Question

A simple pendulum rotates in a horizontal plane with an angular velocity $\omega$ about a fixed point P in a gravity free space. There is a negative charge at P. The bob gradually emits photo electrons(Leave the energy and momentum of incident photons and emitted electrons). The total force acting on the bob is $T$ .

• A. $T$ will decrease, $\omega$ will decrease
• B. $T$ will decrease, $\omega$ will remain constant
• C. $T$ and $\omega$ will remain unchanged
• D. The elastic strain in the string will decrease

Answer 1

Answer: (C), (D)

The correct options are
C $T$ and $\omega$ will remain unchanged
D The elastic strain in the string will decrease

Answer is C and D

As no forces act on the system, ω remains constant. Hence, $T=m{\omega }^{2}l$ remains constant. However, T is the sum of electrostatic force of attraction on the bob and the force exerted by the string due to elastic strain in it. As the positive charge on the bob increases due to photo emission, the electrostatic force on it increases, and the elastic strain on the string must decrease.