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Old Nov 15th 2012, 03:00 PM   #1
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Post Pendulum and Pin issue

Hello, everyone!
I've been given an issue from my colleagues, and I need some help. It's about a particle attached to a string and a particle with a "d" distance underneath the place where the article is attached.
My question is, how should I start solving this issue? Is it really an energy issue or I better start doing it in other way?

Thanks
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Old Nov 16th 2012, 05:43 AM   #2
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Please clarify - is the pendulum a string, or a solid bar? If it's a string is the particle that is 'd' distance below fixed in space, so that when the the pendulum swings down and the string hits it, the string will then tend to wrap around it? Or is it free to move, such that when the pendulum hits it it gets whacked to the right? You called it a "pin" in the subject but a "particle" in the body of your post, so it's unclear.

Energy methods are typically the way to go when you have a particle coliding with another and you want to find velocities (if is an elastic collision), but free-body diagrams are a good way to set up a differential equation to find the position of the pendulum as a function of time. And depending on what you're trying to determine angular momentum concepts may come into play - it all depends what you're trying to solve.

Last edited by ChipB; Nov 16th 2012 at 08:44 AM.
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Old Nov 16th 2012, 11:30 AM   #3
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The pendulum is a string, so it'll wrap around the pin, sorry!
All I need to know is what the string does before and after hitting the pin, so that I can determine the minimun speed the pendulum must have to wrap around it.
Thanks so much!
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Old Nov 16th 2012, 11:40 AM   #4
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The term "wrap around" is a bit nebulous. If you think about the trajectory of the head of the pendulum it could come with sufficient speed so that the string stays taught as it wraps around, and the pendulum string will wind up on the pin. Or it may come in with just enough speed to arc just once past the pin with the string being slack, so that the pendulum head barely clears the pin just once. If you're looking for the former: the velocity required when the pendulum is at the top of the loop must satisfy mv^2/r = mg. Using energy principles you can determine whether it will make it or not. If the latter it gets a bit more complicated - I won't bother with the details unless you verify that this is indeed what you need.

Last edited by ChipB; Nov 16th 2012 at 12:12 PM.
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Old Nov 16th 2012, 11:59 AM   #5
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The pendulum will come with just enough speed to arc once, with the string being slack, as you've already said, but it depends on that "d" distance the pin is held.
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