A man wearing a bullet-proof vest stands still on roller skates. The total mass is 80kg. A bullet of mass 20 grams is fired at 400m/s. It is stopped by the vest and man falls to the ground. What is then, the velocity of man ?
A
1m/s
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B
0.1m/s
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C
2m/s
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D
2.5m/s
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Solution
The correct option is B0.1m/s Mass of the bullet , m1=20g=0.02kg
Total mass of man m2=80kg
Initial velocity of the bullet , ui=400m/s
Final velocity of the bullet, uf=0m/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=0.1m/s