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  4. Two blocks M1 and M2 having equal mass are to move on a horizontal frictionless surface. M2 is attached to a massless spring as shown in figure. Initially M2 is at rest and M1 is moving toward M2 with speed v and collides head-on with M2.

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Q. Two blocks $M_{1}$ and $M_{2}$ having equal mass 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 $M_{2}$.Physics Question Image

Work, Energy and Power

Solution:

While spring is fully compressed, the entire $KE$ of $M_{1}$ is not stored as PE of spring as $M_{2}$ may move. If spring is massless, than as $M_{1}=M_{2}$, velocities of $M_{1}$ and $M_{2}$ are interchanged on collision. $M_{1}$ comes to rest, instead of $M_{2}$. Choice (c) is correct. If surface on which blocks are moving has friction loss of energy is involved . Collision cannot be elastic. Choice (d) is correct.