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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)
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.
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