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

# Why Balloons come Closer?

## Objective

To learn about Bernoulli's principle.

## Introduction

The Bernoulli principle enables aircraft to fly. This experiment demonstrate Bernoulli's principle in a simple way.

## Apparatus

Two balloons, thread

## Procedure

Suspend two balloons a few inches apart. Blow air between them. What happens? Whether balloons come closer or goes farther away? Why? You can also do this experiment by blowing air above a strip of paper.

%\par\medskip\emph{Electrostatic Variant:} Same setup can be used to show electrostatic %attraction and repulsion between balloons when they are charged.

## Discussion

One conclusion from the Bernoulli's theorem is that if velocity of the fluid (air) in a region increases then pressure of the fluid in this region decreases. When we blow air in the gap between the balloons, the speed of air increases due to blowing. Thus, pressure of air in the gap between the balloons decreases. Thus, air from the sides rushes in this low pressure region bringing the balloon along with it. Hence balloons come close to each other.

## Note

Electrostatic Variant: Same setup can be used to show electrostatic attraction and repulsion between balloons when they are charged.