**PURPOSE: **To show what happens when one mass in a collision ball set is different from the others.

**DESCRIPTION: **The collision ball sets photographed are identical except that the third ball from the left in the set on the right is lighter - the original steel ball has been replaced by one made of aluminum. Q: Will the set with one lighter ball work the same as the "normal" set if *one* ball is picked up and released? A: No. Momentum and energy are transferred in a series of elastic collisions between two balls at a time; for identical balls all the momentum and energy are transferred from one ball to the next in each collision. The lighter mass ball breaks the chain, at which point there is no longer complete transfer of all the energy and momentum to the next ball.

**SUGGESTIONS: **Ask your students to vote on the result before performing the experiment.

**REFERENCES: ** (PIRA unavailable.)

**EQUIPMENT: **Equal and Unequal collision ball apparati, as photographed.

**SETUP TIME: **None.