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Homework Help: How to find velocity of a bullet given energy?

  1. Feb 24, 2012 #1
    1. The problem statement, all variables and given/known data
    A bullet with a mass of 15 grams is fired from a rifle with a barrel that is 85 cm long.
    a. assuming that the force exerted by the expanding gas to be a constant 7200N, what speed would the bullet reach?

    2. Relevant equations
    Ek= 1/2mv^2
    Work= Force x distance

    3. The attempt at a solution

    Originally I thought that 7200 N were equal to Energy Kinetic so I solved like that
    but that seems to be incorrect.

    So I found the equation for work
    So I tried doing
    work= 7200N x .85 meters work = 6120 J
    Then setting that equal to Ek so...
    6120J= 1/2(.015)v^2
    v= 903 m/s
    But that also seems to be incorrect.
    Am I missing a step? Or am I using the wrong equation?

    Please Help me, thank you in advance.
  2. jcsd
  3. Feb 24, 2012 #2
    Work is energy, so in the case where the work accelerates a bullet, the work is the same as the change in kinetic energy. W=ΔE_k

    Thus, your calculations should be correct... Are you sure all the values are correct? What is the answer supposed to be?
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