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Homework Help: Conceptual Friction problem

  1. Sep 27, 2009 #1
    1. The problem statement, all variables and given/known data
    There is a box drawn on a horizontal surace. A force is being applied at an unkown angle in the positive x direction to the box. The angle is greater than 0 and less than 90. I'm assuming a standard coordinate system.

    "In the diagram above the box is stationary as the angle [tex]\theta[/tex] is increased. Do the following increase, decrease or stay the same when [tex]\theta[/tex] is increased? Explain each answer."

    1. Fx
    2. Normal Force
    3. fs
    4. fs-max

    2. Relevant equations

    None given.

    3. The attempt at a solution

    Fx decreases when theta is increased. If the force is broken down into vectors, the geometry of a right triangle tells us that as theta is increased the x-component decreases.

    Normal force decreases when theta is increased. Normal force is perpendicular to the surface of contact. When theta increases, the y-component of that force increases. This opposes the force of gravity on the box. The normal force decreases because there is less net force in the negative y-direction.

    Static friction decreases because it is dependent on the force applied in the y-direction(in this case). Less normal force equals less static friction here.

    The max static friction also decreases because it is dependent on the force applied in the y-direction (in this case). Less normal force equals less max static friction.

    Does that seem right to you guys? I feel confident in my answers, but I want to make sure I am not assuming or looking over something. Feedback is always appreciated. Thanks a bunch.

    Last edited: Sep 27, 2009
  2. jcsd
  3. Sep 27, 2009 #2
    Im assuming it looks like this, f is pushing right and downwards.
    f \
    If you apply the same force and the angle increase, cosine decreases, so Fx decreases.
    The normal force is always perpendicular to the surface. You would have mg down and Fy down. Fy is increasing which means what for the normal force?
    fs opposes fx up until fmax. So if Fx gets smaller, fs gets smaller.
    fmax = mu N If N ____ then fmax ____.
  4. Sep 27, 2009 #3
    Thanks for the reply,

    F is pushing right and upwards. Sorry about that. The drawing is an angle of approximately 30 degrees but it is not labeled.
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