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Homework Help: Demode cant simplify equations =(

  1. Oct 23, 2006 #1
    I was given the problem: "What minimum force is required to drag a carton of books across the floor if the force is applied at an angle of 45 degrees to the horizontal? Take the mass of the carton as 40 kg and the coefficient of friction as 0.60.

    Free Body Diagram (Given by physics teacher)
    http://img327.imageshack.us/img327/4383/untitled1yz4.th.jpg [Broken]

    [tex]F\sin(45) + F_n = mg[/tex]
    [tex]\mu_k F_n = F\cos(45)[/tex]

    Now for the life of me, I can't seem how to solve for F since the question asks for the minimum force.. Will i just need to solve the system of equations that I have?

    After some simple math and slapping myself repeatedly for being so blind, i came up with 207.41 as the force.. Would anyone be willing to confirm that answer with me?
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Oct 23, 2006 #2


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    cos 45 and sin 45 are the same.

    If you equate these two now you have just Fn as the unknown - find it and substitute it back to find F*

    Minimum force just means no acceleration ie exactly balanced forces.
  4. Oct 23, 2006 #3


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    Homework Helper

    Express the normal force Fn out of the first equation, and plug it into the second equation. Then solve for F, and you'll get the minimal force.
    Last edited by a moderator: May 2, 2017
  5. Oct 23, 2006 #4
    Alright guys thanks so much.. Perhaps I should have thought harder before turning to the forums for help.. Once again thanks!
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