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PPT Activity
Newton’s 3rd Law of Motion
Developer Notes
1. I'd like to borrow Hewitt's activities from his Practicing Physics workbook.
Goals
 Students should understand that in order for there to be a force, there must be two objects.
 Students should understand that when one object exerts a force on a second object, the
second object exerts an equal and opposite force on the first. "Every action has an equal and
opposite reaction."
 Students should understand that an action-reaction pair is between two objects; that two
equal and opposite forces are not necessarily an action-reaction pair.
Concepts & Skills Introduced
Area
physics
Concept
Newton’s 3rd Law of Motion
Time Required
min
Warm-up Question
There's a nice problem sometimes used in math. If there are 10 kissing cousins, and each one
kisses each other one once, how many total kisses would there be? It's a good math question, but
the interesting part here is what constitutes a kiss. Can you kiss someone without them kissing
back? Is a kiss one kiss or two kisses? Per Newton's 3rd, you can't kiss without being kissed. This
usually promotes a lively discussion.
Presentation
Do a mini-activity. Pair the students. Have one student hold up a hand and the other push on it
with their hand without touching. Impossible, of course. You can't have a force without a
counter-force.
Another mini-activity. Give each pair of students a spring scale. Instruct one student to hold one
end of the scale, but not to pull at all, while the other student pulls the other end. Impossible
again. After playing with that, have the students try to pull different amounts. Also impossible.
Have them pull at an angle. Impossible.
The point is Newton's 3rd Law of Motion. We've looked at inertia (equilibrium), where a net
force of zero means no change in motion. We've looked at F=ma, which defines the relationship
between force and acceleration (and mass). And now we see that a force can't exist by itself there must be something else pushing back.
Have the students stand with their toes against a wall, then have them push on the wall. What
happens? The wall pushes them over backwards. Who pushed who? Student pushes on wall, wall
pushes on student. (Wall wins.)
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PPT Activity
Newton’s 3rd Law of Motion
Newton’s 3rd Law of Motion
Whenever one body exerts a force on a second body, the second body exerts an equal and
opposite force on the first.
This is often stated: For every action, there is an equal and opposite reaction.
In Newton's 1st Law, we looked at the sum of forces on a single object, which could sum to zero.
Here we're looking at the forces between two different objects, which can't be summed in the
same way.
Demonstrate with a balloon. Blow it up and put it on a table. Press on it with your hand. Note
that it is flat by your hand and where it touches the table. Both you and the table are pushing on
it. This is not strictly an action-reaction pair however, it is two separate pairs.
Demonstrate with a balloon. Blow it up and let it go. Obviously, it starts from rest and increases
its speed - it accelerates. So there must be a force. Where does the force come from? The balloon
squeezes (pushes) the air out. In turn, the air pushes the balloon forward. Balloon pushes on air,
air pushes on balloon. That's an action-reaction pair.
Do a POE. Put two spring scales in series. Tell the class you'll pull one until it reads half-scale
(in the middle). Have them write a prediction what the second one will read at that point. When
they're done, demonstrate, then have them write an explanation. The scales will read the same. If
one is pulling with 10 N, the other must be pulling with 10 N.
Newton's 3rd can be related to Newton's 2nd. F = ma. For there to be a force, there must be a
mass accelerating.
Assessment
Writing Prompts
1.
Relevance
Summary
Exercises
1. When you hit a nail with a hammer, the nail goes into the wood, then stops.
a. In the action-reaction pair, if the hammer nits the nail, what does the nail hit?
Answer: The nail hits the hammer.
b. Another pair: if the nail pushes on the wood, what does the wood push on?
Answer: The wood pushes on the nail.
2. Because you like your teacher, you bring her an apple. She puts the apple on her desk. The
force of Earth's gravity pulls the apple down on the desk. The desk pushes the apple back up
so it doesn't move. The two forces are equal and opposite. Are they an action-reaction pair?
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PPT Activity
3.
4.
5.
