# Math Help - "Star Problems" Volume as a function of height

1. ## "Star Problems" Volume as a function of height

Hello Everyone,

I need help on problems like this one:

A water tank is the shape of an inverted cone which has a height of 120 meters and a diameter at the top of 60 meters. Water starts flowing out of the tank at a rate of 5 cubic meters per second.
Find the volume of the water, V, as a function of water height, H.
Find H as a function of time, T, since the water started flowing from a full tank.

Thanks!

2. Originally Posted by wildcurls
Hello Everyone,

I need help on problems like this one:

A water tank is the shape of an inverted cone which has a height of 120 meters and a diameter at the top of 60 meters. Water starts flowing out of the tank at a rate of 5 cubic meters per second.
Find the volume of the water, V, as a function of water height, H.
Find H as a function of time, T, since the water started flowing from a full tank.

Thanks!

http://www.mathhelpforum.com/math-he...ted-rates.html

Unfortunatly that didn't really help me much. I don't know what derivative of w/r means or the If some one could explain these it might help me more. My pre-cal teacher is really frustrating, he didn't explain these problems just handed them out. Can someone explain this to me in pre-cal language?

4. Originally Posted by wildcurls
Unfortunatly that didn't really help me much. I don't know what derivative of w/r means or the If some one could explain these it might help me more. My pre-cal teacher is really frustrating, he didn't explain these problems just handed them out. Can someone explain this to me in pre-cal language?
Volume of the cone V = $\frac{1}{3}\pi{r^2h}$

If H is the height of the cone and R is the radius of the base, then $\frac{H}{R} = \frac{h}{r}$

r = $\frac{hR}{H}$

Substitute this value in V to get V in terms of h.

5. A water tank is the shape of an inverted cone which has a height of 120 meters and a diameter at the top of 60 meters. Water starts flowing out of the tank at a rate of 5 cubic meters per second.
Find the volume of the water, V, as a function of water height, H.
Find H as a function of time, T, since the water started flowing from a full tank.
the water in the tank forms a conical shape similar to the entire cone.

let $r$ = radius of the water surface

$h$ = height of the water surface

$
\frac{r}{h} = \frac{30}{120} = \frac{1}{4}
$

so, $4r = h$ or $r = \frac{h}{4}$

let $V$ = volume of water in the tank

$
V = \frac{\pi}{3} r^2 h
$

substitute $\frac{h}{4}$ for $r$ in the volume equation

$
V = \frac{\pi}{3} \left(\frac{h}{4}\right)^2 h
$

$
V = \frac{\pi}{48} h^3
$

since $V = 5t$ ...

$
5t = \frac{\pi}{48} h^3
$

to get $h$ as a function of time, solve the last equation for $h$.

6. Thank you!!!