Two blocks  and 
 having equal mass are free to move on a horizontal frictionless surface. 
 is attached to a massless spring as shown in Figure. Initially 
 is at rest and 
 is moving toward M2 with speed v and collides head-on with 
.
(a) While the spring is fully compressed all the KE of  is stored as PE of spring.
(b) While spring is fully compressed the system momentum is not conserved, though final momentum is equal to initial momentum.
(c) If spring is massless, the final state of the  is state of rest.
(d) If the surface on which blocks are moving has friction, then collision cannot be elastic.
The answer is the option (c)
Let  come into contact with the spring of 
, such that the velocity of 
 increases and that of m1 decreases. As the velocities adjust to being at par, both the star of the block moves with the same velocity.
a) the kinetic energy of  is not fully transferred to the spring as its potential energy, and hence, option a is incorrect
b) The law of conservation of mass is valid here since the surface is frictionless; hence option b is incorrect.
c) if we consider the case, where the spring is totally massless, then all the kinetic energy of  gets transferred to 
. As a result, m1 comes to rest and 
 acquires a velocity v and starts moving. Hence, option c is correct.
d) even when the force of friction is not involved, a collision can be inelastic. Hence, we reject option d.
The answer is the option (c)
Let  come into contact with the spring of 
, such that the velocity of 
 increases and that of m1 decreases. As, the velocities adjust to being at par, both the star of the block moving with the same velocity.
a) the kinetic energy of  is not fully transferred to the spring as its potential energy, and hence, the option a is incorrect
b) law of conservation of mass is valid here since the surface is frictionless; hence option b is incorrect.
c) if we consider the case, where the spring is totally massless, then all the kinetic energy of  gets transferred to 
. As a result, m1 comes to rest, and 
 acquires a velocity v and starts moving. Hence, option c is correct.
d) even when the force of friction is not involved, a collision can be inelastic. Hence, we reject option d.