A cart of mass m is initially at rest on a smooth horizontal track. A constant horizontal force F is then applied to the cart, causing it to move a distance s along the track.
a. (i) Define mechanical energy of a system.
b. (ii) Explain, with reasons, how the mechanical energy of the cart changes during its motion over the distance s.
c. (iii) Under what specific condition would the total mechanical energy of the cart remain unchanged even though it is set into motion? State the reason briefly.