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Homework Help: Force problem (not for the faint of heart)

  1. Dec 3, 2013 #1
    1. The problem statement, all variables and given/known data

    Bob is pulling a sled up a 50 degree hill (meaning the incline is at an angle of

    fifty degrees above the horizontal). Sitting on the sled is Bob's niece (unknown

    mass). After pulling the sled 30.0 meters up the incline, Bob slips and falls

    releasing the rope attached to the sled. The sled (starting from rest) starts sliding down

    the incline. After traveling the 30.0 meters down the hill, the sled travels 50.0 meters on

    a wide open horizontal surface before finally coming to a stop. Assuming the coefficient

    of kinetic friction is the same throughout the entire problem (hill and level ground), find

    the coefficient of kinetic friction. If the coefficient of static friction between the sled and

    the ground was twice the coefficient of kinetic friction, would the sled move at all?

    Hint #1: Treat this as two different parts with two different coordinate systems

    (Part 1- on the hill Part 2- on the level ground)

    Hint #2: The speed that the sled reaches at the bottom of the hill is the same speed the

    sled starts with when traveling on the level ground.

    2. Relevant equations

    F=ma Ff=μFn Fgx=mgsinθ Fgy=mgcosθ Δx=viΔt+(1/2)aΔt^2 vf^2=vi^2+2aΔx Δx=vfΔt-(1/2)aΔt^2 vf=vi+aΔt Δx=(1/2)(Vi+Vf)Δt
    and any other equation that could be used to solve this problem
    (kinematic and force equations)

    3. The attempt at a solution

    I have no clue on how to solve this problem, and any help would be greatly appreciated
  2. jcsd
  3. Dec 3, 2013 #2


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    Staff: Mentor

    Welcome to the PF.

    You *must* show some effort in working toward the solution before we can offer any tutorial help. That's in the PF Rules that you agreed to when you joined (see Site Info at the top of the page).

    Start problems like this with a free body diagram (FBD), and list all of the forces acting on the FBD. You will use two -- one for on the slope, and one for on the flat.

    Please show us your work...
  4. Dec 3, 2013 #3


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    Gold Member

    The first thing to check with a dynamics problem is whether you can use considerations of energy. If you can, this is often quicker than developing the force and acceleration equations. A good test is whether you are told or asked for the time elapsed. If not, energy is probably the right way.

    Suppose the coefficient of kinetic friction is μk. Can you figure out the magnitude of the frictional force when sliding down the hill? What work is done overcoming that? What does that tell you about the KE at the bottom of the hill?
  5. Dec 3, 2013 #4


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    Bob did well to pull a sled up a 50 degree slope! That would be a grade I winter climb. As the title of the post says: not for the faint of heart!
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