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Pendulum force

  1. Dec 15, 2009 #1
    Initially, a 25 lb. bag is hanging straight down from a 7 ft rope at the center of an object next to a vertical wall. The 25 lbs is raised until it is about 45 degrees or 5 ft from the object and then released. How do I determine the force that hits the object? I am only looking for a simple approximation without the weight of the rope or wind.

    Thank you for your help!!!
  2. jcsd
  3. Dec 15, 2009 #2
    When you release the pendulum it converts gravitational potential energy (GPE) of the raised weight at 45 degrees, to kinetic energy (KE) of impact. You can easily find GPE and KE but if I recall properly the force during collision depends on the elastic properties of the bodies. Also it is not clear what the "object" is in your problem statement.

    Edit: hyperphysics treats the basic concept of impulse force here:


    The formula Favg*d = -(1/2)m*v^2 appears at the link page and might solve your problem if stopping distance "d" can be estimated as the bag comes to a stop as it hits the object.
    Last edited: Dec 15, 2009
  4. Dec 16, 2009 #3
    The object is a plastic prototype secured to the wall. When the bag hits it the object has no substantial elasticity. So given the stopping distance is 0, would it be just a matter of finding the velocity as the weight is about to hit the object?
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