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Homework Help: Pendulum Motion

  1. Nov 27, 2012 #1
    1. The problem statement, all variables and given/known data
    If the period of a 70.8-cm-long simple pendulum is 1.91 s, what is the value of g at the location of the pendulum?

    2. Relevant equations
    I don't know of any that relate the period, radius and the value of g

    3. The attempt at a solution

    The thing is, I'm completely clueless. I'm guessing it has something to do with force, because how else would g be related to the equation? And... probably angular velocity. But I am so clueless.
  2. jcsd
  3. Nov 27, 2012 #2

    Doc Al

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    Staff: Mentor

    Google is your friend.

    Seriously, you should be able to easily find an expression for the period of a simple pendulum.
  4. Nov 27, 2012 #3
  5. Nov 27, 2012 #4
    @tadchem: Um... The link only said "It works!" What works, I don't know...

    @Doc Al: I'm looking to learn how to do it, not simply find an equation.
  6. Nov 28, 2012 #5


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    Science Advisor
    Homework Helper
    Gold Member

    Draw a free body diagram with the pendulum at some angle to the vertical. Show the forces acting on the bob. Assign identifiers (algebraic variables) to the angle, the mass, etc. Obtain an equation for the acceleration.
    At this point, you should hit a snag, a mix of trig and linear functions of the angle. The trick is to assume the pendulum only swings through quite small angles. That allows you to approximate sin(θ) as θ. You should now have a classic simple harmonic motion differential equation.
  7. Nov 28, 2012 #6

    Doc Al

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    Staff: Mentor

    Did you even attempt to Google it? Or use the excellent hyperphysics reference? (Or your text.) Full derivations (similar to what haruspex outlined) are readily available.

    Simple Pendulum
  8. Nov 28, 2012 #7
    My apologies for the incomplete link.
    The home page is http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
    It has a graphic tree navigation system that can quickly get you down to the nuts and bolts of whatever physics problem you are interested in. There is also an index on the right side of the page.
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