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Pick Up a Marble Experiment #447
Grade Level(s): 1-2, 3-5, 6-8
By: Robert Krampf

This week's experiment came from my friend, Mik Jacobs. He has been working on his new program on energy conservation, but took time out to remind me of a fun science experiment.


To try it, you will need:

a marble
a drinking glass with straight sides


Place the marble on a smooth surface. Turn the glass upside down and place it on the table, with the marble inside. Now, the idea is to pick the marble up off the table without touching anything except the glass. How are we going to do that?

Hold the glass and start sliding it across the table in a tight circle. That should cause the marble to spin around the wall of the glass. If you move the glass faster and faster, the marble will spin faster and faster. Once it is spinning very quickly, slowly lift the glass, being sure to keep it spinning. As long as you hold it very level, the marble should stay in the glass as you lift it. Why?

It has to do with inertia. If you roll the marble across the table, inertia keeps it moving in the same direction, at the same speed, until something else pushes or pulls on it. That push or pull could come from friction with the table top, which would slow it down. It could also come from a bump or dip in the surface of the table, or from the side of the drinking glass which could change the direction of the marble. In the case of the drinking glass, it could cause the marble to move in a circle around the side of the glass.

As you move the glass in a circle, it is pushing the marble, to keep it rolling. The marble's inertia tries to keep the marble moving in a straight line, so the marble pushes against the side of the glass. The combination causes the marble to zip in a circle around the inside of the glass.

But something else is happening too. Inertia is causing the marble to push against the side of the glass. As the marble moves faster and faster, it is pushing harder and harder against the side of the glass. When that push gets strong enough, you have enough force to hold the marble against the glass, even when you pick it up off of the table.

This experiment reminded me of a carnival ride that I once rode. It was a large, round room, where everyone stood with their backs to the wall. The room began to spin faster and faster. When it reached the right speed, the floor dropped out from under us. Inertia pushed us against the wall with enough force to hold us in place, suspended on the wall. If it had not made me tremendously dizzy, it would have been a fun ride. Luckily, marbles don't seem to get dizzy.

From Robert Krampf's Science Education Company

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