I was just working on a mechanics problem of finding the stress at a cross 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?