Question 1161285: Calculate the recoil velocity of a 5-Kg rifle that shoots a 0.020-Kg bullet at a speed of 620 m/sec.
A) +2.5 m/s B) -2.5 m/s C) 2.5 N D) - 2.5 N
Answer by jim_thompson5910(35256)   (Show Source):
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Answer: Choice B) -2.5 m/s
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Explanation:
Since they are asking about velocity, the only possible answer choices are either A or B as those units are in velocity units. In contrast, choices C and D are forces (not velocities).
If the bullet has a positive velocity, then the recoil velocity is in the opposite direction, which would give the rifle a negative recoil velocity. Therefore, the answer must be choice B. We don't have to do any math to calculate this since this is the only answer choice with a negative velocity.
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If you want to see how to do this using math, then we'll turn to the idea of conservation of momentum.
Recall that
momentum = mass*velocity
Let's calculate that for the bullet
bullet's momentum = (bullet's mass)*(bullet's velocity)
bullet's momentum = 0.020*620
bullet's momentum = 12.4 kg m/s
This momentum must be equal to the momentum of the rifle recoiling backward, so that the momentum is conserved. We make it negative due to the rifle going in the opposite direction
rifle momentum = -12.4 kg m/s
rifle momentum = (rifle mass)*(recoil velocity)
rifle momentum = 5*(recoil velocity)
5*(recoil velocity) = -12.4
recoil velocity = -12.4/5
recoil velocity = -2.48 m/s
recoil velocity = -2.5 m/s
We round to two sig figs because this is the most number of sig figs we are given (when we were given the values 5, 0.020 and 620)
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