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20. Newton's Third Law

Objectives

Students analyze the relationship between an action force and the resulting reaction force and

recognize that for every action there is an equal and opposite reaction. Toward this end they

investigate that:

Forces occur in pairs, commonly referred to as "action" and "reaction"

Action and reaction forces never act on the same body

Action and reaction forces are always equal in magnitude and opposite in direction

Procedural Overview

Students gain experience conducting the following procedures:

Determining the force exerted on an object as well as the reaction force exerted by the object

Interpreting graphs of Force versus Time

Time Requirement

Preparation time 10 minutes

Pre-lab discussion and activity 15 minutes

Lab activity 20 minutes

Materials and Equipment

For each student or group:

Data collection system Large table clamp (optional)

Force sensor, with hook and rubber bumper (2) Rubber band

Short rod (optional)

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Concepts Students Should Already Know

Students should be familiar with the following concepts:

Free-body diagram

Newton’s first law

Newton’s second law

Related Labs in This Guide

Labs conceptually related to this one include:

Introduction to Force

Newton's First Law

Newton's Second Law

Acceleration

Using Your Data Collection System

Students use the following technical procedures in this activity. The instructions for them

(identified by the number following the symbol: "�") are on the storage device that accompanies

this manual. Choose the file that corresponds to your PASCO data collection system. Please

make copies of these instructions available for your students.

Starting a new experiment on the data collection system �(1.2)

Connecting multiple sensors to the data collection system �(2.2)

Recording a run of data �(6.2)

Adjusting the scale of a graph �(7.1.2)

Adding a note to a graph �(7.1.5)

Displaying multiple graphs �(7.1.11)

Showing and hiding data runs in a graph �(7.1.7)

Finding the coordinates of a point in a graph �(9.1)

Saving your experiment �(11.1)

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Background

When one object exerts a force on another object, the second object exerts a force of equal

magnitude and opposite direction on the first object—sometimes stated as, "for every action

there is an equal and opposite reaction." Newton's third law can be both remarkably clear and

perplexing to students, but a clear understanding of Newton's third law will make it much easier

for a student to draw free-body diagrams and to analyze situations requiring an understanding

of Newton's second law.

It is important to recognize that forces are like shoes—they occur in pairs. It is impossible to

have a single force; for example, one can’t have an action force without having a reaction force.

Students frequently find it difficult to understand that inanimate objects, like walls and floors,

can exert forces. They may also find it preposterous to believe that walls and floors exert a

gravitational force on the earth that is equal in size to the force the earth exerts on them. The

pre-lab discussion and activities address some of these issues. At the end of this activity,

students should realize that for every action there is an equal and opposite reaction, without

exception.

Pre-Lab Discussion and Activity

Select any number of activities for discussion here based on the time and equipment you have available.

Forces occur in pairs

Suspend a 1-kg mass from a large stand using a string and pendulum clamp. Give the mass a small push

with your finger so that it begins to swing. Ask students:

1. In which direction did I exert a force on the mass?

In the direction it began to move.

2. Did the mass exert a force on me?

Yes.

3. If so, in which direction was it?

Toward your finger.

4. How could I tell that it exerted a force on me?

By the pressure on your finger tip.

Inanimate objects can exert forces 1

Hold a meter stick with a small mass on the end and deflect it. Ask the following questions:

1. How can you tell that I am exerting a force on the meter stick?

It is distorted in shape.

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2. Is the meter stick exerting a force back on me?

Yes, but accept all student answers and discuss until consensus is reached.

Release the meter stick by sliding your finger off the end to show the light mass projected into the air.

Inanimate objects can exert forces 2

Place a small mirror on the floor, and shine a laser pointer on it so that the reflected beam strikes the

ceiling. Always direct the laser away from eyes. Now ask a student to walk past the mirror, stepping near

it, as the other students watch the reflected beam. Ask the question,

1. What does the behavior of the reflected beam tell you about the floor?

The fact that the spot on the ceiling moves means the floor was slightly distorted as the student walked past the

mirror.

Objects exert gravitational forces on each other

Ask the questions:

1. What evidence is there that leads us to believe that the earth exerts a gravitational

force on the moon?

