Question

What do you mean by conservation of mechanical energy? State the condition when does it hold.

Answer 1

This means"The total MECHANICAL ENERGY (P.E+K.E) of an isolated system at any instant is equal to the sum of the kinetic ENERGY and the potential ENERGY."
Condition:Condition under which the mechanical energy is conserved is "WHEN THERE ARE NO FRICTIONAL FORCES." In other words the mechanical energy is conserved strictly in vacuum where friction due to air is absent.