question_answer 51)

                  Two blocks \[{{M}_{1}}\] and \[{{M}_{2}}\] having equal mass are free to move on a horizontal frictionless surface.  \[{{M}_{2}}\]is attached to a massless spring as shown in Fig. Initially \[{{M}_{2}}\] is at rest and \[{{M}_{1}}\] is moving toward \[{{M}_{2}}\] with speed v and collides head-on with \[{{M}_{2}}\]. (a) While spring is fully compressed all the KE of \[{{M}_{1}}\] is stored as PE of spring. (b) While spring is fully compressed the system momentum is not conserved, through final momentum is equal to initial momentum. (c) If spring is massless, the final state of the \[{{M}_{1}}\] is state of rest. (d) If the surface on which blocks are moving, has friction, then collision cannot be elastic.                

Answer:

                  (c, d)                 (c) In a head-on elastic collision between two bodies of equal mass, the bodies of equal mass, the bodies exchange their velocities during collision.                 (d) Since there is a loss of KE (when the surface on which the blocks move has friction), the collision cannot be elastic.