A thin homogeneous rod of mass m and length `l` is free to rotate in vertical plane about a horizontal axle pivoted at one end of the rod. A small ballof mass m and charge q is attached to the opposite end of this rod. The whole system is positioned in a horizontal electric field of magnitude `E=(mg)/(2q)`. The rod is released from, shown position from rest.

What is the angular acceleration of the rod at the instant of releasing the rod?

Options

A. `(8g)/(9l)`
B. `(3g)/(2l)`
C. `(9g)/(8l)`
D. `(2g)/(9l)`

Answer

C

Explanation

By taking torque about hinge `Ialpha=mg(l/2)+mg(l)` when `I=(ml^(2))/3+ml^(2)rArr alpha=(9g)/(8l)`