A bob is whirled in a horizontal circle by means of a string at an initial speed of 10 rpm. If the tension in the string is quadrupled while keeping the radius constant, the new speed is:

A 20 rpm

B 40 rpm

C 5 rpm

D 10 rpm

The answer is A.

Solution

For horizontal circular motion, $T=m\omega^2 r$. For constant $m$ and $r$, $T\propto\omega^2$. Given $T'=4T$, so $\omega'=2\omega=2\times10=20$ rpm.

Concept Circular motion - centripetal force

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