# Math Help - electrostatics question...

1. ## electrostatics question...

A small ball of mass m and charge Q is suspended through a string of length l. A point charge is also placed at the centre of the resultant circle with radius l formed by rotating the ball in the vertical plane... the ball is given a velocity v at the bottome of circle.. what should be the minimum velosicty so that it makes a vertical circle? accelecleration due to gravity = g..

2. The minimum velocity is when the ball reaches the top of the motion with the total forces acting on it make the string just taut.

$F_c = F_g + F_e$

The centripetal force is provided by the sum of the gravitational force and the electrostatic force.

$\dfrac{mv^2}{l} = mg + \dfrac{Qq}{4\pi\epsilon_o l^2}$

q is the charge of your point charge.

Make v the subject of formula, and you're done.

Unless there are more information given, there isn't much that can be done.