Question

A block of mass m having charge q is sliding down an inclined plane of inclination and coefficient of friction.When uniform electric field E is applied parallel to the base of inclined plane,the block slides down with constant velocity.The value of E is?

Answer 1

one would have to have an E force which opposed the weight to just enough to make friction eactly the same as the weight down the plane.

weight down the plane mgsinTheta
force of friction= mg*mu*cosTheta
Now E must reduce weight enough to reduce friction, so
force friction=mgsinTheta
(mg-Eq)*mu=mgsinTheta
E= mg(1-Mu*sinTheta)/q*mu
check that.

I assume you know mu, sinTheta,