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Homework Help: Conservation of energy: How do I know?

  1. Mar 22, 2010 #1
    This is not so much a homework problem as it is a question:

    How do I know whether an object has kinetic energy or potential energy?

    I know for potential energy you make a point that is PE(grav) = 0, and then if the object reaches a height you make the other PE = mgh.

    So in (KE_2 - KE_1) - (PE_2 - PE_1) - (S_2 - S_1) = W_other where
    KE = kinetic en.
    PE = potential en
    S = spring en
    W = other forces acting...

    If no other forces are acting on the object W_other = 0; and there is no spring = 0 - 0

    Then you get (KE_2 - KE_1) = - PE_2 + PE_1

    And then if you make a point that is PE_1 = 0

    KE_2 - KE_1 = -PE_2

    Then what about KE ? Does that make KE_2 = 0?

    I have no idea, please help me! I need an answer in 9 hours!
  2. jcsd
  3. Mar 22, 2010 #2
    KE_1 = 0

    At your intial state, no kinetic energy is present, the object only has potential because it is at rest at a certain height(h not 0) from your reference ground which has h=0

    initial: potential = all energy, kinetic = no energy (assumed at rest at point1)
    final: potential = 0, kinetic = all energy
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