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Law of Conservation of Momentum

When an action and reaction occur, momentum is transferred, and total momentum is conserved.

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The Physics of Books: Falling Open or Falling Off?

How can the transfer of objects from one place to another be used to model the Law of Conservation of Momentum?

Credit: Wonderlane
Source: http://www.flickr.com/photos/wonderlane/3909074244/sizes/o/in/photolist-6Xr2zN-7hZVQu-7qiGEg-7rSmD9-7tJZQV-7tK17K-7tK1jg-7tK1un-7tK1Le-7tK2dH-7tNX3J-7tNX8q-7tNXm5-7tNXqW-7tNXCf-7tNYjy-7un7en-7uJtMv-7uJCRF-7uJEYD-7uNiS7-7uNjpC-7uNu6U-7vap8K-7vedkj-7x5pqz-7x9cuJ-7xbrF1-dMTqKC-dMTqNo-dNpGpW-dfkwcN-7Ay9pH-8DG3gb-8DCUdi-9ZBpNF-a1ihS9-dMMSZH-9oWPkU-8fwkL6-9z1qH9-9oQYDV-cdaEDL-bJtVct-c3hdZd-94L4nT-cumDY9-8DFZ7j-b3SXpF-adoaPq-cCq3d9/
License: CC BY-NC 3.0


A librarian is wheeling a cart full of books. The books are stacked so high on the cart that she is unable to see in front of her. Suddenly, her cart bumps into an empty cart that was left in the middle of the aisle, causing books to fall off her cart. Luckily, the books that do fall off land on the cart the librarian had bumped into.

Creative Applications

1. Is the amount of books on the two carts the same after the crash as before the crash?

2. Pretend the amount of books represents momentum rather than the movement of the carts. How does the above scenario model the law of conservation of momentum, which usually occurs between two objects?

3. Now look at the law of conservation of momentum in actual terms — movement. If both carts were empty, what would happen to the movement of both if the crash still occurred?

4. What happens to the energy of the first cart when it collides with the second cart? (Think: Where does the energy go?)

5. This time, the second cart is being pushed by another librarian, and the books similarly are high enough to obstruct his vision. The two librarians with filled carts collide head on at the same speed. What do you expect to happen to the books? To the movement of the carts?

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