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Difference between torque and moment

  1. Jun 1, 2009 #1
    Torque is when a couple is formed...and for a moment, a couple needs not be there right?
     
  2. jcsd
  3. Jun 1, 2009 #2

    tiny-tim

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    Hi dE_logics! :smile:

    "torque" sometimes means a couple, and sometimes not. :rolleyes:

    From the PF Library on https://www.physicsforums.com/library.php?do=view_item&itemid=175"

    Torque is the moment of a force about a point.


    "A torque" is also the name of a pair of equal-and-opposite forces which are not in-line, and so have a purely rotational effect.


    "Torque" vs. "moment":

    The words "torque" and "moment" (of force) mean the same.

    However, "torque" tends to be used when there is an axle or pivot to be turned around, while "moment" tends to be used in essentially non-rotational situations, such as analysis of forces on a beam.
     
    Last edited by a moderator: Apr 24, 2017
  4. Jun 1, 2009 #3
    You topple if you get down from a rapidly moving bus because of the torque exerted by friction alone..
    Moment and Torque are not always the same. Moment is a blanket term. Moment of a force is torque. Moment of momentum is angular momentum. Moment of a vector X, is R x X.
     
    Last edited: Jun 1, 2009
  5. Jun 1, 2009 #4
    Although torque and moment of a force are the same, engineers tend to use moment (I believe an older term) as a way of suggesting motion (a cognitive sticky, if you will).
     
  6. Jun 2, 2009 #5
    You mean in case the moment by each force is not balanced, then it will make a torque.

    So moment is only in statics and it turns to torque in kinetics.

    The 3 answers sound pretty confusing.
     
  7. Jun 2, 2009 #6

    tiny-tim

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    You seem to be expecting English to have the same rigour as maths. :rolleyes:

    In English, "torque" is a bit vague, and in particular, there is a difference between torque of a force and "a torque".

    Don't worry! :blushing: … the context usually makes it clear. :smile:
     
  8. Jun 2, 2009 #7
  9. Jun 3, 2009 #8
    :uhh: Lets just talk science here........


    No 100% English vocab.

    BTW how do you even relate it to the English torque?...or other meanings?
     
  10. Jun 3, 2009 #9
    Balanced or unbalanced, moment of a force about a point is called torque. All you need is a force and a reference point. A force doesn't cease to exist just because it is neutralized by some other force.
     
  11. Jun 4, 2009 #10
    Yeah, that's why we have rotation.

    So torque and moment are the same thing.

    Or is it that, when there is no equilibrium, then its torque, else moment.
     
  12. Jun 4, 2009 #11
    A torque is a torque is a torque.. :smile:
     
  13. Jun 4, 2009 #12

    D H

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    Moment of force is a synonym for torque. They mean exactly the same thing. Using the term "moment", sans the "of force" qualifier, is physicists being a tad lazy. There are lots of other "moments". In addition to those already mentioned, moment of inertia.
     
  14. Jun 8, 2009 #13
  15. Feb 16, 2011 #14
    Moment is the tendency for a force(s) to create rotation about a point.

    In Physics: Moment is Torque is Moment. Done.

    In Engineering: Moment is Moment. Torque is the Moment of a Couple.

    A Couple is a Moment with ZERO NET FORCE.
    A Moment of a Couple can be moved anywhere on the body and cause the same rotation. It is thus sometimes called a "Free Vector". Though this may seem non-intuitive, it is a real characteristic of a Couple.

    "Equilibrium" is not an intrinsic necessity to qualify a Moment or a Couple. If there is rotational motion, there is definitely no Moment equilibrium (sum of Moments = 0). However, if it's a Statics context, there would definitely be translational equilibrium. If there is a resultant Force or Moment, there is spin, and thus no equilibrium. If you are BALANCING a system, then you will set forces and moments to zero and then FIND equilibrium. I hope that clears it up.
     
  16. Feb 16, 2011 #15
    This sems to be a very old thread to resurrect, although the discussion is perennial.
     
  17. Feb 17, 2011 #16
    True, Studiot. I'm in a Statics class and came across this thread just yesterday (and several other similar). There seems to be no real closure to some of them (uncontested misinformation, missing information, questions unanswered) so I thought I'd at least clear this one up.
     
  18. Feb 17, 2011 #17
    There are two real differences between a moment (or couple) and torque.

    Firstly Torque is not limited to a single revolution.

    Secondly moments (and couples) are planar beasts - they exist in a plane. Torque, on the other hand, is three dimensional and has the ability to transfer moments from one plane to another.
     
    Last edited: Feb 17, 2011
  19. Feb 17, 2011 #18
    Moments/couples certainly exist in 3-D. Their resultant vector projects into the third dimension. This is no different than a Torque.

    Couples transfer moments the same as Torques, being free vectors.

    Is a Moment limited to a single revolution? Please, show me where or how.
     
  20. Feb 17, 2011 #19
    Perhaps you can display a non planar couple?

    If the two forces constituting the couple are not in the same plane how can there be a zero force resultant?

    A single force (line) and a single fulcrum (point in space)can only be planar.
     
  21. Feb 17, 2011 #20
    Yes. But we're not talking about a Force and a point, we're talking about a Force and a Position vector, with origin at a point. If we're not talking about two vectors, then we can't be talking about a Moment or a Couple.
     
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