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Homework Help: Sliding box problem

  1. Oct 21, 2007 #1
    1. The problem statement, all variables and given/known data
    A student is pulling a 25 kg trunk up an incline that has an angle with the horizontal of 20 degrees. She has a strap tied to the trunk which is parallel to the surface of the incline. If the coefficient of kinetic friction between the incline and the trunk is 0.5 fine
    a)Force Normal
    b) the force necessary to keep the trunk moving at a constant velocity.
    c)the force she would have to apply to accelerate the trunk up the incline at 0.5 m/s squared.

    2. Relevant equations
    coeff.=(force friction)/(force normal)

    3. The attempt at a solution

    So I think I found (A) to be force normal = 230N by doing Fn=(m)(g)(cos20)
    I tried doing this for B (25)(-9.81)(sin20)-(0.5)(25)(-9.81)(cos20) + ForceX =0
    I got 31 but thats not right...
  2. jcsd
  3. Oct 21, 2007 #2


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    Staff Emeritus
    Science Advisor

    (25)(-9.81)(sin20) + (0.5)(25)(-9.81)(cos20) + ForceX = 0

    The friction force opposes motion, so it must point down the ramp, and the weight component parallel to the ramp must also point down the ramp. So these two force must have the same sign (direction). Both oppose the pulling force up the ramp.
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