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What exactly is moment, and when is it zero?

  1. May 5, 2015 #1
    I've been dealing with the concept of moment this whole year, in statics/dynamics but I've never really completely understood it, just rather memorized it.

    What I think moment is:
    If some point on a body has moment, then this means it wants to rotate. So moment is essentially what causes rotation.

    But here is something that I'm confused about:
    Pins allow rotation, so they have zero moment.

    That is what confuses me. I understand that pins don't resist moment, so there is no unknown moment about a pin. But according to my above definition of moment, if the moment at the point of the pin is zero then this means it will not rotate? But pins are supposed to allow rotation?
     
  2. jcsd
  3. May 5, 2015 #2
    Probably a better definition of a pinned joint is that it has zero moment resistance. So no moment can be transferred to the supports or other connecting bodies. But that doesn't mean it can't rotate about the joint or about any other point for that matter.

    Consider a free body rotating in space. There is no "moment" acting on it but it still rotates at the same speed. Some kind of moment caused this initial rotation but no moment is necessary for it to continue to rotate. So I would amend your definition to say that a moment can cause rotation.
     
    Last edited: May 5, 2015
  4. May 5, 2015 #3
    I am still slightly confused.

    is it possible for the sum of moments about a pin to not be zero?
     
  5. May 5, 2015 #4
    A frictionless surface is not capable of exerting a tangential force on a body in contact with the surface, even though there may be tangential forces from other sources acting on the body (causing it to accelerate). A pin is not capable to exerting a moment on a body attached to the pin, even though there may be moments acting on the body from other sources (causing it to experience angular acceleration).

    Chet
     
  6. May 5, 2015 #5
    Ah okay. Thanks. Then it is possible for the moment at a pin to be nonzero. (However, the pin can't apply any moment to the body).
     
  7. May 5, 2015 #6
    Yes. Exactly,
     
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