How is the slope of the shear force equal to the negative of the load?

AI Thread Summary
The discussion centers on the relationship between shear force and load in beam mechanics, specifically addressing the negative slope of the shear force diagram when a downward point load is applied. It highlights confusion regarding the shear force sign convention, noting that while the slope should be positive according to the equation, the actual representation is negative due to the downward direction of the load. Participants reference a paper that treats the load as positive downwards, clarifying the convention used. The conversation also touches on the importance of understanding shear force in relation to external forces acting on a beam. Overall, the forum serves as a platform for further questions and clarification on this topic.
Svelte1
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Homework Statement
dV/dx=-W
Relevant Equations
dV/dx=-W
Capture.PNG
At any point between A and C the point load is negative (downwards), in the shear force diagram: positive is upwards, so this slope is negative. The equation says the slope should be positive. Is this something to do with shear force sign convention?
 
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Personally, I don't think I've ever understood the shear convention myself. I thought it acts to oppose the external forces on the beam so that the beam remains static, but I'm not sure now. I look forward to hearing other's thoughts on the matter.
 
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So based on the convention given in the paper that @haruspex shared, the shear being positive from ##\rm{A}## until around ##x_1 = 5.7 \rm{m} ## where ##V=0## means the beam is deflecting like:

positive shear.jpg


and after ##x_1## the beam in deforming like the mirror image of that:

Negative shear.jpg
 
Ok, thanks guys. Assuming it's a load makes sens I suppose. Also I have a lot of questions around this level in the coming weeks, is this the correct forum for that? Thanks.
 
Svelte1 said:
is this the correct forum for that? Thanks.
Should be fine, but there is also
https://www.physicsforums.com/forums/engineering-and-comp-sci-homework-help/
 
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