# A ball of mass m with a charge +q can rotate in a vertical plane at the end of a string of length l in a uniform electric field whose lines of force are directed vertically upwards. The horizontal velocity that must be imparted to the ball at top position so that tension in string at the bottom position of the ball is 15 times the weight of the ball is

Arun
25750 Points
5 years ago
Imagine you have one end of the string in your hand, at the other end is the charged sphere. Now you can slowly start swinging the sphere like a pendulum. If you go faster and faster, at some point the sphere will no longer swing back, but complete the circle. It will rotate in a vertical plane. Now you just have to equate gravitational, electric and centrifugal force to find the answer.
Khimraj
3007 Points
5 years ago
Perhaps the ball is on the end of the string with the string's other end fixed in space. Then the ball would be confined to a disk around that point. Also, by "rotation," could it mean that the ball is moving in a circle around that point rather than rotating on its own axis? If that is the case then it would make sense that the tension on the string depends on the ball's velocity because of centrifugal force.