Question

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 ?

Answer 1

mass of man = $m_{1} = 80kg$

velocity of man = $v_1$

mass of bullet = $m_2=20g = \frac{20}{1000}= 0.02kg$

velocity of bullet = $v_{2}$

$m_1 * v_1 = m_2 * v_2$

$80 * v_2 = 0.02 * 400$

$v_2 = \frac{0.02 * 400}{80} = 0.02*5 = 0.1m/s$

the velocity of the man is 0.1 m/s.

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