# Homework Help: Fluid Physics; Finding the Density of an Unknown Liquid

1. Nov 29, 2008

### Jinthe1st

1. The problem statement, all variables and given/known data
I have a problem from the 2002 AP exam; The questions specifically are from 3b, c, and d. ( See the attached image.)

2. Relevant equations
So I'm trying to solve for the unknown density of the liquid (pW). From what I can assume so far, the object is submerged in the liquid, dispersing a volume of water equal to the volume of the object; however, I suppose there will be a different dimension? I believe that the height determined from the object to the surface of the liquid will be essential to this problem?

I believe I am trying to find the weight of the unknown liquid's volume. Would that be the same as the weight of the object?

I think I do not understand some of the concepts at hand, honestly.
3. The attempt at a solution
F=kx
F=Weight

Density of mass is known; use that to find the volume:
Weight = (Density)(volume)(Gravity acceleration)
W= (p)(v)(g)

Volume of object = Volume of liquid
Fb = pL(v)(g)

#### Attached Files:

• ###### 2002AP.jpg
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2. Nov 29, 2008

### Staff: Mentor

What will happen to the stretch of the spring when you dunk the mass into the fluid? Assuming that you found the spring constant in part a, what can you deduce?

3. Nov 29, 2008

### Jinthe1st

Ah, the force should be reduced somewhat; will that be the the buoyant force?

4. Nov 29, 2008

### Staff: Mentor

The change in spring force will tell you the buoyant force. (Draw yourself a free body diagram for the object.)