Suppose a disc of radius aplaced on the groung with uniformsurface charge density s with its axis vertical.A charged particle is dropped from a height H from the ground along the axis of the disc.Initially the particl is at rest so initial kinetic energy is equal to zero.Then see the point when the electric force on the particle is equal to the gravitational force.At that point velocity of the particle is zero.Hence the change in kinetic energy is equal to zero.So the work done by the net external force should be zero.But the resultant force is causing a net displacement in the downward direction.So there has to be some work done by the net external force.Please explain this.

Vinay Arya , 13 Years ago

Grade 12