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Up or down or... (Posted on 2005-07-22) Difficulty: 3 of 5
A jar filled partially with water has an object floating in it. The jar is open. Now, if the jar is closed and a vacuum pump is used to pump the air out, what will happen to the floating object? will it rise up more or sink or ...? and why?

No Solution Yet Submitted by nilshady    
Rating: 2.3333 (3 votes)

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depends on density | Comment 8 of 20 |
Ignore my title.

Upward forces: 
buoyant force of water (due to volume that is submerged)
buoyant force of air (due to volume that is NOT submerged)

Downward forces:
weight of object
weight of a column of air above the object (15 psi)

After the vacuum, both of the air related forces will be gone.

Suppose for a moment that the object is a 1 inch cube.  The downward force of the air pressure is 15 pounds.  The air pressure on the sides of the cube cancel each other out.  The buoyant force of air is only the weight of a fraction of a cubic inch of air.  Now if the object is very tall and thin, so large that the weight of the displaced air is more than 15 pounds (again assuming the cross section is 1 square inch), then the answer is the opposite.

Of course, as Hugo pointed out, the water will start to boil, but before the boiling gets going, the object will move UP due to the loss of downward air pressure.

After the boiling is going fast, there will be bubbles of gasseous water vapor suspended in the liquid, so that the overall density of the liquid will be dramatically decreased.  When this happens, the object will sink like a stone like a ship in the ocean caught in a wash of methane bubbles released from undersea methane clathrate.

My answer:  briefly goes UP(unless the object is hundreds of times taller than its x and y dimensions), then sinks like a stone during boiling.

Edited on July 22, 2005, 11:11 pm
  Posted by Larry on 2005-07-22 23:05:37

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