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Need to calculate force from swinging pendulum

  1. Jan 27, 2013 #1
    First of all, I'm not a physics person, so please forgive my questions. I am a mother of a student that is participating in a program called Destination Imagination. They are required to build a structure that can be "rammed" at 2.2 lbs of force. I don't have the tools or know-how to build the "ram" structure. So, I would like to build something that would simulate something hitting a board with 2.2 lbs of force.

    I'm assuming there is some sort of formula for a pendulum at a certain length with a certain amount of weight if pulled at a specific angle would deliver a given amount of force.

    Can someone point me in the right direction here? It would be much appreciated.

  2. jcsd
  3. Jan 28, 2013 #2
    It’s an interesting assignment, because calculating a ramming force might seem to be easy on the surface, but is anything but. Any time you have some sort of impact or “ramming” it’s very difficult, if not impossible, to calculate. This is due to the number of variables involved. Depending on what you estimate them to be, your answer could easily be a few orders of magnitude off.

    If you REST a pendulum against a board, then it’s quite easy to calculate, because everything is static. Consider the force on a bathroom scale…also easy to predict: it’s simply your weight. But in order to get an accurate reading, you first have to get on the scale and then stand still for a few moments, so that everything settles and becomes static. If you look at the numbers, they jump around wildly the moment you get on the scale…the “ramming” moment. THAT’s the difficult part to calculate. The peak force, the force we’re interested in, will depends on how hard you step on the scale, how fast, what the scale is resting on, what type of scale it is, what it’s made of, how much you bend your knees, the material of your shoes, etc.

    Consider also jumping off a building. How much force do you ram the ground with? Well, again, what do you land on? The concrete? A bush? A pillow? Even if you know these, how you land, on your feet, on just one foot, or on your head? Are your legs straight or bent? Are they long? Are your bones strong? Are you drunk? Any one of these variables and many more can change the peak force enough where it can make the difference between life and death.

    Unless you know all or most of these variables, the only way to get a decent answer is to test it. This is why they perform crash testing of cars.

    Perhaps a pendulum and a board simplifies the problem enough to where it might be possible to estimate an answer, but even then it’s difficult enough to where I doubt that you are actually expected to solve for it.
  4. Jan 28, 2013 #3
    Thanks for your reply. I have done some research, and I was afraid that was the answer. :-)

    I guess we will just do our best and hope our structure survives the ramming impact at the tournament.
  5. Jan 28, 2013 #4
    Yeah I think that's the best you can do. Surely you can estimate your way to some value, but like I said the end result can vary by orders of magnitude.

    Do you know how the strength of the structure will be analyzed? Surely there must be some final test that is performed in from of a judge...
  6. Jan 28, 2013 #5
    Basically, the kids have to build a structure that will be put on a "structure tester". It's basically a platform with a pole in the middle. Then a board is placed on top of their structure.
    At that point, the corner of the board on their structure is rammed with 2.2 lbs of force.

    The idea is that it causes a twisting effect on their structure. Thus the kids should build something that will withstand twisting and not break.

    Anyway, there is a person in our school district that built the actual ram. I will try to borrow it for the kids to test right before the tournament. I was just hoping I might build something really crude they could use for several weeks.

    Thanks anyway.
  7. Jan 28, 2013 #6
    To answer your question. The strength of the structure will be scored on whether or not it still stands after the ram impact and on how much weight is applied to the board. They will get extra points if it survives additional ram hits with more weights.
  8. Jan 28, 2013 #7
    See, this is is the part that I (and you) don't get: how do they control the ram so that it provides exactly 2.2 lbs of force?

    If they use the same ram for all the structures, then I suggest to not necessarily make it as strong as possible, but to add flexibility to the the structure. The more flexible it is, the lower the peak force will be....if I understand the test correctly. With the same principle as jumping off a building onto a huge air bag instead of solid concrete, you could potentially reduce the peak force onto your structure by many times.

    It's misleading though, and outright wrong that they would say that the ram provides 2.2 lbs of force, when in fact it can vary greatly depending on the structure (if I understand the test correctly). It would be more accurate to say that the structure needs to absorb a certain amount of energy.

    It's a nice program, but if they're going to try to teach kids by doing science/ engineering, might as well teach them the correct terms.

    Unless there's something basic I'm misunderstanding here...
  9. Jan 28, 2013 #8
    There have been some clarifications with the challenge. It now simply states that the ram weight is 2.2 lbs. Much simpler. So, that's easy. I'll just cut a board to the size they specify and it should weigh around the same amount. I can figure out some safe way to hang it so that the kids can use it to test.

    Sorry for the confusion.
  10. Jan 28, 2013 #9


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    I'm not sure, from your description, what they mean by "Ram" and how "force" is defined. The only time that the word "force" is really relevant is if the force is applied steadily. If there is any kind of impact, then the force is not particularly relevant (it's actually very hard to evaluate). In collisions, it is the Impulse that counts, which relates to the change in momentum - i.e. more detail about the stress that's applied to your structure is needed.
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