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Friction as B times angular speed?

  1. May 30, 2013 #1
    Friction as B times angular speed??!


    My control systems book uses a weird notation for the moment caused by friction in a rotating motor. The equation is

    Tfriction = B×dθ/dt

    Where does this equation come from? The author assumes the reader is already familiar with this equation.

    The only definition of friction I am familiar with is F = μN. Obviously the torque is given by T = Fd. I have never learned about torques in rotational systems so I assume the equation used in the book is well known and analogous to the translation friction equation.

    What exactly is the B coefficient called? Where does this equation come from?

  2. jcsd
  3. May 30, 2013 #2


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    Gold Member

    That equation for translational motion would be F = B dv/dt. It is termed the damping force.

    B is the viscous damping coeficient. A damper is represented as a pictogram of a dashpot where a viscous fluid such as oil provides the damping force as a linear function of translational velocity dv/dt, or in rotational motion dθ/dt.
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