I was just working on a mechanics problem of finding the stress at a cross(adsbygoogle = window.adsbygoogle || []).push({});

section of a member. The example problems on mechanics give the solution as

follows.

1.cut a freebody diagram at the cross section of interest

2.The solution then says that a force acts normal to the surface(normal force),one more force parallel to the surface(shear force) and force that

turns the section(bending moment and hence a force).

3.Put an arrow representing all the forces.

My worrying and confusing point is point no.3. Inside the cross section where should i put the force. For example for a normal i can put an arrow on the normal to the surface but at any point.

usually all the example problems show the force at the centre of the section

(centroid).

I will now come from the basics. A force is a push or pull. When i put a force

at the centroid the centroid(small area around the centroid) will be pushed to some depth compared to their surrounding. but when i apply a force(push/pull) that point(a small area) only will be pushed

or pulled.

So which is the correct place in the cross section to be considered for a force to act?

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# Force on a surface

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