Q. 603.8( 9 Votes )

A man wearing a bullet-proof vest stands still on roller skates. The total mass is 80 kg. A bullet of mass 20 grams is fired at 400 m/s. It is stopped by the vest and falls to the ground. What is then the velocity of the man?

Answer :

m - Mass of bullet

= 0.02 kg


M - Mass of man


= 80 kg


u - Initial velocity of bullet


= 400 m/s


U - Initial velocity of man


= 0 m/s


v - Final velocity of bullet


= 0 m/s


V - Final velocity of man


Using conservation of momentum


Initial momentum = Final momentum


M * u + M * U = m * v + M * V


0.02 * 400 + 80 * 0 = 0.02 * 0 + 80 * V


8 = 80 * V


V = 0.1 m/s


Therefore, the man moves with a velocity of 0.1 m/s


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