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Homework Help: Forces/coefficient of friction

  1. May 13, 2005 #1
    A box of canned goods slides down a ramp from street level into the basement of a grocery store with acceleration 0.65 m/s2 directed down the ramp. The ramp makes an angle of 35 degrees with the horizontal. What is the coefficient of kinetic friction between the box and the ramp?

    I have been trying to figure this out for a while. Heh.

    So far I drew my picture. One thing I am confused about is what direction friction is in.
    I thought that since the box is sliding downward with a force, then the frictional force is up and opposite of that sliding force. I might have this mixed up. I dont know if there is a force pushing it down.

    Then I broke it up into X component forces:

    Fa - friction (making the downward direction positive) + Wx = ma
    Then I became confused about the Fa and wondered if it even belongs.

    So then:
    -f + wx = ma
    -Mu(mg) + mgsin35 = ma (then cancel out the masses)
    Mu(g) + gsin35 = a

    Mu(g) = a - gsin35
    Mu = a - sin35

    I have done something wrong. If anyone can help me figure this out, I would really appreciate it. I have tried my best! :cry:
  2. jcsd
  3. May 13, 2005 #2


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    There are three forces in the problem, one of which you are going to break into components.

    1) Gravity (weight)- points straight down- break it into a component parallel to the plane and a component perpendicular to the plane

    2) Normal force- points opposite and is equal in magnitude to the gravity component perpendicular to the plane.

    3) Friction- points up the plane and is equal to the magnitude of the normal force times the coefficient of friction. The sum of the opposing forces (difference of magnitude) of the gravity component parallel to the plane and the friction force is the net force acting on the box, so it is equal to the mass of the box times the acceleration.

    If you get the directions of those forces right, and you can do the trig right to break the gravity into two components, you will get the answer.
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