# Homework Help: Fluid Mechanics - find a cavity of a block if it exist?

1. May 24, 2012

### mistax

1. The problem statement, all variables and given/known data

You are given a 10cm Cube of aluminum with Density of 2700kg/m is submerged in water and the weight in water is 13N. If a cavity exist how big is it?

2. Relevant equations

The equation i used were B = V * P_water * g to find buoyancy.

To find the enclosed cavities i used V_0 = V - V_c = B/(p_water * g) - W/(p_m*g)

3. The attempt at a solution

for B i did volume of the cube was .1^3 * 1000KG/M for water * 9.8

and then when plugging in to the equation for enclosed cavities i got
9.8/(9.8 * 1000kg/m) - 13N/(2700KGM * 9.8)

from there i got stuck since i got a number that was really small.

2. May 25, 2012

### truesearch

I prefer to work through stage by stage.
Can you calculate the upthrust (buoyancy) produced by a 0.1m x 0.1m x 0.1 m in water?
I think you have calculated the weight of a cube of aluminium, assuming it has no cavity.
So can you check if there is any difference between the measured weight of the block in water and the predicted weight?