In class we're learning about moments, and I'm not entirely sure what is the difference between using an equation of equilibrium for the moments about a point (in this case C), and the actual moment at C.

From what I understand, taking the "sum" of moments about C is needed to be equal to 0, since at that point, C, if the "sum" is greater than 0, then we are no longer in static equilibrium, and therefore things will move? So, to counteract this we need an "internal moment" provided by the beam at C to counter act this.

Edit: Also, was my professor incorrect here in his direction scheme? It seems like he's taking the moment applied to C from B as negative even though he set the positive moment direction to be in the CW direction. enter image description here enter image description here


1 Answer 1


They mean the same thing to me. Although I suppose you could interpret "moment at a point" to be a specific moment due to a particular cause which composes part of the sum.

As for the sign convention. I think your professor did goof. He carried over the negative sign from the Vc = -6 from the Y-axis linear calculations. Not to mention he also defined his rotation axis (the z-axis) with respect to the X and Y axis contrary to the typical definition of the cross product.

  • $\begingroup$ Thanks for the reply. Yea, this slide really tripped me up since the whole sign convention isn't very intuitive for me in the first place. Quick question, what do you mean by "they mean the same thing to me"? Do you mean, that the sum of moments at C, is essentially equal to the internal moment seen at C (or applied at C)? $\endgroup$ Commented Jun 21, 2021 at 0:56
  • $\begingroup$ To me, "moment at a point" could be used to mean "the sum of all moments about a point" or the net moment about a point in a sentence. Which in this case, is zero since it is static. But I suppose someone could interpret "moment at a point" to mean just one of the terms of the sum (i.e. the internal moment). I would just take it on context for what makes sense. $\endgroup$
    – DKNguyen
    Commented Jun 21, 2021 at 1:13

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