Concepts of Physics

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

Balancing Spoon and Fork on a Matchstick

Introduction

There are various balancing tricks. These tricks are based on the principle of stability.

Apparatus

spoon, fork, glass, matchbox

Procedure

Attach the spoon to the fork by pushing it in between the teeth so that one tooth is held out by the convex surface of the spoon and other teeth are in the concave surface of the spoon. Place a match-stick between two teeth of the fork. The match-stick should be in the same plane with normal axis of the handle of the spoon and fork. Adjust the angle between fork and match-stick, as the fork is above the glass. Once the spoon and fork are in balance on the glass rim, burn the end of the match-stick inside the glass. As the heat of the flame is absorbed by the glass, the temperature drops below the wood's ignition temperature and the burning of the match-stick stops exactly at the glass rim. Burn the other end of the match-stick. The burning will stop at the top of the fork and the heat of the flame is absorbed by the metal. Observe the equilibrium of the fork and spoon about the match-stick on the glass rim.

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Discussion

When you try to balance an object, if the point of support is not at the center of gravity then the object will rotate either clockwise or anti-clockwise depending on which side has more torque. However, if the pivot point is on the same vertical line as the center of gravity, then the object, no matter what shape, is going to balance. It will be stable if the center of gravity lies below the pivot point. If the center of gravity is above the pivot point, even a slight disturbance will pull it off balance. In our case if you want to have a stable situation, the center of gravity of this assembly has to be below the pivot point. The pivot point is where the match-stick rests on the rim of the glass. The actual center of gravity must lie in the empty space between the two forks and below the pivot point to achieve stability.