. You want the net force to be in your direction.

Critical Thinking Questions – Part II

1. Restate Newton’s first law in terms of acceleration.

Acceleration occurs if an unbalanced force acts upon an object

2. If the forces are balanced what is the resulting acceleration?

Zero
3. An object’s resistance to change in motion is dependent solely on what quantity?

Mass
4. Inertia is the resistance to change in motion so inertia depends solely on what?

Mass
5. What is required to cause acceleration?

Unbalanced force
6. What is the net force if you push a cart to the right with 5N of
force and a friend pushes the cart to the left with 5N of force?
0N

7. What is the net force if you start to pull instead of pushing in

#6?
Moving in the same direction. 5N + 5N = 10N
Exercises
1. Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose
that you visit that place (just suppose) and throw a rock. What will the rock do? Why?

The rock will move at a constant velocity in a straight line. Because of Inertia. No unbalanced
force acts on the rock

2. Supposing you were in space in a weightless environment, would it require a force to set
an object in motion? Explain.

Yes. You need force to move mass.

3. Why doesn’t a ball roll on forever after being kicked at a soccer game?

Friction stops it.

4. A 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required
to keep the object moving at this speed and in this direction? Explain.

2.0 kg  4 m/s. None because NO friction acts against it.

5. Ben Tooclose is being chased through the woods by a bull moose which he was attempting to
photograph. The enormous mass of the bull moose is extremely intimidating. Yet, if Ben
makes a zigzag pattern through the woods, he will be able to use the large mass of the moose
to his own advantage. Explain this in terms of inertia and Newton's first law of motion.

It is harder for the moose to move because he is floating in the water.

6. Luke Autbeloe drops an approximately 5.0 kg object (weight = 50.0 N) off the roof of his
house into the swimming pool below. Upon encountering the pool, the object encounters a
50.0 N upward resistance force (assumed to be constant). Use this description to answer the
following questions. (Down is usually considered a negative direction)
a. Which one of the velocity-time graphs best describes the motion of the object? Why?

Graph B
b. True or False: Once the object hits the water, the forces are balanced and the object will
stop. Support your answer with reasoning.
Information KEY
Newton’s Second Law
Forces are Unbalanced
As stated in the first law, the presence of an unbalanced
force will accelerate an object - changing either its speed,
its direction, or both its speed and direction.
Newton's second law of motion pertains to the behavior a≠0
of objects for which all existing forces are not balanced.
The second law states that the acceleration of an object is
dependent upon two variables - the net force acting upon Depends directly Depends inversely
the object and the mass of the object. The acceleration is on the net force on object’s mass
always in the same direction as the net force.
Mathematically this means:
Forcenet a  Fnet
acceleration  ;
mass m

commonly written
as: Fnet  ma

Do not use the value of merely any force in the above equation. It is the net force which is
related to acceleration. The net force is the sum of all the forces acting on an object.

Critical Thinking Questions – Part III

1. What two variables is acceleration dependent on? What is the relationship between these
variables and acceleration? (i.e. if you increase one variable what happens to the
acceleration?)
F = ma
Mass and acceleration are inversely proportional to each other

2. If an object is not accelerating what can you determine about the sum of all the forces on the
object?
They are balanced

3. If the net force on an object is in a negative direction, what will the direction of the resulting
acceleration be?

In the negative direction

4. If you double the net force on an object what is the result on the acceleration?

The acceleration doubles

5. If you double the mass of an object while leaving the net force unchanged what is the result
on the acceleration?
The acceleration is one half (1/2)
2
6. A Cadillac Escalade has a mass of 2 569.6 kg, if it accelerates at 4.65m/s what is the net
force on the car?

F = ma F = (2569.6)(4.65) = 11900 N
7. A 7.26kg bowling ball (16 pounds) is at rest at the end of a bowling lane. Use this
information to answer the following questions.
a. What is the net force acting on the ball?

0 net force
b. You push the ball with a force of 22.8N which induces a -2.3N frictional force.
What is the net force while you push the ball?
Fnet = Fapplied - Ffriction

Fnet = 22.8 - 2.3 = 20.5 N

c. What is the acceleration caused by your push on the ball?

20.5 N = (7.26)(a) = 2.82 m/s2

d. Following your push the ball rolls down the lane at 4.2m/s. What is the net force
on the ball as it rolls down the lane at the constant speed?

O Newtons; the ball is not accelerating

e. CHALLENGE: How long did you push the ball in this situation?

Vf = Vi + at
4.2 m/s = 0 + 2.82 m/s2(t)
t = 1.5 seconds

Exercises
8. Complete the following table. Be sure to include units in your answer.
Net Force (N) Mass (kg) Acceleration

5.0 2.5
2.0
2.32 12
.19
18.2 1.967
9.25
0.87 21.2
0.41
180 1.793
100.4
9. Complete the following table. Be sure to include units.
2
Net Force (N) Mass (kg) Acceleration (m/s )

4.0 4.0
16
25.0 4.998
5.00
53 3
17.7
 172.44 4.665
36.96
1 225 3.43
4201
10. What is the rate of acceleration of a 2,000-kilogram truck if a force of 4,200 N is used to
make it start moving forward?
F = ma
4200 = (2000)(a) = 2.1 m/s2

11. What is the acceleration of a 0.30 kilogram ball that is hit with a force of 25 N?
a. F = ma
25 N = (.30)(a) = 83.3 m/s2

2
12. How much force is needed to accelerate a 68 kilogram-skier at a rate of 1.2 m/sec ?

