Question:

A spherical balloon is filled with $4500\,\pi$ cubic meters of helium gas. If a leak in the balloon causes the gas to escape at the rate of $72\,\pi$ cubic meters per minute, then the rate (in meters per minute) at which the radius of the balloon decreases $49$ minutes after the leakage began is

Updated On: Jul 28, 2023
  • $\frac{9}{7}$
  • $\frac{7}{9}$
  • $\frac{2}{9}$
  • $\frac{9}{2}$
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The Correct Option is C

Solution and Explanation

$V = \frac{4}{3}\pi r^{3}\quad\quad4500 \pi = \frac{4\pi r^{3}}{3}$ $\frac{dV}{dt} = 4\pi r^{2} \left(\frac{dr}{dt}\right)\quad\quad45 ??25 ??3 = r^{3}$ $r = 15\, m$ after $49$ min $= \left(4500 - 49.72\right)\pi = 972\, \pi\,m^{3}$ $972\,\pi = \frac{4}{3} \pi r^{3}$ $r^{3} = 3??43 = 3??3^{5}$ $r = 9$ $72\, \pi = 4\pi ??9 ??9 \left(\frac{dr}{dt}\right)$ $\frac{dr}{dt} = \left(\frac{2}{9}\right)$
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Concepts Used:

Application of Derivatives

Various Applications of Derivatives-

Rate of Change of Quantities:

If some other quantity ‘y’ causes some change in a quantity of surely ‘x’, in view of the fact that an equation of the form y = f(x) gets consistently pleased, i.e, ‘y’ is a function of ‘x’ then the rate of change of ‘y’ related to ‘x’ is to be given by 

\(\frac{\triangle y}{\triangle x}=\frac{y_2-y_1}{x_2-x_1}\)

This is also known to be as the Average Rate of Change.

Increasing and Decreasing Function:

Consider y = f(x) be a differentiable function (whose derivative exists at all points in the domain) in an interval x = (a,b).

  • If for any two points x1 and x2 in the interval x such a manner that x1 < x2, there holds an inequality f(x1) ≤ f(x2); then the function f(x) is known as increasing in this interval.
  • Likewise, if for any two points x1 and x2 in the interval x such a manner that x1 < x2, there holds an inequality f(x1) ≥ f(x2); then the function f(x) is known as decreasing in this interval.
  • The functions are commonly known as strictly increasing or decreasing functions, given the inequalities are strict: f(x1) < f(x2) for strictly increasing and f(x1) > f(x2) for strictly decreasing.

Read More: Application of Derivatives