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Having trouble understanding General moment

  1. Dec 30, 2014 #1
    On the website http://www.real-world-physics-problems.com/physics-of-billiards.html

    they take this equation ∑Mg = IG∝ where ∝ = -agx/r for the rotation of a rigid body where there is no slipping

    They then change the equation to F⋅(h-r) = IG(-agx/r)

    why is ∑MG = F⋅(h-r) when ∝= -agx/r?
  2. jcsd
  3. Dec 30, 2014 #2

    Stephen Tashi

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    Science Advisor

    If we have a lever CD pivoting at point C and with a force applied at D, one way to compute the torque of the force about the point C is multiply the component of the force that is perpendicular to CD times length CD. Another way is to imagine that CD is replaced by a lever that pivots at C and is perpendicular to the line of the force. Let this lever intersect the line of the force at point Y. The moment of the force about C is the length CY times the magnitude of the force. If you use the latter method the length of CY is h-r.
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