# Homework Help: Force and Buoyancy

1. Apr 15, 2005

### srfrgrl727

Can anyone help me out. I don't know of any equation that i can us eto solve this A force 1000N is required to raise a concrete block to the surface of a fresh wate rlake. what is the force required to lift it out of the lake. I know that i need to figure out what the weight outside of wate ris, but i don't know how. no i don't know the volume the 1000N is the only info given. density is 2320 kg/m3.

Last edited: Apr 15, 2005
2. Apr 15, 2005

### FredGarvin

If you draw a free body diagram of the block under water you should have three forces acting on it: The block's weight, the buoyancy force and the 1000N applied force. This should tell you that the force required to lift the block in air will equal the buoyant force + 1000N.

Do you know the volume of the block?

3. Apr 15, 2005

### Staff: Mentor

But are you given the density of concrete?

4. Apr 16, 2005

### shyboy

The Force you need to raise the block with the volume $$V$$ to the surface is
$$F_1=gV(\rho_{concrete}-\rho_{water})$$
the force to raise the block futher is

$$F_2=gV\rho_{concrete}$$

Then $$\frac{F_2}{F_1}=$$

Last edited: Apr 16, 2005