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Homework Help: Kinetic energy (tough algebra problem)

  1. Oct 20, 2009 #1
    1. The problem statement, all variables and given/known data
    Derive an algebraic statement for the fractional loss of kinetic energy in terms of symbols only and show that loss of kinetic energy/inital kinetic energy=M/(M+m)

    this is for a ballistics pendulum lab, and M is the weight of the holder and m is the weight of the metal ball. The task is to prove that the ratio of the kinetic energy lost to the inital kinetic energy equals the ratio of the mass of the holder (M) to the mass of the holder and the ball together (M+m)

    2. Relevant equations

    kinetic energy (.5mv^2)

    3. The attempt at a solution
    i set the two ratios equal to eachother, giving something that looks like this-

    .5mv(initial)^2-.5(m+M)v(final)^2 = M/(m+M)

    but i cant figure out the algebra to prove that the two in fact do equal eachother (and they do, earlier calculations in the lab prove it)
    ps, sorry if the equation looks really bad, its kind of hard to type it using a keyboard
  2. jcsd
  3. Oct 20, 2009 #2


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    Homework Helper

    You need more than algebra! You need some relationship between those velocities - another equation. You haven't made the problem clear, but perhaps you also have a momentum equation?
  4. Oct 21, 2009 #3


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    Homework Helper

    Hi captainsmith1! Welcome back! :wink:

    Yes, you nearly always need two equations for this sort of problem …

    if it's a collision, one of them will be conservation of momentum (it applies to all collisions), and the other will be conservation of energy (if it is conserved), otherwise some geometric constraint. :smile:
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