# Suppose a disc of radius a placed on the groung with uniform surface 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.

509 Points
13 years ago

dear vinay ,

particle is dropped from height H ... electric field due to disk is upward so the force acting on

particle is in upward direction....

gravitational pull by earth is always constant = mg(downward)....

work due to elctric field & gravity is of opposite sign ,

Wg + We = KE change (0)

Wg = -We

Vinay Arya
37 Points
13 years ago

Dear sir

That is what I am saying.If work done by the gravitational force is negative of the work done by the electric force then the net work should be zero.But as there is net displacement there has to be net work done.Please explai this one.