Physics 1
Dynamics Activity
Newton's 3rd Law in an Explosion

Purpose:

To investigate Newton's Third Law:

• How do 2 forces arise in an interaction, when only one object "pushes?"
• How can the accelerations of 2 objects be different if the forces on them are equal?

Discussion:

Newton's Third Law can be difficult to understand. If I push a book, does it really push me back? If so, why does the book move, but I don't seem to?

In this activity, you will use a spring-loaded plunger mounted on a dynamics cart to apply a sudden force (the "explosion") on another dynamics cart. By observing the resulting motions, you should be able to answer these questions.

Equipment:

2 dynamics carts (1 with a spring-loaded "plunger")

some reasonably large masses (bricks, etc.)

Procedure:

1. You can set the spring-loaded plunger by pushing it into the cart and pushing upward so that the metal end of the plunger catches on the end of the cart. You can release the plunger by tapping on it with a pencil or pen.
   
   
2. With the masses of the carts equal (no added mass on either cart), observe and describe the motions of the carts after the "explosion." In particular:
   1. Describe the motion of the carts.
   2. What evidence do you have that there is a force exerted on both carts?
   3. Are the accelerations of the carts equal? Why do you think so?
   4. Is the same amount of force exerted on both carts? How do you know?

    

3. Add mass to one of the carts, so that its mass is at least double the mass of the other cart. Observe and describe the motions of the carts after the "explosion" now. In particular:
   1. Describe the motion of the carts.
   2. Are the forces on the two carts equal? Why do you think so?
   3. Are the accelerations of the carts equal? Why do you think so?
   4. If the accelerations are not equal, which cart has more acceleration? How can this happen?

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last update November 10, 2007 by JL Stanbrough