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Question

Physics Question on Circular motion

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

Answer

20 rpm

Explanation

Solution

Tension (T) provides the centripetal force: T = mv 2/r. With r and m constant, T ∝ v2. If

T 2 = 4 T 1, then v 22 = 4 v 12.

Given v 1 = 10 rpm, then v 2 = √4 × 102 = 20 rpm.