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A person holding a rifle (mass of person and rifle together is 100 kg) stands on a smooth surface and fires 10 shots horizontally, in 5 s. Each bullet has a mass of 10 g with a muzzle velocity of 800 m/s. The final velocity acquired by the person and the average force exerted on the person are:

1. -1.6 m/s; 8 N
2. -0.08 m/s; 8 N
3. -1.6 m/s; 16 N
4. -0.8 m/s; 16 N

Correct answer: -0.8 m/s; 16 N

Solution

Using conservation of momentum, the total momentum of the bullets equals the momentum of the person and rifle. The velocity of the person is calculated as (-10 × 0.01 × 800) / 100 = -0.8 m/s. The average force is found using F = Δp/Δt = (100 × 0.8) / 5 = 16 N.

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