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Homework Help: Normal force

  1. Aug 20, 2010 #1
    1. The problem statement, all variables and given/known data
    How to know when one is suppose to add or subtract to get the normal force of an object.

    2. Relevant equations
    FN = Fg, FN = F sin(-)

    3. The attempt at a solution

    I believe to get the normal force of a 2 kg object when it is being pulled upwards by a force of 45 Newtons at an angle of 40 degrees is:
    FN = 45 N sin(40) - 19.6 N
    FN = 9.32 N

    or is it:

    FN = 45 N sin(40)
    FN = 28.9 N

    Which one is correct? How do I know when to add or subject the normal force and gravitational force?
  2. jcsd
  3. Aug 20, 2010 #2
    I usually draw the FBD and look at my positive and negative values.

    Fn is perpendicualr to the slope of the incline, therefore it's equal to the y component of mg.
  4. Aug 20, 2010 #3
    How is it the normal force equal to the force of gravity? That is impossible with an incline. It is only possible if the mass is on a flat surface. However, I see your point of labeling the FBD. Therefore, my example that I used, the mass was being pulled upwards because the normal force was higher than the force of gravity?
  5. Aug 20, 2010 #4


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    Homework Helper

    If you push a cart with a force making an angle θ with the horizontal on the horizontal surface, then the normal force = mg + Fsinθ.
    If you pull the cart in the above case, the normal force = mg - Fsinθ.
    In your problem two forces are acting on the body.Net normal force is the sum of the components of two forces perpendicular to the surface of the inclined plane. 45 N is acting parallel to the inclined plane. So it has no vertical component with respect to the inclined plane. Hence the normal reaction is mg*cos(θ)
    Last edited: Aug 20, 2010
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