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How a non uniform force, converted to an equivalent point force acts on an object.

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MatthewMansfield

Civil/Environmental
Aug 11, 2012
47
Hello all

I am trying to understand how a non uniform force, converted to an equivalent point force acts on an object.

I was hoping I could get the communities opinion on the following sketch I have made:-

fd_fxfjrn.jpg


In Diagram A have a non uniform force that is being applied to my shape.
The centre of mass & centroid are the same location as the density of the material is uniform.

In Diagram B I have calculated the X,Y,Z position of the force distribution shown in blue.
I now have 2 x centroids, one for my shape and one for my force distribution.

In Diagram C I have projected the centroid of my force distribution onto my shape shown in yellow.

If I converted my non uniform distributed load into an equivalent point load then would it be correct to say that the point force would act at the projected centroid (yellow) and not the centroid of my shape - in other words will the equivalent load act on the projected centroid (shown in yellow) or through the centroid of the shape (shown in black)

Is this correct?

Thank you.
 
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In terms of free body diagram (ie net forces/reactions on the body) it’s the yellow point, however be aware that in terms of internal forces (eg forces on the wall) it’s not the same internal reactions.
 
MatthewMansfield:
You have to be aware, stay aware, of how a given loading works on, or interacts with the structure. The centroid of your actual structure may not actually have much to do with it. You should really study your Statics and Engineering Mechanics textbooks a bit, to help clear this up. This is pretty elementary stuff. Maybe you should ask your Prof. to explain this better, so the entire class benefits from the explanation.
 
draw a free body diagram of the loaded element and make sure that the boundary forces are the same. Yellow dot is right.

That is, a 'tapered' force field will generate a torque and a force on the face of the element, or if you prefer, the force is offset from the centroid of the element, in your case by 1/6 of the length of the element, making obvious assupmtions.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
if you're only interested in internal stresses away from the square plate (like at its boundary and beyond) then yes, any loading that produces the same reactions along the boundary is acceptable. The way you express yourself "center of my object", I don't think so.

if you're looking at the square plate, your object?, you'll be hopelessly mislead. it is almost certainly conservative in some places, and unconservative in others. Does this matter, who knows. You should work through some examples to see for yourself, and so you don't defend your work (if you have to) with "some guys on the interwebs said this approach was good".

another day in paradise, or is paradise one day closer ?
 
Your loading condition does not create a point load but a line load. Then for the retaining structure it is analyzed as a load per foot or per unit depending on your preferred units of measure.
 
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