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Posted by on Monday, March 29, 2010 at 9:46pm.

A 0.47 kg pendulum bob is attached to a string 1.2 m long. What is the change in the gravitational potential energy of the system as the bob swings from point A to point B where θ = 31°?

I've tried using PE = mgh but I don't know where the 31 degrees comes in there. I'm confused as to what formula to use.

  • Physics - , Tuesday, March 30, 2010 at 5:01pm

    for simplicity i am going to do this symbolically.

    m = .47Kg
    l = 1.2m
    g = 9.81m/s²
    è = 31 degrees

    the change in potential energy is

    mgÄh

    in this problem,
    Äh = l-lCos(è)
    = l(1-cosè)

    (it is 1-cosè becaus cos 90=1.. and its cos 90-cosè)

    so

    ÄE = mgÄh
    = .47Kg * 9.81m/s² * 1.2m * (1-cos31)
    and the answer is in units of joules
    answer: .79

  • Physics - , Tuesday, March 30, 2010 at 5:02pm

    mgh*

    cos theta*

    sorry

  • Physics - , Tuesday, March 30, 2010 at 5:03pm

    also Deta E*

  • Physics - , Tuesday, March 30, 2010 at 5:03pm

    Delta* wow i cannot type

  • Physics - , Tuesday, September 11, 2012 at 11:52pm

    @kelly..may i know why have to l-l cos delta?

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