The moon orbits the earth.

2. What evidence is there that leads us to believe that the moon in turn exerts a

gravitational force on the earth?

The moon causes tides. On the side of the earth nearest the moon, the water bulges towards the moon.

3. Why does the gravitational force of the earth on the moon have a much greater

effect on the moon's motion than does the gravitational force of the moon on the earth

have on the earth's motion?

The earth is much more massive than the moon.

Lab Preparation

Although this activity requires no specific lab preparation, allow 10 minutes to assemble the equipment

needed to conduct the lab.

Safety

Follow all standard laboratory procedures.

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Sequencing Challenge

The steps below are part of the Procedure for this lab activity. They are not in the right order. Determine

the proper order and write numbers in the circles that put the steps in the correct sequence.

Procedure with Inquiry

After you complete a step (or answer a question), place a check mark in the box () next to that step.

Note: Students use the following technical procedures in this activity. The instructions for them (identified by the

number following the symbol: "�") are on the storage device that accompanies this manual. Choose the file that

corresponds to your PASCO data collection system. Please make copies of these instructions available for your

students.

Part 1 – Pushing

Set Up

1. Start a new experiment on the data collection system. �(1.2)

2. Connect the force sensors to the data collection system. �(2.2)

3. Display two graphs simultaneously, the first with Force, push positive on the y-axis and

Time on the x-axis, the second with Force, pull positive on the y-axis and Time on the

x-axis. �(7.1.11)

Note: Your force sensor may use the measurements “force” and “force ( inverted)” rather than “push

positive” and “pull positive.”

4. Why do you think one sensor uses “push positive” and the other uses “pull positive”?

When the sensors face each other, they will be pointed in opposite directions.

5. Attach the force sensor’s rubber bumper attachment to the front of each force sensor.

Before pushing or

pulling, predict

which student will

push or pull the

hardest.

2

Determine which

student pushed

or pulled the

hardest and how

that relates to

Newton’s third

law.

4

Two students

record the force

applied as they

first push the

sensors against

each other and

then pull them

from each other.

3

Connect 2 force

sensors to the

data collection

system. Zero

both force

sensors before

recording data.

1

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6. Decide which sensor will be held by Student A and which will be held by Student B.

Teacher Note: Alternatively, if you do not want students pushing and pulling one another, one of the sensors

can be mounted to a lab bench with the table clamp and small rod.

7. If Student A and Student B push the force sensors against each other, bumper to

bumper, which student will exert the larger force?

Student answers will vary largely depending on misconceptions regarding stronger or larger students exerting

more force, but the forces are equal and opposite.

8. Student A should face Student B with the force sensors held level in front of each

student, using the finger loops.

9. Press the zero button on each force sensor, and then position the force sensors so their

bumpers touch.

Collect Data

10. Start data recording. �(6.2)

11. Pushing procedure:

a. Student A pushes Student B’s force sensor while Student B tries to hold the sensor in

place for 5 to 10 seconds.

b. Student B pushes Student A’s force sensor while Student A tries to hold the sensor in

place for 5 to 10 seconds.

c. Both Student A and Student B push at the same time for 5 to 10 seconds.

12. Stop data recording. �(6.2)

Analyze Data

13. Adjust the scale of the graphs to include all three peaks and align the Time axes of both

graphs. �(7.1.2)

14. Find the peak force for each force sensor at each stage and record the peak values in

Table 1 in the Data Analysis section. �(9.1)

15. Annotate the peaks of each graph to show which student was pushing at each

peak. �(7.1.5)

16. Sketch your graphs of Force versus Time in the “Force versus Time – Pushing” blank

graphs in the Data Analysis section.

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Part 2 – Pulling

Set Up

17. Remove the rubber bumpers from the force sensors and attach a hook to each sensor.

18. Student A and Student B face each other again. Hold the sensors level and press the zero

buttons.

19. Connect a rubber band between the two hooks.

20. To avoid confusion, hide all previous runs of data on your graph. �(7.1.7)

21. Will Student A or Student B exert more force when pulling away from each other?

Student answers will vary largely depending on misconceptions regarding stronger or larger students exerting

more force, the forces are equal and opposite.

