# Homework Help: Help me to understand.

1. Nov 7, 2007

### smhaq

My question is that if i had put a penny over a piece of a card paper placed on a glass(empty).then i pushed a card with a sudden stroke of finger,card moved ahead while the coin falls in a glass.Why it does so?
1. The problem statement, all variables and given/known data

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2. Nov 7, 2007

### catkin

The only force acting on the penny is caused by friction between penny and card. Try pushing the penny along the card by the edge to get a feel for now much that is. Because of the high speed your sudden stroke of finger gives to the card, this friction force only acts for a short time.

Intuitively you are expecting the penny to move sideways with the card. The penny starts at rest; for the penny to move sideways it has to be accelerated. The small friction force and short time that the small friction force is applied to the penny mean the penny is accelerated very little.

The penny is accelerated so little that it appears to to stay where it is, dropping straight into the glass. Watch carefully from the side and you will see the penny does move slightly in the directon of the card.

It may be that the penny is in poor contact with the card; it might be slightly flicked up by shock waves in the card from your your sudden stroke of finger; air may be dragged into the gap between penny and card. Both these effects would reduce the friction resulting in even less acceleration.