Concepts of Physics

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

Blow Air in a Long Air Bag

Introduction

This is a very interesting demonstration which is thoroughly enjoyed by the audience. It is based on the Bernoulli's principle. A consequence of this principle is that if the velocity of a fluid in a region is high (or low), the pressure in that region becomes low (or high)

Apparatus

A very large polythene bag

Procedure

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  1. Call a volunteer from the audience and ask him to blow up the polythene bag by blowing air into it just once.
  2. Generally we see that the volunteer puts his mouth on the open end of the bag and blows hard.
  3. Close the mouth of the bag immediately.
  4. Not even half the bag blows up with the amount of air the volunteer manages to blow into the bag.
  5. Now, keep the open end of the bag in front of your mouth at some distance (say 30 cm) and blow hard into the bag and close the bag
  6. We see that this time almost full bag blows up with the amount of air we manage to blow into the bag.

Discussion

When the volunteer blows into the polythene bag by putting his mouth on the open end, even though he blows hard, the air he manages to blow out from his lungs is insufficient to fill the bag.

But when we blow into the bag from a certain distance into the open end of the bag, the velocity of air in this region increases. According to Bernoulli's principle the pressure in this region decreases and hence becomes less than the pressure of the surrounding air. So, surrounding air also rushes into this region and is blown into the bag along with the air we blow out from our lungs. The amount of air entering the bag is much greater this time and hence the bag blows up to almost its full capacity.