Question:medium

The surface area of a spherical balloon is increasing at the rate of \( 2 \, \text{cm}^2/\text{sec} \). Then the rate of increase in the volume of the balloon, when the radius of the balloon is \( 6 \, \text{cm} \), is:

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To solve such problems, relate \( \frac{dV}{dt} \), \( \frac{dS}{dt} \), and \( \frac{dr}{dt} \) using their respective equations and carefully substitute the given values.
Updated On: Mar 28, 2026
  • \( 4 \, \text{cm}^3/\text{sec} \)
  • \( 16 \, \text{cm}^3/\text{sec} \)
  • \( 36 \, \text{cm}^3/\text{sec} \)
  • \( 6 \, \text{cm}^3/\text{sec} \)
Show Solution

The Correct Option is D

Solution and Explanation

The surface area \( S \) of a sphere is \( S = 4\pi r^2 \), where \( r \) is the radius. The volume \( V \) of the sphere is \( V = \frac{4}{3} \pi r^3 \). We are given \( \frac{dS}{dt} = 2 \, \text{cm}^2/\text{sec} \) and \( r = 6 \, \text{cm} \). We need to find \( \frac{dV}{dt} \). Step 1: Find \( \frac{dr}{dt} \). Differentiate \( S = 4\pi r^2 \) with respect to \( t \) to get \( \frac{dS}{dt} = 8\pi r \frac{dr}{dt} \). Rearrange to \( \frac{dr}{dt} = \frac{\frac{dS}{dt}}{8\pi r} \). Substitute the given values: \( \frac{dr}{dt} = \frac{2}{8\pi \cdot 6} = \frac{1}{24\pi} \, \text{cm/sec} \). Step 2: Find \( \frac{dV}{dt} \). Differentiate \( V = \frac{4}{3}\pi r^3 \) with respect to \( t \) to get \( \frac{dV}{dt} = 4\pi r^2 \frac{dr}{dt} \). Substitute \( r = 6 \) and \( \frac{dr}{dt} = \frac{1}{24\pi} \): \( \frac{dV}{dt} = 4\pi (6)^2 \cdot \frac{1}{24\pi} \). Simplify: \( \frac{dV}{dt} = 4\pi \cdot 36 \cdot \frac{1}{24\pi} = \frac{144}{24} = 6 \, \text{cm}^3/\text{sec} \). Final Answer: \[\boxed{6 \, \text{cm}^3/\text{sec}}\]
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