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Question: On dropping a stone in stationary water circular ripples are observed. Rate of flow of ripples is 6 ...

On dropping a stone in stationary water circular ripples are observed. Rate of flow of ripples is 6 cm/sec. when radius of the circle is 10cm, then fluid rate of increase in its area is :

A

120 cm2/sec

B

p cm2/sec

C

120p cm2/sec

D

None

Answer

120p cm2/sec

Explanation

Solution

drdt\frac{dr}{dt} = 6cm/sec

A = pr2

dAdt\frac{dA}{dt}= 2pr .drdt\frac{dr}{dt} = 2p × 10 × 6

= 120p cm2/sec