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Question

# A man (mass=50 kg) and his son (mass=20 kg) are standing on a frictionless surface facing each other. The man pushes his son so that he starts moving at a speed of 0.70 msâˆ’1 with respect to the man. The speed of the man with respect to the surface is:

A
0.20 ms1
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B
0.47 ms1
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C
0.14 ms1
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D
0.28 ms1
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Solution

## The correct option is A 0.20 ms−1Let vsm be the velocity of son with respect to man, vm and vs be the velocity of man and son respectively with respect to surface. As the son and the man will be moving in opposite direction, vsm=vs+vm vs=0.7−vm According to conservation of momentum, Pi=Pf ⇒Pi=msvs−mmvm ⇒0=20(0.7−vm)−50vm ⇒70vm=14 ⇒vm=0.2 ms−1 Hence, option (B) is correct.

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