Answer to Question #89734 in Mechanics | Relativity for Sandra Sangai

Question

A popular magic trick consists of placing a coin on a card and the card on the top of a glass. The edge of the card is flipped briskly with the forefinger, causing the card to fly off the top of the glass as the coins drop into the glass. Explain. What law does this illustrate

Answer 1

This trick demonstrates Newton's first law: a body is at rest or moves with constant velocity unless some force do not accelerate it or cause it to stop. In this trick, the coin is resting. The friction between the card and the coin is negligible. So when the coin is on the card, force of gravity pulling the coin downward is compensated by the force of card's elasticity acting on the coin upward.

When you flip the card, the force acting upward disappears, the force of gravity is still pulling the coin down, that is why it falls.