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Unbalanced force - at rest

  1. Oct 26, 2014 #1
    Hi,

    i'm stuck on a mechanics problem where the initial conditions are a mass at rest on an inclined plane, but the forces are seemingly unbalanced.

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

    A 5.17 kg box sits at rest at the bottom of a ramp that is 8.56 m long and that is inclined
    at 40.0◦ above the horizontal. The coefficient of kinetic friction is 0.40, and the coefficient
    of static friction is 0.50.
    What constant force F, applied parallel to the surface of the ramp, is required to push the
    box to the top of the ramp in a time of 4.22 s ?

    2. Relevant equations


    3. The attempt at a solution

    I have drawn a freebody diagram with the weight of the box acting straight down. perpendicular to the slope, we have the component $$mg \cos(\theta)$$ and the normal force being $$-mg \cos(\theta)$$. parallel to the slope, a component of the weight acts down the the slope $$mg \sin(\theta)$$ and the force of static friction opposes this motion.

    Static friction $$f_{s} \leq \mu_{s} n$$ where $$n = -mg \cos(\theta) = -19.43 N$$

    This is the max force that can be applied parallel to the slope without the box beginning to move, but the component of weight acting down the slope is greater than this, so why is the box not moving?

    Was I supposed to consider "at the bottom of the slope" to mean the normal force = weight?
     
  2. jcsd
  3. Oct 26, 2014 #2

    PhanthomJay

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    Science Advisor
    Homework Helper
    Gold Member

    Maybe it is just being held there before the force parallel to incline is applied. It is just a pre-condition to start the problem.
     
  4. Oct 26, 2014 #3
    It seems to me that I don't need this information to solve the problem.

    The applied force is parallel to the slope, so kinetic friction remains constant. Using the equations of kinematics, I can determine the acceleration required to cover the distance in that time. Using newton's second law, I can determine what the net force must be, and therefore the applied force is equal to the difference between the kinetic friction and resultant force.
     
  5. Oct 26, 2014 #4

    Doc Al

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    Staff: Mentor

    You need to know that it starts at the bottom and that it starts from rest. No hidden meaning beyond that.
     
  6. Oct 26, 2014 #5

    Thanks
     
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