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Wrote general equation for a free body diagram

  1. Nov 10, 2012 #1
    So I derived this equation for the net force/acceleration of a motorcycle on a flat surface. I beleive this equation could practically be used for any rolling object.
    ƩF = Fp - μ*Fn - FD
    Where: FP is the force of the bike acting against the ground, tangent to the ground
    FD = [itex]\frac{1}{2}[/itex]pv2CDA (air drag)
    μ*Fn is the frictional force of the tires, Fn is the normal force.

    Now to find a:

    ma = Fp - μFn - FD

    a = (Fp - μFn - FD) / m

    Is there anything wrong with this?
  2. jcsd
  3. Nov 10, 2012 #2

    Simon Bridge

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    Woah! Let me turn down the volume a bit there:
    So you'd use this for something like a rolling ball? I guess you'd factor in the mass-distribution in one of the F's there ... that is pretty general indeed. How about for irregularly shaped objects like an apple?
  4. Nov 11, 2012 #3
    Your kidding. Irregulaly shaped objects like apples?
    Well you would need to run a model of a 3D apple through a simulator. There can't possibly be an equation to model that.
    My bad, that's why you can't use this for every rolling object. Only for surfaces that are nearly perfectly circular.
    Last edited: Nov 11, 2012
  5. Nov 11, 2012 #4

    Simon Bridge

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    How about a regular shaped object then, like a polyhedron?
    A hollow sphere with another, much smaller but heavy, solid sphere free to roll inside it?
    You did claim that the equation could be "used for any rolling object".
    You asked "in there anything wrong with this?"
    Now you know.
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