Demos: 1H-01 Action - Reaction

This demonstration illustrates Newton’s Third Law in a way that focuses on the physiological forces called “pulls”. It shows that when the student thinks that he/she is not actually pulling, the force is still there. It also illustrates conservation of momentum resulting from internal action-reaction forces.

Directions: Two flat carts, fitted with skateboard wheels to reduce friction, are spaced about 2 meters apart and their positions noted. A student stands on one cart, the professor on the other (the greater the mass difference, the better). Each holds one end of a rope and as they pull on the rope, the carts come together.

Suggestions for Presentation: To make this activity quite dramatic in its illumination of Newton’s Third Law, first tell the student to follow your directions carefully. The first time, say, “You don’t pull. You just hold on and I’ll do the pulling.” Then start pulling on the rope. Note where the carts come together. Set everything back as it was and this time say, “I’ll hold on and you do the pulling.” Again note the location of the collision. It will be the same. Finally (after telling the student to stand sideways with legs spread considerably for balance), say “Now let’s both pull.” The carts will move quite quickly, but should collide in the same place as before. Act chagrined and say, “You can’t follow directions. I told you not to pull.” Then explain that it really wasn’t the student’s fault--as long as the rope was being held, he/she had to pull on it.

Note carefully that no matter how the pulling was done, the carts ended up in the same place. To explain that phenomenon requires the introduction of momentum conservation. However, you can persuade the students that the fact the carts ended up in the same place is evidence that the three situations were not different and the forces were equal and opposite each time.

Applications: Use this demo as a way to introduce the idea that “pulling” and “pushing” don’t necessarily mean movement of the arms.

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