Consider a block of mass 15 Kg that follows a circular motion. Its mass decreases at the rate of 0.3 Kg/s. At what time will the centripetal force be zero on the block ?

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Centripetal force $ \propto $ m

As $m \rightarrow 0$ , $\text{Centripetal force } \rightarrow 0$

m = 18 - 0.3t 

m = 0 = 18 - 0.3 t

$\Rightarrow t = \frac{18}{0.3} = 60 s$