F = ma
F = (68)(1.2) = 81.6 N

2
13. What is the mass of an object that requires a force of 30 N to accelerate at a rate of 5 m/sec ?

F = ma
30 N = (m)(5) = 6.0 Kg

14. What is the force on a 1 000 kilogram-elevator that is falling freely under the acceleration of
2
gravity only (9.8m/s )?

F = ma
F = (1000) (9.8 m/s2) = 9800 N

15. What is the mass of an object that needs a force of 4 500 N to accelerate it at a rate of 5
2
m/sec ?

F = ma
4500 N = m(5) = 900 Kg

16. What is the acceleration of a 6.4 kilogram bowling ball if a force of 12 N is applied to it?

F = ma
12 N = (6.4)a = 1.9 m/s2

17. CHALLENGE: What is the mass of an object that needs a force of 6 600 N to increase its
speed from rest to 107 m/s in 2.3 seconds?
F = ma

6600 N = (m) ( 107-0/2.3) = 141 Kg

 Key

Information
Newton’s Third Law
A force is a push or a pull upon an object. This implies there must be two objects; one being
pushed and one doing the pushing. Thus, forces result from interactions between objects.
According to Newton’s Third Law, whenever objects interact with each other they exert forces
upon each other. These two forces the objects exert on each other are called action and reaction
forces. Friction is one type of reaction force.
Newton's third law states:
For every action, there is an equal and opposite reaction.
The statement means that in every interaction, there is a pair of forces acting on the two
interacting objects. The size of the forces on the first object equals the size of the force on the
second object. The direction of the force on the first object is opposite to the direction of the
force on the second object. Forces always come in pairs - equal and opposite action-reaction
force pairs.
Examples:

The horse pushes back on the ground and the ground pushes forward on the horse. The horse pulls forward on the wagon and

The man pushes on the refrigerator, and the refrigerator pushes back on the man.

Critical Thinking Questions – Part IV

1) All forces result because of a push or pull (interactions) between objects.

2) Forces come in pairs. What are these pairs called?

Equal and opposite Action Reaction Pairs

3) In the diagrams below the action forces have been labeled with arrows. In each diagram
label the reaction forces with arrows.
a) b) c)
4) In the drawing to above:
a. How much force is the physics student exerting on the rope? In what direction?

500 N to the left (at 1800)

b. How much force is the wall exerting on the rope? In what direction?

500 N to the right (at 00)

c. What is the net force on the force gauge?

Zero- THE WALL IS NOT MOVING

d. Would the Physics student be moving? If yes, is he accelerating?

No

e. What would happen if the wall stopped exerting any force?

It would move to the left with the student

f. What happens if the Physics student is standing on ice?

He couldn’t pull with the same force

g. What is the force between the student and the ground called?

Normal force (weight)

h. Draw and label arrows on the diagram to represent three sets of force pairs.

5) On the drawing below draw at least 5 force pairs.

 Reaction 6) The force pairs are drawn on the diagram to the left.
a. What is the action force acting on?
The Rocket

b. What is the reaction force acting on?

The Exhaust
c. If the forces are equal and opposite why does the rocket
Action ship move?

Because the force is large enough to overcome gravity

It produces a “net force”

7) Manuel is holding a 5 kg box.

a. How much force is the box exerting on him? In what
direction?

F = ma F = 5 kg x 9.8 m/s2 = 49 N down

b. How much force is he exerting on it? In what direction?

49 N up
c. If Manuel weighs 375 N and is holding the box. How much
force is the floor exerting on him? (include direction)

375 N (Manual) + 49 N (box) up (he is not falling through the floor) = 424 N
d. How much force does each of his legs exert?

424/2 = 212 N down

e. Draw and label the force arrows between Manuel and the box.

Draw a free body diagram

8) While driving down the road, a firefly strikes the windshield of a bus and makes a quite
obvious mess in front of the face of the driver. This is a clear case of Newton's third law of
motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater:
the force on the firefly or the force on the bus? Explain.

They are equal and opposite

9) Many people are familiar with the fact that a rifle recoils when fired. This recoil is the result of
action-reaction force pairs. A gunpowder explosion creates hot gases which expand outward
allowing the rifle to push forward on the bullet. Consistent with Newton's third law of motion,
the bullet pushes backwards upon the rifle. The acceleration of the recoiling rifle is ...
a. greater than the acceleration of the bullet.
b. smaller than the acceleration of the bullet.
 c. the same size as the acceleration of the bullet.
Explain your answer. F = ma; the mass of the
rifle is much greater so the acceleration is
much less