A spherical balloon is to be deflated so that its radius decreases at a constant rate of 12 cm/min. At what rate must air be removed when the radius is 5 cm? Must be accurate to the 5th decimal place.

It seems like an easy question but no matter how many times i attempt it I get it wrong.

1 answer

Most questions of this type appear to disregard pressure, so in the following solution I will do the same (physics expert will surely groan)

V = (4/3)π r^3
dV/dt = 4πr^2 dr/dt

so when r = 5 and dr/dt = -12

dV/dt = 4π(25)(-12)
= appr -3769.91118