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Homework Help: Ratio for Kindetic and Potential energy

  1. Feb 5, 2010 #1
    If the phase angle for a block-spring system in SHM is φ and the block's position is given by x = xm cos(ωt+φ), what is the ratio of the kinetic energy to the potential energy at time t = 0? State your answers in terms of the given variables.



    x =xm Cos(wt + phi)
    v = -w(xm)sin(wt +phi)
    K = (1/2 mv2)
    P = (1/2 m w2 x2)




    dividing K/P = [v/wx]2
    substitute v and x we get: [(-w(xm)sin(wt +phi)/(w)(xm Cos(wt + phi))]2
    simplifying we get: [-wtan(wt + phi)/w]2 = [tan(wt + phi)]2
    at t = 0 we should get the following: [v/wx]2 = [tan(phi)]2


    The answer is marked wrong, am I interpreting the question incorrectly? I know what ratio is.. and ratio for Kinetic energy and Potential should be K/P right?
     
  2. jcsd
  3. Feb 5, 2010 #2
    you are absolutely right !! its odd that it is marked wrong .. maybe you should change the form of your answer? try different form maybe that will work ..
     
  4. Feb 5, 2010 #3
    oh good I thought I was going crazy. I was only allowed three attempts. so I used three different formats that were all the same thing.. still marked wrong.
     
  5. Feb 5, 2010 #4
    it happens, sometimes you have the right answer but i.e you have to express it in a certain form or in a certain sigificance .. :) anyway, dont worry you had the right answer ..
     
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