question_answer 75)

                  A curved surface is shown in Fig. The portion BCD is free of friction. There are three spherical balls of identical radii and masses. Balls are released from rest one by one from A which is at a slightly greater height than C.                 With the surface AB, ball 1 has large enough friction to cause rolling, down without slipping; ball 2 has a small friction and ball 3 has a negligible friction.                 (a) For which balls is total mechanical energy conserved?                 (b) Which ball (s) can reach D?                 (c) For balls which do not reach D, which of the balls call reach back A?                

Answer:

                  (a) In case of ball 2, a part of mechanical energy is converted into internal energy of ball and the surface oil which it slides. Therefore, mechanical energy (i.e. K.E. + P.E.) is conserved for balls 1 and 3.                 (b) Only ball 3 can reach D.                 (c) Balls 1 and 2 cannot reach point A.