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Help with pendulum question

  1. Jan 23, 2005 #1
    I dont know how to calculate the magnitude of the unbalanced force.

    A pendulum bob with a mass of 1.80 kg is displaced from its equilibrium position, A, to an unstable position, B, by means of a string which is kept horizontal. The force applied to the string is, F. The length of the pendulum is 1.25 m. The angle through which the pendulum is displaced is 30.0 degrees as measured from the vertical. Determine the following.


    Draw a free body diagram for the pendulum bob when it is at an angle of 12.0 degrees as measured from the equilibrium position. Ca1culate the magnitude of the unbalanced force acting upon the bob. In what direction must the unbalanced force act?

    I've calculated the speed which is 1.66m/s. And from the speed I've got the centripedal force which is 3.96N. I know the unbalanced force acts toward the center.

    Ok, how do show how i calculated the unbalanced force.
    I was thinking T- mg(cos something)= centripedal force but unsure of how and what to do next.
  2. jcsd
  3. Jan 23, 2005 #2

    Doc Al

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    Staff: Mentor

    Careful. While there must be an centripetal force (which you calculated), it is only a component of the unbalanced (net) force. (If the unbalanced force only acted towards the center, then the speed of the pendulum would not change--it would just keep going in a circle--but you know it speeds up as it falls!)

    First find the tension in the string by applying Newton's 2nd Law in the centripetal direction: [itex]T - mg cos\theta = ma_c = m v^2/r[/itex]. (Solve for T.)

    There are two forces on the pendulum bob: the weight and the tension in the string. Add them (as vectors, of course) to find the net unbalanced force.
  4. Jan 24, 2005 #3
    Does Fnet=5.4N 35 degrees above horizontal sound right?
  5. Jan 24, 2005 #4

    Doc Al

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    Staff: Mentor

    Sounds good to me.
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