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Marble gravitron

Challenge a friend to lift a marble or round lolly (or any spherical object) off a table touching it only with a glass. They'll never succeed of course. You however, are an intergalactic master of the centripetal force and laws of nature, so you achieve this piddly feat with ease. Stand back Earthlings!

Marble gravitron - tricks

1. This is very important: it only works with a glass that is narrower at the top like the one on the left in this picture. Wine glasses work best - the glass on the right in this picture won't work at all.

2. Trap a marble (or any spherical object) under the glass on a smooth surface.

3. Wiggle the glass and the marble will start rolling around inside really fast. Look carefully and you'll see it starts 'climbing the walls'.
Note: the marble is just a faint blur in this photo

4. When the marble is really hooting along, lift up the glass and the marble stays inside. With some glasses, you can keep the marble inside as long as you keep wiggling.
Note: you can just see the blur of the marble between my middle and ring fingers in this photo.

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What's going on?

This trick reminds me of that motorbike stunt inside a huge spherical cage. When the riders get up enough speed they can ride around sideways or even upside down.

The force that keeps these daredevils and your marble stuck to the glass is called the centripetal force and it results from the marble's circular path and the shape of the glass.

You experience the centripetal force pushing you 'sideways' inside a car every time you go around a corner. Inside a car, the physics is very intuitive: the faster you go or the sharper the corner, the greater the 'sideways' force. And you'd rather be sitting next to a little sister than a big uncle because the heavier an object, the greater the sideways force.

The reason you experience this force is because any object in motion tends to stay in motion in a straight line unless something gets in the way. Once moving, the marble wants to keep moving in a straight line but the curved walls of the glass make it change direction. As a result, the marble pushes against the glass. From the marble's perspective, your upside down glass has walls which bank up and outward, so it rises up. As long as you keep the marble racing, it keeps climbing the glass and stays put.


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Published 24 March 2008

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