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# 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?

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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?

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Mass of the man, m1 = 80 kg

Speed of man, v1

Mass of bullet m2 = 20 g = 0.02 kg

Speed of bullet v2 = 400 m/s

Acc. to law of conservation of momentum

m1v1 = m2v2

80 x v1 = 0.02 x 400

v1 = $$\frac{400 \times 0.02}{80}$$ = 0.1 m/s

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