# Friction as B times angular speed?

1. May 30, 2013

### tamtam402

Friction as B times angular speed??!

Hi,

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?

Thanks.

2. May 30, 2013

### 256bits

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.