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Homework Help: Find least force?

  1. Aug 14, 2010 #1
    find least force?????

    1. The problem statement, all variables and given/known data
    a safe weighing 10 tons is to be loaded on a truck 5.0 ft high by means of planks 20 ft long if it requires 350lb to overcome friction on the skids, find the least force nescessary to move the safe.


    2. Relevant equations



    3. The attempt at a solution
    i dont know where to start pls help me....
     
  2. jcsd
  3. Aug 14, 2010 #2
    Re: find least force?????

    what is the formula for finding least force help me pls????????????????????
     
  4. Aug 14, 2010 #3

    Doc Al

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    Re: find least force?????

    What force parallel to the planks is required to just overcome gravity and friction?
     
  5. Aug 14, 2010 #4
    Re: find least force?????

    sir doc al will i use the cosine law???? thx for the repz..
     
  6. Aug 14, 2010 #5
    Re: find least force?????

    will i use conversion with this prob???? ton??
     
  7. Aug 14, 2010 #6
    Re: find least force?????

    i convert ton in kg???
     
  8. Aug 15, 2010 #7

    Doc Al

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    Re: find least force?????

    Keep everything in terms of pounds. 1 ton = 2000 lbs (at least in the US).

    You'll need to find the component of the weight parallel to the incline.
     
  9. Aug 15, 2010 #8
    Re: find least force?????

    i convert 10 tons to 20000 lbs? then i get the angle 5/20=0.25cos=14.50degrees? i calculate force of gravity 20000 x 9.8 = 196000.. then the normal force 196000/cos14.50=184757.7.....then i get the parallel to the incline 196000/sin14.50=49074.5 is that correct??? whats the next step??? pls reply thxxxxx...
     
  10. Aug 16, 2010 #9

    Doc Al

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    Re: find least force?????

    Yes, that's the weight.
    OK. The sine of the angle is 5/20 = 0.25; thus that angle must be 14.5 degrees.
    No, you already have the force of gravity. 20000 lbs is a force, not a mass.
    No, not correct. You have the wrong value for the force. But beyond that, you are calculating the components incorrectly. Read this: http://www.physicsclassroom.com/Class/vectors/U3l1e.cfm#trig"

    Once you get the correct parallel component of the weight, you need to add the friction force to get the total force needed.
     
    Last edited by a moderator: Apr 25, 2017
  11. Aug 16, 2010 #10
    Re: find least force?????

    ok normal force= 20000sin14.50=5007.6 and theparallel component 20000cos14.50=19363 is this correct?????
     
  12. Aug 16, 2010 #11

    Doc Al

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    Re: find least force?????

    You have them mixed up. The angle is with respect to the horizontal, so the normal component of the weight would be Wcosθ and the parallel component would be Wsinθ.
     
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