Momentum Transfer: Yogurt Cup Ping Pong Ball Launcher
[Shopping List: single-serving yogurt cups; ping-pong balls; buckets of water, a location that can get very wet!]
- Fill a yogurt cup 1/2 to 2/3 full of water.
- Put a ping-pong ball into the cup, and try to swirl the water so that the ping-pong balls spins in the center of the cup without touching the sides. This may take some practice, and while it's not absolutely necessary, it will help make overall success more probable.
- With the ping-pong ball spinning in the center of the cup, drop the cup straight onto the floor or table top. You want it to land as flat as possible.
- If everything works, the ping-pong ball should shoot 10-20 feet or more into the air, much more than you could even bounce the ball throwing it as hard as you can. Warning: you will probably get wet as well.
What's Happening: When you drop a ping-pong ball on the ground it will bounce back to a lower height than that from which you dropped it. Because it has little mass, it also has little momentum. The water in the yogurt cup has much more mass, so when it falls to the ground it has much more momentum as it hits. It also bounces a little, but because the water is a liquid and flows easily, it splashes around but doesn't bounce much. When a ping-pong is floating on the water surface, both will fall to the ground at the same speed when dropped, but since the water is on the bottom, it will hit the ground first and actually begin moving upward while the ping-pong ball is still moving downward. This transfers much of the very large momentum in the water to the very small mass of the ping-pong ball, giving it a much larger velocity than either the ball or the water had even at the moment of impact. This is enough velocity to move the ball 10-20 feet in the air, even though it was only dropped 3 or 4 feet to the ground.
Many things can happen to prevent achieving maximum height however, and there are plenty of parameters to study in further investigations. If the cup strikes the ground tipped, then the water, and thus the ball will probably move more sideways, maybe even hitting the sides of the cup. The depth of water in the cup may have some effect on your results. Also, if the ping-pong ball is resting against the cup's sidewall as it hits the ground, it may also bounce off at a sideways angle and perhaps strike the cup instead. This is why it's best to keep the ball centered in the cup by swirling the water first. When you do this, the water flows to the sides of the cup so that the level in the center is slightly lower and the ball will move towards the lower level in the center. This does take some practice however, as swirling the water smoothly while the ball is floating on the surface can be difficult. The type of surface on which you drop the cups also seems to affect the results. A smooth hard surface seems best, and a typical fiberglass bathtub actually works very well, which is also for a good place to experiment anyway given the high probability of splashing.
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