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Homework Help: Very Basic Pendulum problem

  1. Mar 5, 2005 #1
    A bowling ball hangs from a rope with length L of 5.00 meters.
    Initially, the bowling ball is at a position such that the rope makes a 20 degree angle with respect to vertical. The ball is released from rest.

    What is the speed of the bowling ball when it swings down and the rope makes an angle of 10 degrees?

    Just looking for help on setting the problem up.

    I have done every other problem on my homework so far with relative ease. I'm sure I've learned what I need to know to do this problem, but it's just not happening :frown:
    Whoever can point me in the right direction, the chapters are all focused on Newton's 2nd law, and thus any responses should be pretty basic :smile:It's just a physics I class, but my book has no pendelum problems in it for examples thus far.
  2. jcsd
  3. Mar 6, 2005 #2
    Have you covered potential energy yet? Two concepts that will make this problem easy are potential energy and conservation of energy. Finding the answer using only Newton's 2nd law is way too much trouble (requires calculus).

    The idea is, the total energy of the bowling ball (kinetic energy plus potential energy) is the same at all times throughout the ball's motion. At first, all its energy is potential--it is at rest. The ball loses potential energy on the way down. That loss is exactly balanced by a gain in kinetic energy. Kinetic energy is what you are looking for.
  4. Mar 6, 2005 #3
    Yes we have, and I understand that the energy shifts from potential to kinetic (and back on the up swing). But, everything we've done so far involves the mass of the object. Without the mass, I'm sort of loss. I'm sure somewhere there's a conversion from energy to the variables I am given in this problem, but I'm not finding it.

    A little more push in the right direction :redface:

    I realize that the height is L-Lcos20 initially and L-Lcos10 final . So I know the amount of drop in height. However, all of my books problems involve a mass (which then I could do this), but I don't know where to go right now.
  5. Mar 6, 2005 #4
    Dont worry about mass - put "m" in for mass, and you'll find that in equating GPE and KE the masses will cancel each other! Numbers are not neccesary to do physics - you can find a general case for many things by plugging in a variable and solving anyway.
  6. Mar 6, 2005 #5
    oh crap, well that was extremely simple!! Sometimes I just look for too much.

    Thank you :)
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