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.