WBJEE 2017 Physics Solved Paper

A particle with charge Q coulomb, tied at the end of an inextensible string of length R meter, revolves in a verticalplane. At the centre of the circular trajectory there is a fixed charge of magnitude Q coulomb. The mass of the movingcharge M is such that Mg=

Q2

4πε0R2

. If at the highest position of the particle, the tension of the string just vanishes,the horizontal velocity at the lowest point has to be