Understanding Moment Direction: Analysis and Solution for Homework

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Discussion Overview

The discussion revolves around the interpretation of a moment diagram related to a beam in static equilibrium, specifically addressing why a moment of -400 Nm is indicated at certain points and how it relates to moments at other points on the beam. Participants explore the implications of moment direction and magnitude in the context of beam mechanics, with a focus on homework-related queries.

Discussion Character

  • Homework-related
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants question why the moment -400 Nm is only acting at regions B and C and not from A to C.
  • Others explain that the moment at B is a couple acting on the beam, and a moment reaction develops at C to maintain static equilibrium.
  • There is a discussion about the direction of the moment at B being counterclockwise (CCW) and at C being clockwise (CW), leading to confusion over their signs in the diagram.
  • Some participants assert that the moment at B is positive 400 Nm, while the reaction at C must be -400 Nm to ensure the net moment is zero.
  • Questions arise regarding the representation of the moment in the diagram and why it appears as negative from B to C, with some participants suggesting misinterpretation of the diagram's presentation.
  • Participants discuss the nature of the moment diagram, including how it represents changes in moment values and the necessity of not overlapping diagrams for clarity.

Areas of Agreement / Disagreement

Participants express differing interpretations of the moment diagram and the signs of the moments at points B and C. There is no consensus on the correct interpretation of the diagram, and multiple competing views remain regarding the representation and understanding of the moments involved.

Contextual Notes

Participants note that the moment diagram does not plot discrete values in isolation and that certain allowances must be made in the diagram's presentation to avoid confusion. There are also indications of potential misinterpretations of the moment values and their signs.

  • #31
chetzread said:
Why the moment is restricted about A, but not at C?

I don't know what you mean by "restricted about A, but not at C".

The beam is fixed at C, so a reactive moment is supported there. The beam is free at A.
 
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  • #32
SteamKing said:
I don't know what you mean by "restricted about A, but not at C".

The beam is fixed at C, so a reactive moment is supported there. The beam is free at A.

https://www.physicsforums.com/threads/moment-diagram.878559/#post-5520349
I am sorry for the previous post...
I mean why in this thread, the moment of the beam is not restricted about B , but is restricted about A ?
I
 
  • #33
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  • #34
SteamKing said:
I still don't know what you mean by the moment being "restricted" about A. There is no moment about A.
As you can see, the moment 400Nm graph is drawn from A to the point where moment 400Nm act, so there's moment restriction about point A, right? Why there's no moment restriction about point B?
https://www.physicsforums.com/threads/moment-diagram.878559/#post-5520349
 
  • #35
chetzread said:
As you can see, the moment 400Nm graph is drawn from A to the point where moment 400Nm act, so there's moment restriction about point A, right? Why there's no moment restriction about point B?
https://www.physicsforums.com/threads/moment-diagram.878559/#post-5520349
There is no moment applied at point A, so there won't be one showing up on any moment diagram.

There is a moment applied at point B, which is why the moment diagram starts there.
 
  • #36
SteamKing said:
There is no moment applied at point A, so there won't be one showing up on any moment diagram.

There is a moment applied at point B, which is why the moment diagram starts there.
Sorry, I think u misunderstood me... I refer to the thread below...
https://www.physicsforums.com/threads/moment-diagram.878559/#post-5520349
why the moment of the beam is not restricted about B , but is restricted about A ?
 
  • #37
SteamKing said:
If you look at the moment plot at C, it clearly indicates that the reactive moment there is -400 N-m, which, when added to the moment at B of +400 N-m, equals zero net moment.
Do you mean that since the moment at B is positive 400Nm, so the moment -400Nm must be restricted at any points along beam BC to keep it in equilibrium?
Your interpretation of what the plot would look like would require a net change in moment of -800 N-m, which is not supported by the loading of the beam.
 
  • #38
  • #39
SteamKing said:
It's not clear what you mean here.

The moment diagram doesn't plot discrete values of BM in isolation, like you apparently want to do.

At point B, the moment goes from 0 to +400 N-m, which is why there is a jump at this location.

The moment remains constant at +400 N-m from point B to point C, where the reactive moment of -400 N-m is added to the +400 N-m, to bring the net moment back to zero.

You should review how shear force and bending moment values are plotted for regular beam problems.
it it because of the reaction moment and moment causes the part of beam to become concave downwards, so bending moment is negative?
 
Last edited:

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