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Homework Help: Kinetic energy question

  1. Jan 11, 2010 #1
    kinetic energy question!!


    I have the following question to complete, and dont have the slightest clue where to start! :(

    A sphere of mass 1kg, moving at 10 m/s, overtakes another sphere of
    mass 5 kg moving in the same line at 3m/s. Find the loss of kinetic
    energy during impact, and show that the direction of motion of the
    first sphere is reversed. Assume the coefficient of restitution e = .75

    should i use the following forumla?: 2(m1 + m2)E = m1m2(u1-u2)2(1-e2)

    any help would be greatly appreciated,


    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jan 11, 2010 #2


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    Homework Helper

    Re: kinetic energy question!!

    Start by doing the first question: the loss in kinetic energy. Since the coefficient of restitution is 0.75, 25% of the initial kinetic energy is lost, correct?

    Then write out the conservation of momentum & conservation of energy equations, remembering that 25% of the original kinetic energy is lost.
  4. Jan 11, 2010 #3

    Doc Al

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    Staff: Mentor

    Re: kinetic energy question!!

    I don't recognize that formula. In any case, it's usually best to derive your own formula from first principles.

    Hint: What's the definition of the coefficient of restitution? That will give you one equation. Conservation of momentum will give you the other.
  5. Jan 11, 2010 #4


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    Staff Emeritus
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    Homework Helper

    Re: kinetic energy question!!

    Just a friendly correction, that isn't what is meant by coefficient of restitution; it involves the relative velocities of the objects before and after the collision. Presumably it's in the OP's textbook or lecture notes so I will reiterate Doc Al's advice and let the OP look it up.
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