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Homework Help: Finding velocity and momentum of a bullet and recoiling gun

  1. Apr 13, 2016 #1
    1. The problem statement, all variables and given/known data
    A bullet leaves the barrel of a gun with a kinetic energy of 90 J. The barrel of the gun is 50 cm long. The gun has a mass of 4 kg, the bullet 10 g. (a) Find the bullets final velocity. (b) Find the bullets final momentum. (c) Find the momentum of the recoiling gun. (d) Find the kinetic energy of the recoiling gun.

    2. Relevant equations
    KE = 1/2mv^2
    p = mv

    3. The attempt at a solution
    (a) Since I know the kinetic energy of the bullet as it leaves the barrel, I attempted to find the velocity with this information.

    90 J = (1/2)(.01 kg)v^2
    which gives v = 134.16 m/s

    (b) To find momentum of the bullet I plugged this value for v into p= mv, this gives the result p = 1.3416 kg m/s.

    for (c) I used conservation of momentum, since the initial momentum of both the bullet and gun are zero.
    0 = (1.3416 kg m/s) + (4 kg)v
    solving this give v = -.3354 m/s

    (d) KE =(1/2)mv^2 so the kinetic energy of the gun is KE = (1/2)(4 kg)(-.3354 m/s)^2 = .225 J

    I'm not entirely sure if I solved this correctly. My main problem is that it gives the length of the barrel of the gun and I did not use this in the solution. Is this unnecessary information? or did I do this incorrectly? I thought that the 50 cm could be used to find the work done on the gun, but 50 cm is how far the bullet moved, not how far the gun moved.
  2. jcsd
  3. Apr 13, 2016 #2


    User Avatar

    Staff: Mentor

    Welcome to Physics Forums!

    Your calculations look good. The barrel length is irrelevant as you have surmised. You might want to round your results to better reflect the number of significant figures provided in the given data.

    In future, please retain the formatting template headings when you put together your post. I've re-inserted the headers for you this time.
  4. Apr 13, 2016 #3
    Thank you very much! I planned to round my final results but just kept them that way in case I needed to use them in further calculations. Thanks again for the input!
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