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J. Perelman
"Entertaining physics". Book 1.
Chapter 5. Properties of liquids and gases

WHAT IS HEAVIER?

One Cup of scales put a bucket filled to the brim with water. On the other - the same bucket is full to the brim, but it floats a piece of wood. What bucket will pull?


Both buckets are the same and filled with water to the brim; one floating piece of wood. Which will pull?

I tried to set this task to different people and got conflicting answers. Some answered that must drag the bucket, in which floats the tree, because in addition to water in the bucket there is a tree”. Others think that, on the contrary, will pull the first bucket, “because water is heavier than wood.”

But neither that, nor another is not true: both buckets have the same weight. In the second bucket, however, water is smaller than in the first, because a floating piece of wood displaces a certain volume. But, according to the law of navigation, all floating body displaces its immersed part exactly as much liquid (by weight), what is the weight of all this body. This is why scales and must remain in balance.

Decide now another task. I put on the scales a glass of water and next put the weight. When the scales are balanced by weights on the Cup, I drop the weight in a glass of water. What will become with weights?

By Archimedes ' principle, the weight in water is easier than it was out of the water. You can seemingly be expected that a Cup of scales with a glass rises. Meanwhile in reality, the scales will be balanced. How to explain this?

A weight in the glass replaced some of the water, which was higher than the initial level; because of this increased pressure on the bottom of the vessel so that the bottom is experiencing incremental force equal to the weight loss the weight.

Entertaining physics J. Perelman

 




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