Concepts of Physics

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

The Air has a Mass?

Objective

To show that the air has a mass.

Apparatus

a football or a volleyball, a pump, weighing balance

Procedure

Measure the weight of an un-inflated ball. Now, pump sufficient air into the ball and measure its weight again. Observe if the ball has gained weight. Repeat the experiment by pumping in more air. You will observe that the weight of the ball goes on increasing as more air is pumped in.

Discussion

As the air has mass, the weight of the ball increases when air is pumped into it. When a balloon or a ball is inflated, the buoyant force of air acts upward on them. In the case of an ordinary balloon, the buoyant force is nearly equal to the weight of the air that is pumped inside the balloon. Hence, the difference in weight cannot be found out by a common balance or a spring balance. In the case of a ball, a large mass of air can be pumped in without a comparable change in the volume of the ball. Hence, after the ball gets inflated, there is not much increase of buoyant force. Buoyant force depends upon the volume of the ball. In the case of a balloon you cannot pump in large volume of air, for if you do so the balloon will burst.