Demos: 1F-05 Coin, Hoop & Milk Bottle (Inertia)


A flexible cardboard ring is placed on the lip of a glass milk bottle (see picture). On the ring is placed a coin so that it sits directly above the bottle opening. A sharp blow on the ring knocks it out from under the coin with the latter dropping into the bottle. But this happens only if you hit the ring in the proper way.

Directions: After placing the coin in position, ask how you might get the coin into the bottle without touching it. Students will likely suggest hitting the ring with a quick blow from the side. What you do next depends on how you want to present the demo. If you strike the ring directly from the side, the coin will be projected into the air and miss the bottle completely. If you pretend to do the same thing, but bypass the outside of the ring and let your finger catch the INSIDE of the ring on the other side, the ring will flatten vertically and the coin will not be thrown off.

Suggestions for Presentation: If you feel lucky, make a “bet” with a student on how to get the coin in the bottle. Let the student try. Usually the student will strike the ring on the outside. You might then say, “I don’t why that didn’t work. Here, let me try.” If you move your hand fast enough, no one will likely notice that you passed by the first edge and hit the inside of the ring.

Applications: Relate this to the other inertia demos. But you should be honest and tell them that this demo is as much about deception as it is about inertia!

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Last Updated: May 9, 2016 11:44 AM