Collect Data

22. Start data recording. �(6.2)

23. Pulling procedure:

a. Student A pulls Student B’s force sensor while Student B tries to hold the sensor in

place for 5 to10 seconds.

b. Student B pulls Student A’s force sensor while Student A tries to hold the sensor in

place for 5 to 10 seconds.

c. Both Student A and Student B pull at the same time for 5 to 10 seconds.

Note: Be careful not to break the rubber band.

24. Stop data recording. �(6.2)

Analyze Data

25. Adjust the scale of your graphs to show all three peaks of the Force versus Time graph,

and align the Time axes. �(7.1.2)

26. Find the peak force for each force sensor at each stage, and record the peak value in

Table 2 in the Data Analysis section. �(9.2)

27. Annotate the peaks of each graph to show which student was pushing at each

peak. �(7.1.5)

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28. Sketch the graphs of Force versus Time in the “Force versus Time – Pulling” blank graph

in the Data Analysis section.

29. How accurate were your predictions?

Answers will vary. In all cases, though, the force was the same magnitude but opposite in direction.

30. Save your experiment as instructed by your teacher. �(11.1)

Data Analysis

Table 1: Maximum force applied while pushing

Condition Student A Student B

Student A pushing (N) –28.7 28.1

Student B pushing (N) –41.1 39.1

Student A and Student B pushing (N) –51.4 53.8

Force versus Time – Pushing

Student A

Student B Students A and B

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Table 2: Maximum force applied while pulling

Condition Student A Student B

Student A pulling (N) 17.8 –17.9

Student B pulling (N) 13.2 –13.3

Student A and Student B pulling (N) 21.8 –21.8

Force versus Time – Pulling

Analysis Questions

1. If you look at all of the interactions studied in this activity, how would you

summarize the results in a single sentence?

For every action there is an equal and opposite reaction. Or: When Student A exerts a force on Student B,

Student B exerts a force on Student A equal in size and opposite in direction.

2. It is likely that the results from pushing are not as consistent as the results from

pulling? Can you think of reasons that this might be true? What might you do to

improve the consistency of the pushing part of the Procedure, if this is the case?

The sensors tend to self-align along the axes of the sensors when pulling, but pushing the force sensors against

each other has a higher potential to be off-axis. If the sensors were mounted on carts on a track, or some other

form of force alignment was used, the results would be more consistent.

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Synthesis Questions

Use available resources to help you answer the following questions.

1. A 65.0 kg Olympic diver dives from a 10 m high tower. Consider the instant that

the diver is in the air 1 m above the platform. Draw a free-body diagram showing the

forces acting on this diver at this instant. In the table below, describe these forces in

the "Action" column. Give the size and direction of the force as well as stating what

exerts the force. In the "Reaction" column, give a similar detailed description of the

reaction force. Assume that the gravitational field strength is 9.80 N/kg.

Table 3: Forces acting on the diver

Action Reaction

A gravitational downward force of 637 N is exerted by

the earth on the diver.

A gravitational upward force of 637 N is exerted by

the diver on the earth.

2. A 65.0 kg tourist is standing in line waiting to get into a theatre. Draw a free-body

diagram to show the forces acting on the tourist. In the table below, describe these

forces in the "Action" column. In each case, give the size of the force, show its

direction, and specify what exerts the force. In the "Reaction" column, describe the

reaction for each of the action forces again, giving size, direction, and the object that

exerts the force.

637 N

637 N 637 N

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Table 4: Forces acting on the tourist standing in line

Action Reaction

A gravitational force of 637 N down is exerted by the

earth on the tourist.

A force of 637 N up is exerted by the ground on the

tourist.

A gravitational force of 637 N up is exerted by the

tourist on the earth.

A force of 637 N down is exerted by the tourist on the

ground.

3. A parent pulls a sled with children at a constant velocity of 0.8 m/s across a level

snow covered lawn by exerting a force of 225 N to the west. The mass of the sled and

children is 85.0 kg. Draw a free-body diagram to show the forces acting on the loaded

sled. In the table below, describe these forces in the "Action" column. In the "Reaction"

column, give the size of the force, show its direction, and specify what exerts the force.

