Inertia means that a rolling ball on a smooth, level surface will roll forever if nothing stops it. In fact, friction and air pushing against the moving ball will eventually bring it to a stop. But interesting things happen when a motionless object gets in the way of a moving one. Try this experiment and see for yourself.
- Tape the yardsticks to a tabletop so they're parallel and about 1/2 inch apart (if you are under 5 then we feel compelled to remind you that mom and dad will not appreciate the artistic appeal of 2 yardsticks glued to the kitchen table).
- Put 2 marbles in the middle of the sticks (our 'track') a few inches apart.
- Flick a marble so that it rolls and hits the other one. Notice that the one that had been rolling stops while the one that had been still now rolls! The momentum of the rolling marble transfers to the other one, stopping the first and setting the second in motion.
- Now put two marbles on the track so they touch, and a third several inches away. Flick the single marble into the other two. Notice that the rolling marble stops, the middle one stays put, and the third one rolls. The momentum went through the second marble into the third.
- Try other combinations: two marbles into three still marbles, or three into three. You'll find that however many marbles you set in motion, the same number will be made to roll when they hit.
This experiment introduces 3 concepts about momentum :
- Momentum can transfer from one object to another.
- Momentum can pass from one object, through a second, and into a third.
- The total amount of momentum at the beginning will stay the same.
Supplies: Yardstick, Marble
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