Two blocks `M_1 and M_2` having equal masses are to move on a horizontal frictionless surface. `M_2` is attached to a massless spring as shown in figure. Initially `M_2` is at rest and `M_1` is moving toward `M_2` with speed v and collides head-on with `M_2`.

Options

A. While spring is fully compressed, all the kinetic energy of `M_1` is stored as potential energy of spring
B. While spring is fully compressed, the system's momentum is not conserved, though final momentum is equal to initial momentum.
C. If spring is massless, the final state of the `M_2` is state of rest.
D. If the surface on which blocks are moving has friction, then collision cannot be elastic

Answer

D