59) 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 ?
When both man and bullet are at rest, then total initial momentum =0….(i)
Later, Let the velocity of the man = v
Final momentum: 80 v + (20/1000) x 400 ………….(ii)
As per law of conservation of momentum:
80 v + (20/1000) x 400 =0
v = -8/80 =- 0.1 m/s
velocity of man is 0.1 m/s.
direction is opposite to the direction of the bullet.
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Problems Set – grade 9