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Inertia of a card

How you can flick a card off a glass and let the coin on the card drop into the glass.


This has to do with impulse (force x time = change of momentum).

There will be a frictional force between the coin and the cardboard. The size of this force depends on the nature of the two surfaces and the weight of the coin.

If the card is flicked quickly this frictional force will only act on the coin for a short time and so it will only get a small impulse and so not move much in the horizontal direction. It will therefore drop into the bottle.

However if you move the card slowly the force will act for a much longer time and so the coin will move with the card in the horizontal direction.

© Keith Gibbs 2011