# Rate of Change

1. Apr 21, 2010

### icystrike

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

2. Relevant equations

3. The attempt at a solution

v=1/3 pi r^2 h
dv/dt = 1/3 pi (2rh dr/dt + r^2 dh/dt)
now i sub r=15/3 and h=3 .
Now i express dr/dt=dr/dh x dh/dt
and find dr/dh .
But it fails

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2. Apr 21, 2010

### sylas

The equation for v looks okay, as long as h is <= 4.

The substitution r = 15/3 is wrong.

You should be able to express r in terms of h. What is the relation between r and h? Note that if you use this, you can get just get rid of r altogether, which will be nice.

Cheers -- sylas

3. Apr 21, 2010

### icystrike

could it be r=h tan (a) ? thanks btw =D

4. Apr 21, 2010

### sylas

No.

Look again at the figure. Draw some water in it. Mark h and r for the given volume.