1.
6.
Newton’s 3rd Law of Motion
Answer: They are not an action-reaction pair, even though they are equal and opposite.
Action-reaction refers to the forces back and forth between the same two objects.
When you run, what force is it that pushes you forward?
Answer: The road pushes you forward. If you were on ice, you'd have a hard time running.
When a car accelerates, what force pushes it forward along the road?
Answer: The road pushes it forward. This is a bit of a difficult idea to see. To demonstrate,
place a piece of paper on the floor, then take a little car with a wind-up or battery motor, and
lower it onto the paper while you still hang onto the car. The paper will go backward. So, the
car pushes the road backward, the road pushes the car forward.
When a gun is fired, it recoils (kicks). What causes the recoil?
If you play your ukulele so much that you get a blister, is that blister the result of an actionreaction pair? Explain.
Draw a picture of a man dragging a box with a rope. Name as many action-reaction pairs as
you can.
Challenge/ extension
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PPT Activity
Newton’s 3rd Law of Motion
Background / History
Newton took Galileo's concept of inertia, that an object wouldn't change its motion unless there
was a net force on it, and created his 1st Law of Motion. Then, in his 2nd Law, he defined what
force is: F = ma. In his 3rd Law, he noted that a force can't exist all by itself - there must be an
equal and opposite force. It takes two to tango. You've probably heard, "For every action, there is
an equal and opposite reaction." That's Newton's 3rd Law of Motion.
Newton’s 3rd Law of Motion
Whenever one body exerts a force on a second body, the second body exerts an equal and
opposite force on the first.
It is important to realize that in Newton's 1st Law, we were looking at all of the forces acting on
one object, so they could be summed and might equal zero - equilibrium - . In his 3rd Law, we're
looking at equal and opposite forces on two objects. They can't be summed to find equilibrium.
Forces may be equal and opposite, but that doesn't make them an action-reaction pair. An actionreaction pair must be between the same two objects. (It doesn't matter which you call action and
which you call reaction.)
Could a bat hit a ball without the ball hitting the bat? Could you jump off the floor if the floor
didn't exert a force on you? When a horse pulls a wagon, the wagon pulls back. The horse pushes
backward against the ground, and the ground pushes the horse forward.
Problem
Materials
This worksheet.
Procedure
Imagine you're standing on a scale. The scale is on the ground (Earth). Your weight is
500 N. Answer the following questions:
If the action force is you pushing on the scale with 500 N, then
1. What is the reaction force?
Earth
If the action force is Earth pulling you down with 500 N, then
2. What is the reaction force?
If Earth pulls you down with 500 N, and the scale pushes you up with 500 N, then
3. Are you in equilibrium?
4. Are the two forces equal and opposite?
5. Are the two forces part of an action-reaction pair?
Summary
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PPT Activity
Newton’s 3rd Law of Motion
Exercises
2. When you hit a nail with a hammer, the nail goes into the wood, then stops.
a. In the action-reaction pair, if the hammer nits the nail, what does the nail hit?
b. Another pair: if the nail pushes on the wood, what does the wood push on?
3. Because you like your teacher, you bring her an apple. She puts the apple on her desk. The
force of Earth's gravity pulls the apple down on the desk. The desk pushes the apple back up
so it doesn't move. The two forces are equal and opposite. Are they an action-reaction pair?
4. When you run, what force is it that pushes you forward?
5. When a car accelerates, what force pushes it forward along the road?
6. When a gun is fired, it recoils (kicks). What causes the recoil?
7. If you play your ukulele so much that you get a blister, is that blister the result of an actionreaction pair? Explain.
8. Draw a picture of someone dragging a box with a rope. Name as many action-reaction pairs
as you can.
Challenge/ extension
1.
Vocabulary
 Newton’s 3rd Law of Motion
Whenever one body exerts a force on a second body, the second body exerts an equal and
opposite force on the first.
This is often stated: For every action, there is an equal and opposite reaction.
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