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Calculating Force from N m

  1. Oct 27, 2008 #1
    HI all

    I wonder if someone could help me out here. I run a structure borne noise & vibraton control company and have been passed an enquiry that is little off our usual field. We use a lot of springs in our business. The idea being that if something is vibrating and we support it on something with a significanlty lower natural frequency than the vibration frequency then we get isolation.

    The problem we have been given is this:

    A train wagon is feeding a glass furnace. The train runs round a track & up to the furnace door. If everything is in sync the door opens as the wagin arrives, it deploys it's load and moves off again. The probl;em is that the door occasionally fails to open and the wagon is hitting the doorcasing a lot of damage. The idea is to use one of our springs as a buffer. The info we have been given is that the wagin hits the door with a force of 140 N m. The problem I have is that I'm not sure whet the units are saying. As i understand them the definition would be 140 N of force acting through a distance of 1 metre along the direction of the force.

    What I am trying to do is calculate what the force the wagon hits the door with is in Kg. I can then select a spring with an appropriate spring rate and problem solved. Can anyone translate this to a recognisable unit for me?
  2. jcsd
  3. Oct 27, 2008 #2


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    They're doing the engineering equivalent of mixing metaphors. The units are units of energy (N*m) and stating it is a force. The first thing I would do is corner them to nail down what they meant. Did they mean force or energy?
  4. Oct 27, 2008 #3
    Um, actually I think thats work or maybe in this case it could possibly be torque. Anyway, as Fred stated you need to confront them and figure out what they are trying to tell you. But to pick a spring you don't really need the force that they are hitting the door do you? I would imagine that you only need to know the mass and velocity of the wagon.
  5. Oct 28, 2008 #4
    Thank you for the help. I'll get back to my client and get him to clarify.

    Topher, yes I can and probably will calculate the force as a cross check but I'd prefer to be given the value by my client because then he remains responsible for the answer, particularly as we don't always get given the correct input data as it seems in this case. A sorry state of affairs but we live in a litigous world. I wasn't sure what the unit N m was and thought perhaps there was a simple way of getting to a unit I recognised but now I know differently.

    Thanks again
  6. Oct 28, 2008 #5


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    A N*m is a Joule.
  7. Oct 28, 2008 #6
    Ok so the wagon's mass is 250 Kg and it's velocity at the point of impact is 0.8 m/s.

    Could someone please correct/ confirm;


    so p=200 Ns

    F=delta p/t

    So if it takes 0.1 secs for the wagon to come to rest F = 2000N

    Is 0.1 sec a reasonable assumption? This is a very heavy furnace door so I assume it is very rigid.
  8. Oct 28, 2008 #7


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    Eh, it would more than likely not be safe to make an assumption for this. If you assume that the time is 0.05 instead of 0.10, only one-twentieth of a second difference, the force is doubled.
  9. Oct 28, 2008 #8


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    I would start with using what the customer gave and assume they knew what they were talking about in terms of energy. Then I would look at your springs' force vs. deflection plots. The area under the curve, in a perfect world, is the energy absorbed-produced when the spring is compressed or extended. A starting point would be a spring that has that area under it's curve that meets that energy requirement.

    I would still ask the customer to verify the units given.
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