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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 man falls to the ground. What is then, the velocity of man ?

A
1 m/s
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B
0.1 m/s
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C
2 m/s
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D
2.5 m/s
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Solution

The correct option is B 0.1 m/s
Mass of the bullet , m1=20 g=0.02 kg
Total mass of man m2=80 kg
Initial velocity of the bullet , ui=400 m/s
Final velocity of the bullet, uf=0 m/s
Let the velocity of man with which he falls is vf

We know that in the process of collision external force applied on the system is zero. Therefore, the momentum of system remains conserved.
pi=pf
0.02×400+0=0+80×vf
vf=400×0.0280
vf=0.1 m/s

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