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After this activity, students should be able to:

• Understand that momentum depends on both mass and velocity.
• Explain the difference between an elastic and inelastic collision.
• Recognize that difference surfaces and materials promote different types of collisions.
• Use numbers to count and measure.
• Examine equivalent forms, such as decimals.
• Collect data to solve equations.
• Confidently perform multiplication and division operations.
• Use calculations to discover that objects fall at the same rate.

Momentum can be thought of as 'mass in motion' and is given by the expression:

The amount of momentum an object has depends both on its mass and how fast it is going. For example, a heavier object going the same speed as a lighter object would have greater momentum. Sometimes when moving objects collide into each other, momentum can be transferred from one object to another. There are two types of collisions that relate to momentum: elastic and inelastic.

An elastic collision follows the Law of Conservation of Momentum, which states "the total amount of momentum before a collision is equal to the total amount of momentum after a collision." An elastic collision example might involve a super-bouncy ball; if you were to drop it, it would bounce all the way back up to the original height from which it was dropped. Another elastic collision example may be observed in a game of pool. Watch a moving cue ball hit a resting pool ball. At impact, the cue ball stops, but transfers all of its momentum to the other ball, resulting in the hit ball rolling with the initial speed of the cue ball.

In an inelastic collision, momentum is not conserved and the energy is transferred to another kind of energy such as heat or internal energy. A dropped ball of clay demonstrates an extremely inelastic collision. It does not bounce at all and loses its momentum. Instead, all the energy goes into deforming the ball into a flat blob.

In the real world, there are no purely elastic or inelastic collisions. Rubber balls, pool balls (hitting each other), and ping-pong balls may be assumed extremely elastic, but there is still some bit of inelasticity in their collisions. If there were not, rubber balls would bounce forever. The degree to which something is elastic or inelastic is dependent on the material of the object.

This chart illustrates the range of bounciness for eight types of balls. click for copyright

Many sports incorporate collisions and momentum as part of game play. Can you think of some? Certain sports rely primarily on elastic collisions that conserve momentum, such as pool or billiards, while others use inelastic collisions to make the game more challenging. What would happen if a baseball and a bat had an elastic collision like a golf ball and club? (Answer: There would be a lot more home runs during a game!)

This activity is best done in groups, because while one person drops the ball, another person must watch the ball and meter stick to note how high the ball bounces. Additional team members could hold the meter stick steady and/or record the data.

For the momentum calculation, note that all the balls will have the same velocity because any object dropped from the same height will fall at the same constant rate due to gravity.

Students could investigate the materials used to make balls as a way to better understand why they bounce the way they do. For example, if you cut open a golf ball, you will find a mass of rubber bands wound around a core that is also usually rubber. All that rubber (and the hard plastic cover) explains its bounciness. A baseball has a similar construction, but with very different materials. A baseball's inside is a mass of yarn wound around a cork core, and its cover material is leather. These materials make for a less bouncy ball. (Note: safety precautions should be taken when opening these balls and should be done under adult supervision.)

Momentum: http://www.physicsclassroom.com/Class/momentum/momtoc.html

Momentum and energy loss of balls colliding against different surfaces: http://www.iit.edu/~smile/ph8709.html

Science of Baseball, The Exploratorium: http://www.exploratorium.edu/baseball/index.html

Science of Baseball, The Exploratorium: http://www.exploratorium.edu/baseball/howfar7.html

Science of Baseball, The Exploratorium: http://www.exploratorium.edu/baseball/howfar5.html