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Think It Over

This section of Resonance presents thought-provoking questions, and discusses answers a few months later. Readers are invited to send new questions, solutions to old ones and comments, to ‘Think It Over’, Resonance, Indian Academy of Sciences, Bangalore 560 080. Items illustrating ideas and concepts will generally be chosen.

Balloons in Balance

The following experiment is suggested in a 4th standard science textbook to show that air has mass:

“Take a ruler, 30 cm long. Tie a thread at its centre and hold the ruler by the thread. This is the beam of your balance.

Take two inflated balloons of the same size and hang them from the two ends. Adjust the position of the thread so that both sides are balanced.

Ask a friend to prick one of the balloons with a needle. The air will escape from the balloon and the other side will go down.

It is the mass of the air in the inflated balloon that makes that side heavier. This shows that air has mass.”

Is the above explanation really correct? Does the experiment show air has mass? For, if we consider the buoyancy force of the ambient air acting on each balloon then the weight of the air inside is exactly cancelled by the buoyancy force. Then both before and after pricking the balloons should be in balance: the net force will be only the weight of the balloon material both on the inflated balloon and on the pricked one. Is there an explanation that shows the inflated-balloon side goes down?

What would happen if the experiment is done with balloons filled with a lighter gas (eg. helium) instead of air?

Jaywant H Arakeri
Department of Mechanical Engineering
Indian Institute of Science
Bangalore 560 012, India.

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