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?

As given, the mass of the man, $m_1=80\ kg$

Mass of bullet $m_2=20\ g=0.02\ kg$


Speed of bullet $v_2=400\ m/s$

Let $v_1$ be the velocity of the man.

According to the law of conservation of momentum

$m_1v_1=m_2v_2$

Or $80\times v_1=0.02\times 400$

Or $v_1=\frac{8}{80}$

Or $v_1=0.1\ m/s$

Therefore, the velocity of the man is $0.1\ m/s$.

Updated on: 10-Oct-2022

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