Mechanical Energy...?

Question

Can mechanical energy ever be transformed completely into heat or internal energy? Can the reverse happen? In each case, if your answer is no explain why not: if yes give an example.

Answer 1

Mechanical energy can be converted entirely to heat, but the reverse cannot happen. The laws of thermodynamics demand that there is no such thing as a heat engine (one that converts heat energy to mechanical energy) that is 100% efficient.

Example for 1: If a block of mass m is sliding on a frictional surface with velocity v, it's mechanical energy is 0.5*m*v^2. Eventually it will stop (v=0), and its mechanical energy will be 0; all of it is converted to frictional heat.