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B Torque vs Moment of a Force

  1. Oct 18, 2016 #1
    I'm confused when it comes to the rotational version of Newton's Second Law:

    τ = Iα
    Is the torque τ the same thing as the moment of a force? I'm not sure if there is a discrepancy between British and American schools but I was always taught that a torque was a special case of moments and is calculated when two equal forces act in opposite directions on a new object. All the rotational dynamics question questions seem to suggest that torque is actually what I know as the "moment of a force". Am I missing something?
  2. jcsd
  3. Oct 18, 2016 #2


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  4. Oct 18, 2016 #3
    This can be a fuzzy area of physics. The MOMENT of a force is F x perpendicular distance from the pivot ===Fxr. In my experience the term 'MOMENT' is used in balancing lever type applications!!
    TORQUE is force F x distance from pivot (spot any difference?) tends to be used in rotational dynamics applications
    COUPLE is when 2 forces produce a turning effect (2 torques?) couple = 2F x r or F x 2r or...(just to add to confusion) F x D (D =diameter)
    This is more about usage of terms than physics principles. You have to get used to it !!
  5. Oct 18, 2016 #4


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    In my experience, "torque" is never about force times non-perpendicular distance. Only the perpendicular distance is ever relevant. If I push my torque wrench end-on, its reading remains stubbornly at zero.
  6. Oct 18, 2016 #5

    Philip Wood

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    Agree broadly with lychette. There's no real distinction, but I'd say that torque is used for moment about an obvious or intended axis of rotation, like the centre of a nut that's being loosened on a bolt, or about a shaft in an engine or motor. We tend to use the word 'moment' when we're considering force x perp distance from some arbitrary point in a structure, say, in order, perhaps, to determine conditions for equilibrium.
  7. Oct 19, 2016 #6


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    +1 Torque and Moment are frequently used to mean the same thing although general use seems to be..

    Torque: May not have an associated force and distance or both force and distance are unknowns.

    Moment: You can usually "factorise" a Moment into a force and a distance because one or both are known. A moment may or may not cause a resulting linear force as well.

    Couple: Combination of forces and distances that result in a pure torque without an additional resulting linear force. eg special case of a Moment where there are two equal but opposite forces.

    That would be a couple.
  8. Oct 20, 2016 #7
    Here we go....Like moment, TORQUE is force x perpendicular distance from the pivot. If the line of action of the force is not perpendicular to the pivot then the perpendicular distance from the line of action of the force from the pivot must be used.
    Sigh !!!!....'what other distance would you use' ...
  9. Oct 20, 2016 #8


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    You had written...
    The difference that I spotted and commented on is the lack of the word "perpendicular". If you meant something else, perhaps you could be less coy and spit it out.
  10. Oct 20, 2016 #9
    I see what has happened....you did not see the word 'perpendicular' in the previous statement or you thought it applied to only that statement.
    we have a semantics problem....sorry
    I hope that we have not confused anybody
  11. Oct 20, 2016 #10


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    Yes indeed, I had parsed it as if "perpendicular" applied to only the one and not the other.
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