# Archimedes Principle Quick Question

1. Apr 18, 2010

### jumbogala

1. The problem statement, all variables and given/known data
In air, Bob's weight is 687 N. In water, it is 34.7 N.

Determine Bob's average density.

2. Relevant equations

3. The attempt at a solution
Is it correct to say that (buoyancy force) + 34.7 N = 687 N?

If so then buoyancy force = (density of water)(volume)*9.81 m/s

I can solve for the volume.

Then I can use (density of Bob)(volume)(9.81 m/s) = 687 N do solve for Bob's average density.

Is that right? I just want to make sure it's okay before doing the rest of the problem.

2. Apr 18, 2010

### kuruman

You are on the right track.

3. Apr 18, 2010

### jumbogala

I was just kind of confused because Bob isn't necessarily all under water. If he was half submerged, wouldn't that change things?

I guess because the question doesn't specify, I should just assume Bob's entirely under water...

4. Apr 18, 2010

### kuruman

Yes, you should assume that. You cannot solve the problem otherwise, unless you know what percent of his volume is under water.

5. Apr 18, 2010

### jumbogala

Okay, thank you!