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Max velocity for simple pendulum oscillation

  1. Feb 1, 2013 #1
    1. The problem statement, all variables and given/known data
    Show that v_max = w_max * Length of string where v_max is the velocity of the simple pendulum and w_max is the maximum angular velocity.


    2. Relevant equations
    [itex]\omega_{velocity} = -\theta_{max} \cdot \omega_{frequency} \cdot sin(\omega_{frequency} \cdot t + \phi)[/itex]


    3. The attempt at a solution
    The closest resemblance I could find was using energy:

    [itex]\frac{1}{2}kx^2 = \frac{1}{2}mv_{max}^2[/itex] where x = some displacement. The displacement in this pendulum's clase would be the Length of the string * angular displacement because s = Lθ.

    Solving for v_max gives: [itex]v_{max} = \omega_{frequency} \cdot x[/itex]

    But the 'x' in this case represents the displacement made by the pendulum (the arclength subtended by the angular displacement).

    Is there another equation I'm unaware of?
     
  2. jcsd
  3. Feb 1, 2013 #2

    Simon Bridge

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    Well yes - via ##v_{T}=R\omega## perhaps?

    I think, to understand the problem, you need to say where you have to start from
    ... possibly by deriving the equation for "v" - however it has been defined.
    I don't think you have been very careful with the definitions of terms.
     
  4. Feb 1, 2013 #3

    rock.freak667

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    You may need to solve the SHM DE of

    d2x/dt2 + ω2x = 0

    Are you able to solve this DE or know the solutions to this DE? (it is similar to your relevant equation)
     
  5. Feb 1, 2013 #4
    Oh, so there is an equation that states ##v_{T}=R\omega##. I did not know this. How do you derive this?

    My relevant equation is just the derivative to this solution. I didn't think the angular position equation was more useful than its derivative so I only included the equation for angular velocity.
     
  6. Feb 1, 2013 #5

    rock.freak667

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    Yes and that is why I thought by show, you would have had to show the steps in the derivation towards the DE.
     
  7. Feb 1, 2013 #6

    Simon Bridge

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    <puzzled> the pendulum is an example of circular motion:

    The distance ##s## on an arc, radius ##r##, changes with angle ##\theta## as ##ds=r d\theta## (follows from the definition of "angle"). To get the tangential velocity ##v_{T}## just divide through by ##dt##.
     
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