A man wearing a bulletproof 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 the velocity of the man?

1 Answer

"0.1 m/s"

Explanation:

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 implies V = "0.1 m/s"

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