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Normal Force ?

  1. Jan 26, 2015 #1
    I know that normal force is a reaction force.

    http://www.sumoware.com/images/temp/xzhhpaacgkdktjna.png [Broken]
    In the leaning ladder problem, the normal force is perpendicular to the floor and the wall.
    But, in the cable and beam problem, why isn't the normal force perpendicular to the wall ?

    http://www.sumoware.com/images/temp/xzeanhxxmcalmnkn.png [Broken]

    Hmm.. Then, I imagine what the system (cable and beam) will be without the wall.
    I think that the beam will fall down.
    And, because the beam touches the wall, the wall will exert a force back to the beam at right direction.
    So, the normal force will have up and right direction.

    But, I imagine the ladder problem.
    But, Why the N1 doesn't have the upward direction ?
    I think if there is no wall, then the ladder must fall and rotate counter-clockwise.

    Please explain me how to determine the normal force direction.
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Jan 26, 2015 #2


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    A normal force is by definition normal to the surface. If there is nothing else holding the beam at its contact point, it will fall down. The "something else" can be a frictional force due to a rough surface (in which case the resultant force will not be normal to the wall) or that the beam has been attached to the wall.
  4. Jan 26, 2015 #3
    So, the force in up direction on the hinge is the frictional force
    And the force in right direction on the hinge is the normal force
    The resultant is the frictonal force + the normal force
    Do I get this right?

    And, if the beam is attached to the wall, is there any normal force ?
  5. Jan 26, 2015 #4


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    If you have a hinge, it is a completely different situation. There is a normal force from the hinge, but the hinge contact point is not tilted in the same direction as the wall. It is therefore misleading to talk about it as a normal force to the wall.
  6. Jan 26, 2015 #5
    From the picture above, I have no idea about 'not tilted in the same direction as the wall'.
    Then, how to know that the normal force is up and right in direction ?
    What will the condition be if the normal force is down and right, or down and left, or etc ?
  7. Jan 26, 2015 #6


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    You do not, you can guess it (based on the other forces present) and define your force components in such a way, but you will not know for sure until you do the equilibrium analysis and find out what the components of the force are. The condition is that the beam is in equilibrium and the rest should follow from there.
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