A spherical snow ball is forming so that its volume is increasing at the rate of $8 \mathrm{~cm}^3 /…

A spherical snow ball is forming so that its volume is increasing at the rate of $8 \mathrm{~cm}^3 / \mathrm{sec}$. Find the rate of increase of radius when radius is $2 \mathrm{~cm}$
  1. $\pi \mathrm{cm} / \mathrm{sec}$
  2. $\frac{1}{8 \pi} \mathrm{cm} / \mathrm{sec}$
  3. $2 \pi \mathrm{cm} / \mathrm{sec}$
  4. $\frac{1}{2 \pi} \mathrm{cm} / \mathrm{sec}$

Solution

$\mathrm{V}=\frac{4}{3} \pi \mathrm{r}^3$ $\therefore \frac{\mathrm{dV}}{\mathrm{dt}}=\frac{4}{3} \pi\left(3 \mathrm{r}^2\right) \frac{\mathrm{dr}}{\mathrm{dt}}=4 \pi \mathrm{r}^2 \frac{\mathrm{dr}}{\mathrm{dt}}$ $\therefore 8=4 \pi(2)^2 \frac{\mathrm{dr}}{\mathrm{dt}} \Rightarrow \frac{\mathrm{dr}}{\mathrm{dt}}=\frac{1}{2 \pi}$

Asked in: MHT CET 2021 (22 Sep Shift 1)

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