# Simple buoyancy problem

1. May 13, 2012

### mps

1. The problem statement, all variables and given/known data
An object suspended in water is lifted out of the water. How does the absolute pressure at the bottom of the water change?

2. Relevant equations

3. 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?

2. May 14, 2012

### flatmaster

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. May 14, 2012

### mps

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. May 14, 2012

### flatmaster

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. May 14, 2012

### mps

6. May 14, 2012

### flatmaster

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. May 14, 2012

### mps

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. May 14, 2012

### flatmaster

In that case, the pressure at the bottom will remain the same.

9. May 14, 2012

### mps

Thank you flatmaster!