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Homework Help: Force with kinetic energy question

  1. Feb 11, 2008 #1
    [SOLVED] Force with kinetic energy question

    A 2.5-g bullet traveling at 350 m/s hits a tree and slows uniformly to a stop while penetrating a distance of 12 cm into the tree's truck. What force was exerted on the bullet in bringing it to a rest?

    Not sure on this one. I know to find to the kinetic energy would be KE=1/2mv^2. I'm not sure if that is needed though. And to find force would be F=W/D or F=MA.

    So far I only have KE=153.125 using the equation above. I plugged in 1/2(.0025kg)(350m/sec)^2. Now I'm not sure what to do. I want to try to find W since it includes distance. I don't know if there is an equation to find work by using only M,V,and X. Hmm..I'm confused now.
  2. jcsd
  3. Feb 11, 2008 #2
    You know the initial and final velocity, and you know the distance over which the acceleration took place. From these three you can calculate the acceleration with [tex]v^{2} = u^{2} + 2as[/tex].

    From there you can determine the force exerted on the bullet.
  4. Feb 11, 2008 #3

    Doc Al

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    Staff: Mentor

    You're doing fine by calculating the KE. How does work done (FD) relate to the change in KE?
  5. Feb 11, 2008 #4
    They are equal aren't they? W=K-Ko
  6. Feb 11, 2008 #5
    Ahh I think figured it out. The W would be equal to =-153.125J. Then I would just use F=W/D or F=-153.125 J/.12m and that comes out to be....-1276.04. Is that correct?
  7. Feb 11, 2008 #6
    Yep, that's right.
  8. Feb 11, 2008 #7
    Thank you again!
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