Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Card-Coin Experiment

Introduction

Card-coin experiment is a very old and widely used experiment to show the property of ``Inertia of Rest''. A card is placed on an empty glass tumbler and coin is placed on the card right above the center of the tumbler. A flick is given to the card with finger, the card flies away and the coin falls in the tumbler. It is explained by saying that due to inertia, the coin remained at its own place and the card went away due to the flick.

While the experiment itself is beautiful, the explanation has loopholes. Our experiment brings out these loop holes.

Apparatus

A beaker, cards, coin

Procedure

  1. You are given several cards. Place the smallest one on the beaker and put the coin on it at the center.
  2. Flick the card with your finger as hard as you can. Did the coin fall in the beaker?
  3. Do the same with other cards. Write your observations.
  4. Going again with the small card, flick the card with a smaller force. Write your observations.

Discussion

You find that whether the coin falls in the beaker or not depends on the length of the card and the force with which you flick. It also depends on the friction at the surfaces of contact.

As soon as the card begins to move it slips against the coin and exerts a forward force on the coin. By the time coin runs out of card’s surface below, the coin also moves a little in forward direction. The coin falls in the beaker only if this forward displacement is smaller than the radius of the beaker. Otherwise it falls outside the beaker. Thus in no case, the coin ``remains at its own place'' as the card moves out.