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
L.Shane John Paul N. · Stefan V.
Nov 18, 2015

#"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"#

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