Newton's Third Law

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In this lesson, you will investigate Sir Isaac Newton's third law of motion.

Before you get started, let's review what you have already learned about Newton's first two laws of motion in the previous Related Lessons, found in the right-hand sidebar. Explain each of these laws to a teacher or parent. Also, provide a real-world example of each while giving your explanation.

Newton's third law is probably his most well-known law.

The third law has to do with how forces affect motion. A force is a push or a pull on an object. Test Newton's third law by printing the worksheet Forced Motion from Downloadable Resources in the right-hand sidebar.

For this activity, you will investigate force by observing the movement of a ball. You will need one ball to complete this activity, preferably a small ball with a good bounce, such as a bouncy ball or tennis ball. Follow the instructions and record your observations on the worksheet.

The Forced Motion activity allowed you to observe the movement of a ball when it was thrown up into the air and when it was bounced against a hard surface.

• In each activity, what force(s) were applied to the ball?
• What similarities did each demonstration have in common

You should have made the following observations and conclusions:

• For each activity, multiple forces were acting on the ball.
• In the first activity, when the ball was thrown into the air, you applied a force to the ball by throwing it up. The ball came back down because gravity was pulling down on the ball.
• In the second activity, when the ball was bounced against a hard surface, you applied force to the ball when you threw it down. The ball was able to bounce back up because the floor pushed the ball up.
• In each demonstration, the ball always changed direction when a new force was applied to it.

What you observed during this activity has to do with Newton's third law of motion.

Newtown's third law is called the action/reaction law, because it states that every action has an equal and opposite reaction. This means whenever an object comes into contact with another object, both objects exert a force on one another. The amount of force being exerted by each object is the same, but the direction of the force being exerted by each object is opposite.

Let's continue experimenting with Newton's third law.

1. Lightly push on a wall.
   • What happens? Does it hurt?
   • The wall prevents you from continuing to push your hand forward because it is pushing back on your hand with the same amount of force you are exerting.

2. Now, pretend you punched the wall as hard as you are able.
   • What would happen? Would that hurt?
   • The wall would still prevent you from pushing your hand forward, but your hand would probably hurt.
   • In this example, your hand would probably hurt because you are using greater force to punch the wall. The wall is responding by exerting the same amount of force on you. Therefore, the greater force is going to cause your hand to hurt.

Learn more about Newton's Third Law of Motion: Action and Reaction from Professor Dave Explains:

Then, keep learning about Newton's third law of motion by relating it to one of America's favorite sports.

Watch the National Science Foundation's video, Newton's Third Law of Motion- Science of NFL Football.

• How does Newton's third law of motion relate to football?

Go back to the image from the start of this lesson.

• What does Newton's third law have to do with the fish swimming?

Think about your response.