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Question:

A point mass of 1 kg collides elastically with a stationary point mass of 5 kg. After their collision, the 1 kg mass reverses its direction and moves with a speed of 2 m/s. Which of the following statement(s) is (are) correct for the system of these two masses?

Momentum of 5 kg mass after collision is 4 kg m/s

Total kinetic energy of the system is 4J

Kinetic energy of the centre of mass is 0.75J

Total momentum of the system is 3 kg m/s

Solution:

By conservation of linear momentumu=5vð•’¶.. (i)By Newton's experimental law of collisionu=v+2 (ii)using (i) and (ii) we havev=1m/sandu=3m/sKinetic energy of the centre of mass=12msystemV2cm=0.75J