Assume that the gravitational field strength is 9.80 N/kg.

Table 5: Forces acting on the sled

Action Reaction

A downward gravitational force of 833 N is exerted on

the sled by the earth.

A westward force of 225 N is exerted by the man on

the sled.

An eastward frictional force of 225 N is exerted by the

snow covered lawn on the sled.

An upward force of 833 N is exerted by the snow

covered lawn on the sled.

An upward gravitational force of 833 N is exerted by

the sled on the earth.

An eastward force of 225 N is exerted by the sled on

the man.

A westward frictional force of 225 N is exerted by the

sled on the snow covered lawn.

A downward force of 833 N is exerted by the sled on

the snow covered lawn.

4. A horse that is hitched to a stationary cart begins to pull on the cart. If the force

exerted by the cart on the horse is always equal in size and opposite in direction to

the force exerted by the horse on the cart, how can the horse move the cart?

The force exerted by the horse on the cart and the force exerted by the cart on the horse act on two different

bodies, namely the cart and the horse. Consider the cart. If the horse exerts a force that is greater than the force

of friction acting on the cart, then the cart will begin to move.

5. A student holds a force sensor from which a 500-g mass hangs. Suddenly, the force

sensor slips out of the student’s hand and falls to the floor. What reading does the

force sensor show as it falls to the floor?

The force sensor would show a value of 0 N. Earth's gravitational field accelerates the sensor downward at the

same rate as the mass so it is impossible for the mass to exert a downward force on the sensor.

225 N

225 N

833N 833N

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Multiple Choice Questions

Select the best answer or completion to each of the questions or incomplete statements below.

1. A student attaches a force sensor to a large block on a level surface. The student

gradually increases the horizontal force exerted by the force sensor on the block until

the block begins to move. If the force sensor records a value of 23.5 N the instant the

block begins to move, what is the value of the size of the force exerted by the block

back on the force sensor?

A. Less than 23.5 N

B. 23.5 N

C. More than 23.5 N

D. Either more than 23.5 N or less than 23.5 N depending on the amount of friction

present

2. The earth exerts a downward gravitational force on an automobile. The reaction to

this force is:

A. The upward force of the ground on the automobile

B. The downward force of the automobile on the ground

C. The upward gravitational force of the automobile on the earth

D. None of the above

3. A soccer player exerts a force of 86 N on a 300 g soccer ball by kicking it. The force

exerted by the soccer ball on the player's foot is:

A. 0 N

B. 29.4 N

C. 43 N

D. 86 N

E. 170 N

Key Term Challenge

Fill in the blanks from the list of randomly ordered words in the Key Term Challenge Word Bank.

1. If a quarterback exerts a force of 150 N [to the south] on a football, then the reaction force

is a force of 150 N [to the north] exerted by the football on the quarterback. If the quarterback is

in the air when he makes the throw, the ball will move toward the south and the quarterback

will move to the north. The motion of the ball will be much more significant because the mass of

the ball is much less than that of the quarterback.

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2. Newton's third law is commonly stated as: To every action there is an equal and opposite

reaction. These forces always occur in pairs that are equal in magnitude but opposite in

direction.

Extended Inquiry Suggestions

The diagram below shows two different setups using a dynamics system, super pulley, force

sensor, and 500-g mass. The purpose of the cart is to support the force sensor and ensure that

friction is negligible. Assuming that g = 9.80 N/kg, have students predict what the reading the

force sensor will record in each of the two situations. Teachers might want to first set up

arrangement A, and ask students make a prediction. Then, after predictions are made, the

arrangement can be switched to B. Again, ask students to make a prediction. At this point, some

students may want to change their prediction for arrangement A. After giving predictions and

discussing reasons for the predictions, the measurements can be made.

A

B

In each case, the force sensor will record a reading of 4.9 N. In situation B, the rod pulls to the

left with a force of 4.9 N on the force sensor just as the 500 g mass on the left did in situation A.

500 g

500 g 500 g