A riddle with 3 possible answers

When I went for my interview for possible admission to London University, the interviewer asked me to state Archimedes’ Principle.

It goes like this “When a body is immersed in a fluid, the upthrust is equal to the weight of the fluid displaced”.

Hey, I was thinking this is going to be easy!

Then he asked me this question –

If you have a large glass bottle about half full of water and there’s a cork floating in the water. The bottle is sealed but there’s a tube going through the seal. Now if we pump air through the tube into the bottle, will the cork sink, stay put, or rise?

Three possible answers, I chose the two wrong ones before I got the right one. After each failed attempt, he made me repeat Archimedes’ Principle.

All the Principle means is that if, for instance, you are under water, you will float up if your body weight is less than the weight of the water that would take up the space that your body it taking up. If you weigh more, then you’ll sink.

What I failed to take into account is that air is also a fluid. As the pressure increases, so the density of the air increases, the weight of the air displaced by the cork increases and therefore the upthrust increases.

The cork rises up!

Despite my miserable attempts, I was admitted, probably because I finally realised why he kept getting me to repeat Archimedes’ Principle.



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