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Most effective Pulling

  1. May 15, 2014 #1
    1. The problem statement, all variables and given/known data

    An initially stationary box of sand is to be pulled across a floor by means of a cable in which the tension should not exceed 1100 N. The coefficient of static friction between the box and the floor is 0.35.
    (a) What should be the angle between the cable and the horizontal in order to pull the greatest amount of sand, and
    (b) What is the weight of the sand and box in that situation?

    2. Relevant equations

    fs-max = FNμs

    3. The attempt at a solution

    Because the block has to move after all,
    the applied force's x-component must be greater than the frictional force that will oppose it by acting in the opposite direction with a maximum magnitude as static frictional force max fs-max.
    So, T cosθ > μs(mg-T sinθ)
    rewrite, T(cosθ + μssinθ)> 3.43 m
    Besides 0 < T < 1100 N
    So, 0 < T(cosθ + μssinθ) < 1100(T(cosθ + μssinθ))
    And if 3.43 m is already lesser than T(cosθ + μssinθ),
    It surely is lesser than 1100(T(cosθ + μssinθ))..
    rewriting i get,
    cosθ + 0.35 sinθ > 3.1*10-3 m------------------------(1)
    Here, I got a gut feeling :rofl: and a bit of crooked logic telling me that since the RHS is lesser than the LHS, I should probably be finding the maximum value of the LHS, which would give me the corresponding maximum value for the RHS.
    So, I did the maxima for the LHS and got θ=19.3°
    this, i resubstituted in (1) to get
    m < 341.9 kg.
    And so the (a) 19.3°
    and (b) weight of the load so (341.9)(9.8)=3.35*103
    which tallied with the answers at the back.
    BUT is there another method to attack the problem? Is my logic crooked? Any better explanation for the key step where i find θ using maxima? Any advice to tidy up my reasoning in the rest of the solution is also highly welcome! Thanks!

    Attached Files:

  2. jcsd
  3. May 15, 2014 #2


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    looks real good! I guess you could take the second derivative to be sure you have a max and not a min, which maybe you did, as you did not show calcs for determining the angle for max mass.
  4. May 16, 2014 #3
    Yes, @PhantomJay! Thanks!
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