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

# Suspending a Cup by a Balloon

## Objective

This demonstrate the concept of air pressure and Boyle's law, $$PV=constant$$.

## Apparatus

balloon, paper cup

## Procedure

Blow up a balloon to less than half of its potential size. Place the rim of a paper cup against the side of the balloon. Hold the cup and blow the balloon to its full size. You will find the cup becomes `fastened' to the surface of the balloon and will actually hold on fairly tightly.

<h3>Discussion</h3> Because of the curvature of the balloon surface, a part of the balloon extends into the cup when balloon is half blown. When balloon is blown to its full size, the curvature of the balloon changes leading to increase in the volume of the air trapped inside the cup. By Boyle's law, the pressure of the air inside the cup reduces and become less than atmospheric pressure. The pressure of the air outside the cup is equal to the atmospheric pressure. This pressure difference causes sticking of the cup to the balloon.