A particle with the total energy E is moving in a potential energy region U(x). Motion of the particle is restricted to the region when

total energy TE =KE+PEconsider a body dropped from a height h, at the dropped point , TE=PEat the point where the body reaches the surface, TE=KE, here PE =0hence PE ≤TE ⇒ U(x)≤E