# Homework Help: Archimedes (Buoyant force) Not EZ!

1. Jun 21, 2007

### Jguth

I tryied but failed miserabely, i need to prove this formula!

Archimedes (Buoyant force)

Lets say that a rock is attatched to a string, the string has a tention of 5.0N out of the water, and has a tention of 3.2N submerged in water. Now to find the density of the rock, the formula used is:

p rock=(5.0/5.0-3.2)p water

To explain...

density of rock=(wt in air/ wt in air-wt in water)*density of water

Now i need to prove this formula, i would apreciate any help, thanks

Note:
(p=density)

What i know:
p=mv
Density of water is 1000kg/m^2

I tried many different ways but never got a definite answer

Last edited: Jun 21, 2007
2. Jun 21, 2007

### siddharth

You can start by drawing the freebody diagram for the rock, and calculate the net force on the rock in and out of the water.

3. Jun 21, 2007

### Jguth

I know all that, i know the density of the rock and everything, i just need to prove the formula (show why it works)

4. Jun 21, 2007

### Jguth

i need to prove

density of rock=(wt in air/ wt in air-wt in water)*density of water

5. Jun 21, 2007

### arunbg

Start from lost weight in water = buoyant force of water = ......