Physics Help Forum Moment diagram

 Oct 29th 2016, 04:52 AM #1 Senior Member   Join Date: Jun 2014 Posts: 306 Moment diagram for the moment diagram, why the 400Nm straight line is drawn from the point of moment to A only? why shouldnt the 400Nm straight line is drawn from the point of moment to B only? Attached Thumbnails   Last edited by ling233; Oct 29th 2016 at 08:22 AM.
 Oct 29th 2016, 06:47 AM #2 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,287 In drawing moment diagrams for each point along the x-axis you pretend that the beam is cut and you determine the moment you would have to apply to either the left or right sections at that point to keep the beam stable. The technique that most people use is to work from left to right, but it would be equally valid to work from right to left if you want. ling233 likes this.
Oct 29th 2016, 08:30 AM   #3
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 Originally Posted by ChipB In drawing moment diagrams for each point along the x-axis you pretend that the beam is cut and you determine the moment you would have to apply to either the left or right sections at that point to keep the beam stable. The technique that most people use is to work from left to right, but it would be equally valid to work from right to left if you want.
do you mean it 's also correct if i wanna cancel off the (400Nm act on A up to 2m from A ) and move to (from the point from 2m from A up to point B ) ? just like what i did in the photo attached here ?
Attached Thumbnails

 Oct 29th 2016, 09:43 AM #4 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,287 If you work right to left then at point A you have the sum of 600N x 6 m - 1000N x 4m + 400N-m, which equals zero, so all is good. But be careful of signs an inverted moment diagram - working right to left will give you negative values where left-to-right gives positive. I'm not sure what you mean by "cancel off" the 400N-m moment. ling233 likes this.
Oct 29th 2016, 04:36 PM   #5
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 Originally Posted by ChipB If you work right to left then at point A you have the sum of 600N x 6 m - 1000N x 4m + 400N-m, which equals zero, so all is good. But be careful of signs an inverted moment diagram - working right to left will give you negative values where left-to-right gives positive. I'm not sure what you mean by "cancel off" the 400N-m moment.
What i mean is move the 400Nm moment from A to point of moment to point of moment to point B , just like what i did ....

Can i do so ? is my concept correct ?
Attached Thumbnails

 Oct 30th 2016, 08:59 AM #6 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,287 If you work right-to-left the 400 N-m moment only exists in the region between A and 2m to the right of A (where the moment is applied) - it's zero from that point to B. See the attached figures - the one on the left is the moment diagram working left-to-right, and the one on the right is the diagram working right-to-left. Note how the one on the right is upside down. This is why by convention it's best to work left-to-right. ling233 likes this.

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