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Homework Help: Dynamics Question - Simple but stuck

  1. Nov 29, 2007 #1
    1. The problem statement, all variables and given/known data

    http://img511.imageshack.us/img511/3732/dynamics01fm7.jpg [Broken]

    2. Relevant equations

    F = M x A
    F = u (coefficient of friction) x N

    3. The attempt at a solution

    http://img508.imageshack.us/img508/7526/dynamics02ry1.jpg [Broken]

    W sin 30 = 50000
    W cos 30 = 86602.54

    1Mg = 10,000KG

    86,602.54 = N + P sin 20
    50,000 + F = P cos 20

    Im left with two unknowns, P and F, and i dont know what i need to do to get either one of them.
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Nov 29, 2007 #2

    Doc Al

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    Staff: Mentor

    Looks to me like you have two equations and three unknowns. Get rid of F by using what you know about friction (one of your equations listed). Then you'd have two equations and two unknowns--which works out just right.
  4. Nov 29, 2007 #3
    Are you saying i should combine the two equations into one? Like this?

    u x N = M x A

    Since its steadily moving up a = 0 so:

    0.15 x N = 0

    Surely that cant be right? If its any help, answer is listed as 6230 N, but i cant get it myself. It isnt accelerating, so a = 0, but when i put that into the equation it says F and therefore N = 0 which i know cant be right.
    Last edited: Nov 29, 2007
  5. Nov 29, 2007 #4

    Doc Al

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    Staff: Mentor

    You lost me on that last post! All I'm saying is to use [itex]F = \mu N[/itex] to eliminate F from your two equations. Then you can solve them together.

    Also: Shouldn't 1Mg = 1,000KG (not 10,000)? (Mg is a rather odd unit!)
  6. Nov 29, 2007 #5
    Yeah your right, im sorry, its 1000KG. Im not quite sure how to use F = [tex]\mu[/tex] N

    I dont know F, and i dont know N, I only know that [tex]\mu[/tex] is 0.15, and i dont know how i can work out F or N.
    Last edited: Nov 29, 2007
  7. Nov 29, 2007 #6

    Doc Al

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    Staff: Mentor

    Once you eliminate F from your equations (from post #1, after you correct them) by replacing it with [itex]\mu N[/itex], you will have two equations and two unknowns (N and P). Solve those two simultaneous equations and you'll find P.

    (To solve them, if that's the problem, take one of them and rewrite it to solve for N in terms of P. Then use it to replace N in the other equation. Then you'll have one equation with only P as your unknown.)
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