# 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?

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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