A curved surface

(a) ball 1 and 3 the total mechanical energy is conserved.

For ball 1, friction is so large that starts rolling without slipping and gains rotational kinetic energy.

For ball 3, friction is negligible so there is no external force responsible for energy loss ∴ the energy is conserved.

(b) ball 1 acquires some rotational kinetic energy ∴ it will not be

able to reach C and thus will not reach point D

ball 2 loses its energy due to friction, thus it cannot reach point D

ball 3 has exactly the amount of energy to reach C and D because it does not roll and does not lose energy due to friction.

∴ only ball 3 will reach point D

(c) ball 1 and 2 starts moving in the back direction before

reaching point C.

for ball 1, it loses its energy while slipping back to point B, thus it will not reach back to point A

for ball 2, it will have clockwise rotation as it moves backs to point B due to which there will be kinetic friction coming to play due to which it will lose energy and won’t be able to reach point A.