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Homework Help: Simple Harmonic Motion (Mass on a Spring)

  1. Apr 19, 2008 #1
    [SOLVED] Simple Harmonic Motion (Mass on a Spring)

    1. The problem statement, all variables and given/known data
    A solid cylinder of mass M= 10.8 kg is attached to a horizontal massless spring so that it can roll without slipping along a horizontal surface, as shown in the Figure. The force constant of the spring is k= 346 N/m. The system is released from rest at a position in which the spring is stretched by a distance x= 17.0 cm. What is the translational kinetic energy of the cylinder when it passes through the equilibrium position? What about the rotational KE?


    2. Relevant equations
    Translational Energy-.5mv^2
    Rotational Energy- .5Iw^2
    Angular Velocity (spring)- sqrt(k/m)

    3. The attempt at a solution

    Alright... I only have this problem left and it's frustrating me. For the first part, I used translational Energy for a spring (.5kx^2), but I am not taking the mass into account. All the example problems we were given have amplitudes and other useful things to use to compute velocity. So, I moved onto what I thought would be the easier one, rotational. For this, I tried using .5(r^2)(k) when I realized, much to my dismay, that I had no radius for the cylinder. Any alternative ways I am not thinking of? Thanks so much.
  2. jcsd
  3. Apr 19, 2008 #2
    You actually don't need radius of cylinder. At the begining you have just energy of spring
    0.5kx^2 , that equals sum of rotational and translational energy in equilibrium position. The you should know that

    [tex] \omega =v/r ~,~E_t=0.5mv^2~,~E_r=0.5I~\omega^2 [/tex]

    Now since I for cylinder is 0.5mr^2 , r falls out. You have only equation for v then.
  4. Apr 19, 2008 #3
    Thank you very much!
  5. Dec 1, 2010 #4
    Re: [SOLVED] Simple Harmonic Motion (Mass on a Spring)

    Can someone please explain to me where the formulas are coming from for translational energy, etc?
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