A0.100-ma electron beam with kinetic energy 54.0 ev enters a sharply defined region of lower potential where the kinetic energy of the electrons is increased by 10.0 ev. what current is reflected at the boundary? (this simulates electron scattering at normal incidence from a metal surface, as in the davisson–germer experiment.)

For the above situation we will draw a force diagram first

Now as per force diagram we can see that there is a component of force parallel to the surface due to which sled is pulled forward.

Now this force must be counter balanced by friction force as the sled is moving with constant speed.

So if sled is moving with constant speed it will have net force ZERO

by Newton's II law

so we can say

So friction force on it is 48 lb.