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Force on a surface

  1. Apr 12, 2005 #1
    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

    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

    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?
  2. jcsd
  3. Apr 12, 2005 #2


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    Last edited: Apr 12, 2005
  4. Apr 13, 2005 #3
    the first one in the image, the tensile force on the surface is of confusion to
    me. How can we say that the force acts at the centre of the surface.
  5. Apr 15, 2005 #4
    Do you have a picture of the member the problem refers to, which shows the external force(s) acting on it?
  6. Apr 16, 2005 #5
    my question is as follows. To analyse force components on a surface
    we usually put the force symbol at the centroid.

    Imagine that we push(i.e give a force) at the centroid of a surface then
    the small area at the centroid only will be pushed and not the entire surface

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  7. Apr 16, 2005 #6


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    It is an assumption that is made in most cases that is very much justifyable. Most times when dealing with basic stress analysis, the loading is in the elastic range of the material so no permanent deformations occur and the loading is gradually applied. It is a simplifying assumption that the load is placed in the centroid and the material to facilitate an equal stress distribution across the section you are looking at. This is not always possible or wise, but when learning the basics, it is perfectly acceptable to assume that the stress will be evenly distributed across the face.
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