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Homework Help: Incident Kinetic Energy?

  1. Oct 31, 2007 #1
    1. The problem statement, all variables and given/known data
    A projectile mass (mass=0.20 kg) is fired at and embeds itself in a target (mass=2.50 kg). The taget (with the projectile in it) flies off after being struck. What percentage of the projectile's incident kinetic energy does the target (with the projectile in it) carry off after being struck?

    2. Relevant equations

    3. The attempt at a solution
    I have no idea what this question is even asking me to do, how do you do a question like this when they only give you masses?
  2. jcsd
  3. Oct 31, 2007 #2
    That's a bit of a tricky question. I believe what you want to look at is the conservation of energy of the system, the system being the projectile and the target. You should end up with a ratio, as in part of the kinetic energy over the whole kinetic energy, and convert that to a percentage. Can you show me how you would set up the equation?
  4. Nov 4, 2007 #3


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    Homework Helper

    Believe it or not, this will become clear when you work toward the answer. Have you dealt with elastic and inelastic collisions yet? If so, what type of collision is this between the target and bullet? You will want to start from the conservation of linear momentum.
  5. Nov 4, 2008 #4
    In case anyone's managed to figure this one out since it was originally posted, I'm taking Physics now, and am having trouble with this same problem. I figured out that the collision is completely inelastic [can use equation: (m1+m2)vf = m1v01+m2v02)] , and think that mass and velocity have to be transformed somehow into kinetic energy (Ke=1/2mv^2), but I don't know how to set up the equations so that you get the projectile's incident kinetic energy (what is that, anyway?) Any ideas?
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