Volume of a Ball

• November 28th 2010, 07:05 PM
Dotdash13
Volume of a Ball
I need help with this problem:
The radius of a spherical balloon is increasing by 2 cm/sec. At what rate is air being blown into the balloon when the radius is 10 cm?
So far what I have is that the volume of the ball is $\frac4 3\pi r^3$ and that the radius $r=2t$ where $t$ is the time in seconds after the balloon began to inflate. Then the substitution could be made that the volume is $\frac4 3\pi (2t)^3 = 8\pi t^3$. Then to find the rate of change, take the derivative, giving $24\pi t^2$. Then the radius will be 10 cm when t=5 so I plug in 5 for t, giving $24*25*\pi = 600\pi$ $cm^3/min$. This answer feels wrong to me, as the total volume of the balloon is only about 4188.79 cubic cm. Could somebody point out where I went wrong if I have done so, thanks.
• November 29th 2010, 01:53 AM
dwsmith
Pauls Online Notes : Calculus I - Related Rates

If that didn't help:

$\displaystyle V=\frac{4}{3}\pi r^3 \ \mbox{and} \ \frac{dr}{dt}=2cm/sec$

Derivative of V $\displaystyle \frac{dV}{dt}=4*\pi r^2\frac{dr}{dt}=4*\pi*10^2*2=.....$