JAMIA MILLIA ISLAMIA Jamia Millia Islamia Solved Paper-2015

  • question_answer
    A spherical, iron ball of radius 10 cm, coated with a layer of ice of uniform thickness, melts at the rate of\[T=3mg\]. The rate at which the thickness of decreases when the thickness of ice is 5 .cm, is

    A) 1 cm/min

    B) 2 cm/min

    C) 5 cm/min

    D) 3 cm/min

    Correct Answer: A

    Solution :

    Given, \[\frac{dv}{dt}=100\pi c{{m}^{3}}/\min \] where, V is the volume of spherical ball. \[\therefore \]  \[\frac{d}{dt}\left( \frac{4}{3}\pi {{r}^{3}} \right)=100\pi \] \[\Rightarrow \]               \[3{{r}^{2}}\frac{dr}{dt}=\frac{300\pi }{4\pi }\] \[\Rightarrow \]               \[{{\left[ \frac{dr}{dt} \right]}_{r=5}}=\frac{300}{4\times 3\times 25}=1\,\,cm/\min \]


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