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Hands off! (Posted on 2017-05-16) Difficulty: 3 of 5
A scale balances a cup of water with a certain weight.

Will the balance be upset if you put your finger in the water, if you’re careful not to touch the glass?

No Solution Yet Submitted by Ady TZIDON    
Rating: 3.0000 (1 votes)

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One way or another Comment 3 of 3 |
For every action there is an equal an opposite reaction.  The water presses on the finger, including an upward force, so it follows that the finger exerts some downward force on the water.  There is no question in my mind that the cup would go down, IF THIS EXPERIMENT TOOK PLACE IN A VACUUM.

I think there is a complication, though, if we are not in a vacuum.  The air exerts pressure on all sides of the cup, including a downward pressure on the surface of the water.  The surface area is reduced when a finger is inserted, and so is the downward component of the air pressure on the cup.  I guess that in normal air pressure, the increased pressure from the finger outweighs the decreased air pressure, and the cup will move down.  I suspect that you could have the opposite reaction in a high-pressure bariatric chamber.

But, in any case, no matter what the air pressure, THE BALANCE IS UPSET, one way or another. 

Edited on May 16, 2017, 2:31 pm
  Posted by Steve Herman on 2017-05-16 14:29:50

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