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How to solve general forces equations as variables?

  1. Nov 14, 2012 #1
    Okay, so we were doing forces, and my teacher had this question:

    Code (Text):

    In an atwood machine, a massless, non stretching string passes over a frictionless peg,
    one end of the rope is connected to object m1, and the other end to object m2 (heavier).
    When the system is released from rest, m2 goes down and m1 goes up, find acceleration.
    So she started out with

    Fnet = ma
    Then here is where I got confused: She goes to:
    Fnet = FT - Fg.

    For the second equation, how do you just pull this out, and why does it work, i mean i get the math and can do it, but i don't know why?
  2. jcsd
  3. Nov 14, 2012 #2
    Draw a free body diagram for both mass m1 and mass m2 then sum up the forces acting on each.
  4. Nov 14, 2012 #3


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    Welcome to PF!

    Hi PhoniexGuy! Welcome to PF! :smile:
    For one object, there are two forces acting on it:

    its own weight (mg), and the tension in the rope (FT) :wink:
  5. Nov 14, 2012 #4
    Re: Welcome to PF!

    Oh, okay, then thank you, i get it now. Is it negative Fg since gravity is downwards?
  6. Nov 14, 2012 #5


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    not exactly

    Fg and FT have opposite signs because they act in opposite directions :wink:

    (since signs obviously worry you, i'll anticipate the next question …

    in your two F = ma equations for the two different objects, depending how you set them up, it needn't be the same a in both equations … one may have a and the other may have minus a)
  7. Nov 14, 2012 #6
    Okay, because that's what our teacher did, is this because the objects are attached by the rope?

    Also, still related to forces:

    I have a 10 Kg block that is pushed with force F at a angle θ (in degrees) to the horizontal. I have a few questions about the equations for certain things and there is μs and μk:

    1) I know that to find normal force
    Code (Text):
    F[SUB]n[/SUB] = F[SUB]w[/SUB] - F(sin θ)
    2) I also know then to find frictional force?
    Code (Text):
    F[SUB]f[/SUB] = μ[SUB]s[/SUB]F[SUB]n[/SUB]-[b]F[/b]cosθ
    I understand how to use them, but how do you get these equations? Like how can you derive them from something else?

    3) Find the acceleration of the object when F is doubled.

    For this I know that Fnet = ma, so a = Fnet/m Where do i go from there? I get how forces work, and drew diagrams, but I need help in understanding how the equations themselves will be used, only as variables (which our teacher gives on tests), i know how to use the math, and can do it with calc, but i want to learn how to manipulate the equations and use Fnet = ma to solve them.
  8. Nov 14, 2012 #7


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    Pushed on a horizontal plane with a force that's angled? Up or down?
    Or pushed up or down a slope that's at θ to horizontal?
    Please define your variables.
  9. Nov 15, 2012 #8


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    Hi PhoniexGuy! :smile:

    (just got up :zzz:)
    yes, the length of the rope is constant, so the displacement of the two ends must have the same magnitudes …

    and of course, by differentiating, that means that the velocities of the two ends, and the accelerations of the two ends, must also have the same magnitudes

    (this equation, a1 = -a2, is known as a constraint

    it's a geometry equation, not a physics one! :wink:)
    you mean, the teacher has given you the formulas, but you want to be able to derive them yourself?

    1) this comes from good ol' Newton's second law (F = d(mv)/dt)

    (as indeed does nearly all of mechanics!)

    … you know that the acceleration in the normal direction is 0 (obviously! :biggrin:)

    so, applying Newton, no matter what is going on on the surface, Fnet in the normal direction must be zero

    there are only three forces with normal components, Fn, Fw, and F, and if you apply the relevant cosines, equation 1) is what you get! :smile:

    2) this equation seems to be written wrong :confused:

    it looks like the equation Fnet, horizontal = Fcosθ - µsN
    i'm not sure exactly what the question is

    anyway, can you show us how far you've got on it?
  10. Nov 15, 2012 #9
    Actually, nevermind. Thank's for all the help, i understand it now! (asked teacher)
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