Newton's Third Law of Motion: What Is a Reaction?

Newton's third law of motion, the law of action-reaction, says that for every action there is an equal and opposite reaction. If you use your arms to push against a concrete wall, you are exerting an action force on the wall. The wall in turn exerts a reaction force upon your hands. Since the wall does not cave in or move, the action force is equal and opposite to the reaction force. You will also notice that the action and reaction forces act on different objects. Your hands act on the wall, and the wall exerts a force on your hands. In this activity you will demonstrate Newton's third law of motion using spring scales.

Materials

Two spring scales (both of which should read from 0 to 10 Newtons)

Activity

1. Place the palms of your hands in front of you. Push your right hand hard against your left hand, but resist moving with your left hand. Do you feel the force of your right hand on the left? Do you feel the force of your left hand pushing back on the right? Are they equal forces? Opposite?
2. Hook the top portion of one spring scale around your right thumb. Position your right hand on the right edge of a table.
3. Hook a second spring scale to the first scale so the bottom portion of the second scale is hooked to the bottom portion of the first scale.
4. With the two scales attached, hold your right hand steady so it does not move. With your left hand pull horizontally on the top portion of the second spring scale until the reading on the scale measures 5 Newtons.
5. Look at the reading on the other spring scale. What does it read?

Follow-Up Questions

1. What did you experience when you pushed your palms together at the beginning of this activity?
2. What reading was showing on the spring scale that was being held steady? Was the reading on both scales the same? Explain the reason for this.

Answers

1. When you pushed forward with the left hand, the right hand exerted a force back on the left. The action had an equal and opposite reaction.
2. Five Newtons. Yes, the same force was being exerted on both scales in the activity due to the third law of motion.

Extension

Look at Figure 3.4, which shows the hands holding the two spring scales. Underneath the drawing, add arrows to show all of the forces at work. Remember to draw the forces interacting between the hands and the spring scales as well as the spring scales interacting with each other.