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Normal force acting on a ladder lying against a wall

  1. Aug 1, 2010 #1
    Hi.. I am just confused normal forces. The definition of normal force is always perpendicular to the surface. But when we draw a free body diagram for a ladder lying against the wall. The normal force is like slanted towards the wall. Please explain thanks.
     
  2. jcsd
  3. Aug 1, 2010 #2

    Doc Al

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    No, the normal force between the wall and ladder is always perpendicular to the wall. By definition! But if there's friction on that wall, the wall may also exert a vertical force on the ladder.
     
  4. Aug 1, 2010 #3
    Hi there. Here is my two diagrams. The correct answer is the first one. Why cant it be the 2nd one? the normal force acting on the ladder on the ground,shouldnt it be perpendicular to the ground ( normal force definition)? for the normal force on the wall due to the wall also shouldnt it be perpendicular too?Thanks
    Untitled.png
     
  5. Aug 1, 2010 #4

    Doc Al

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    Who says?

    I'd say that the second diagram is the correct one. (Assuming no friction between ladder and wall.) The first diagram doesn't make sense.
     
  6. Aug 1, 2010 #5
    The qn given was that it has friction between the ladder and the ground and also between the ladder and the wall cos to prevent the ladder from slipping. So if we taken into account of friction then the normal force will not be perpendicular? y is it so?
     
  7. Aug 1, 2010 #6

    Doc Al

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    Are you sure? You don't need friction between ladder and wall to prevent slipping, only between ladder and floor.

    Why don't you post the exact problem that those diagrams are supposed to describe.
    No. The normal force is always perpendicular to the wall.

    (If I were drawing a diagram, I would always show the forces at wall and floor in terms of vertical and horizontal components. Much cleaner that way.)
     
  8. Aug 1, 2010 #7
    QN: A ladder of weight W rests against a vertical wall. Friction between the ladder and the ground and also between the ladder and the wall prevents the ladder from slipping. Which diagram shows the directions of the forces on the ladder? The answer given was C. So the forces that is shown is not the normal forces? both on the ground and wall?
    Physics.png
     
  9. Aug 1, 2010 #8
    anyway if we like draw the net force, wouldnt we be drawing the net force instead of the forces that is acting on it? for free body diagrams we are always drawing the indidivual forces? am i right?
     
  10. Aug 1, 2010 #9

    Doc Al

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    Correct. They are showing the total force of wall on ladder and floor on ladder, not just the normal force.

    Do this. Show the components (normal force and friction), then add the components to see how the total force points at some angle.
     
  11. Aug 1, 2010 #10
    Ok thanks. Here is another qn on free body diagram. A crane is made from a uniform rigid boom of length 30m and weight 400N, as shown. The boom is supported at its lower end by a frictionless hinge on the ground. ITs upper end is attached to a support case that is fastened on the ground behind the boom. The support cable has variable length, hence allowing the boom to elevate the load. In a particular application, the crane is used to support a crate of weight 2.0 kN. The boom makes and angle of 45 degrees with the horizontal and the support cable makes an angle of 30 degrees with the horizontal. Complete a free body digram of the boom showing all forces acting on it. Label each force clearly. My qn: I thought they are merely asking for the forces not NET force. how come the reaction force on the floor is the first one and not the 2nd one? thanks
    Physics 2.png

    Physics 3.png
     
  12. Aug 1, 2010 #11

    Doc Al

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    Yes. In a free body diagram, one shows all the forces acting on the ladder: gravity, force of wall, force of floor.

    But you can represent the force of wall on ladder by its components (normal force and friction) or by a single force vector that includes everything (like they do in this example). If you were actually solving a numerical problem, most likely using the components would be more useful. In this particular problem, they just want to test if you know which way the forces are acting.
     
  13. Aug 1, 2010 #12

    Doc Al

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    They are just asking for the forces acting on the boom, not the components of those forces.
    The floor is attached to the boom. Why would you assume that it can only exert a vertical force? (There'd better be a horizontal component, otherwise what will balance the horizontal component of the cable tension?)
     
  14. Aug 1, 2010 #13
    ok thanks! appreciate it!
     
  15. Jun 23, 2011 #14
    woahhh thanks for this! i was getting stuck in my own physics and found this online :) anw just to clarify, in real life there isnt such thing as a frictionless wall so the correct resultant force on the ladder from the wall is never perpendicular to the wall right?
     
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