First slide
Collisions
Question

When target is very light and at rest then after head on elastic collision it moves with double speed of projectile i.e. the velocity of body of mass m will be 2v.

Moderate
Solution

In head on elastic collision velocity get interchanged (if masses of particle are equal). i.e. the last ball will move with the velocity of first ball i.e 0.4 m/s

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