First slide

Second law

Question

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 $800 m s^{- 1}$ . The final velocity acquired by the person and the average force exerted on the person are

Moderate

A

$- 0.08 {ms}^{- 1}, 16 N$
B

$- 0.8 {ms}^{- 1}, 8 N$
C

$- 1.6 {ms}^{- 1}, 16 N$
D

$- 1.6 {ms}^{- 1}, 8 N$

Solution

$0 = n m u + (M - n m) v \Rightarrow 0 = 10 \times \frac{10}{1000} \times 800 + (100 - 10 \times \frac{10}{1000}) v \Rightarrow v = - \frac{80}{99.9} = - 0.8 m / s A v e r a g e f o r c e e x e r t e d = \frac{10 \times 0.01 \times 800}{5} = 16 N$

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