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Homework Help: Missing sumthing really dumb. Please help?

  1. Aug 8, 2007 #1
    1. The problem statement, all variables and given/known data

    A block of mass M = 3 kg is released from rest and slides down an incline that makes an angle q = 32° with the horizontal. The coefficient of kinetic friction between the block and the incline is µk = 0.15.

    What is the acceleration of the block down the inclined plane?

    2. Relevant equations
    this is the easiste part of 4 part question and i cant get it.

    3. The attempt at a solution
    OK heres what i know
    Normal = 3kg * (9.8sin32)
    so friction is

    fs = .15*15.58 = 2.34N
    so i'm taking x axis as the incline plane, (pos x = down the plane)
    friction opposes the motion, so force runs along neg axis = -2.34N

    forces in pos x direction are
    3 * 9.8cos32 = 24.93N

    24.93N - 2.34N = 22.59N (this is the net force)

    Finally F/m = a
    22.69/3 = 7.53m/s/s

    = wrong.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    Last edited: Aug 8, 2007
  2. jcsd
  3. Aug 8, 2007 #2


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    Science Advisor

    Looks to me like you have sine and cosine reversed. You draw the inclined plane as a right triangle having angle 32 degrees at the bottom. The force of gravity is straight down, the two legs of that triangle are perpendicular and parallel to the inclined plane- the 32 degree angle is the bottom of that triangle. The normal force is given by cos(32), the force along the incline by sin(32).
  4. Aug 8, 2007 #3
    yeah yer right
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