Why is the 400Nm straight line drawn from the moment point in moment diagrams?

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Homework Statement

for the moment diagram, why the 400Nm straight line is drawn from the point of moment to a only?

Homework Equations

The Attempt at a Solution

Is it wrong? I think it should be drawn throughout the entire length of beam? since moment can act at anywhere on the beam...[/B]

 

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or the moment 400Nm will only causes the region from A to 2m away from A to break ? So , in the moment area diagram , it's drawn from A and 2m away from A ?

 

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https://www.physicsforums.com/threads/direction-of-moment.878803/#post-5520313

In the previous thread , i was told that to keep the beam in equilibrium, the moment has to be applied at the another end (wall)in opposite direction
But, in this question, i have 2 ends, namely A and B, but both point doesn't restrict moment, am i right?
In the previous therad, it's wall, so it can restrict moment...But , how about this case?@SteamKing

 

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