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Homework Help: Kinetic Energy and Time Problem

  1. Jan 28, 2013 #1
    1. The problem statement, all variables and given/known data
    A ball with mass m projected horizontally o the end of a table with an initial kinetic energy K. At a time t after it leaves the end of the table it has kinetic energy 3K. What is t? Neglect air resistance.
    (answer: (2/g)(√K/m)

    2. Relevant equations
    E = 1/2 mv^2

    3. The attempt at a solution
    I understand what the problem is asking, but I don't get how to relate time and kinetic energy
  2. jcsd
  3. Jan 28, 2013 #2


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    What equations do you know for the motion of a projectile under gravity?
  4. Jan 28, 2013 #3
    x = vo(t) + 1/2at^2
    v^2 = v0^2 + 2ah

    Okay, so does that mean the final velocity was tripled then? and I apply the above equations?
  5. Jan 28, 2013 #4


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    No. Not tripled. Think conservation of energy. You have to gain energy 2K by losing potential energy mgh. Put time into it by figuring how long it takes to fall a distance h with zero initial vertical velocity.
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