Simple buoyancy problem

  • Thread starter mps
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  • #1
mps
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Homework Statement


An object suspended in water is lifted out of the water. How does the absolute pressure at the bottom of the water change?


Homework Equations




The Attempt at a Solution


Wouldn't the absolute pressure not change if the object is still directly over the water? Because I thought the entire air column weighed down on the water, so the object would as well. No?
 

Answers and Replies

  • #2
501
2
If you take the object out of the water, how does the height of the water change? Also, the pressure at the bottom of a collum of water is the product of it's density, g, and the height of the water in the container.

P = dgh

If you have a scale, do the experiment for yourself. Put a container of water on the scale, suspend an object in the water, then pull it out and see if the weight changes.
 
  • #3
mps
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but doesn't the air column above push on the surface of the water which pushes on the bottom?
also, say submerging an object makes the water height h0. Is the pressure at the bottom with submerged object higher, lower or the same as the pressure at the bottom of a water column at height h0 (no submerged object)
gracias!
 
  • #4
501
2
but doesn't the air column above push on the surface of the water which pushes on the bottom?
also, say submerging an object makes the water height h0. Is the pressure at the bottom with submerged object higher, lower or the same as the pressure at the bottom of a water column at height h0 (no submerged object)
gracias!

Yes, the air above the water will push on the surface, but it is just going to be at atmospheric pressure. It won't change.
 
  • #5
mps
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thanks! could you please answer my other question too?
 
  • #6
501
2
When you submerge the object, it will displace some water and push up the water a bit. The greater depth of water causes a greater pressure at the bottom.
 
  • #7
mps
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What i mean is if the depth of water is the same in both situations but in one situation, the water encloses an object.
 
  • #8
501
2
In that case, the pressure at the bottom will remain the same.
 
  • #9
mps
27
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Thank you flatmaster!
 

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