Question

is the fan rotating with constant speed in equlibrium according to 2nd condition of equilibrium?

Answer 1

nce the rotation speed is constant, the angular acceleration is zero. Therefore the net torque is zero. Friction and wind drag add up to be equal and opposite the torque the motor is applying.

Net torque = I*alpha
where I is Moment of Inertia (also know as Rotational Inertia),
alpha is angular acceleration. I is equivalent to mass and alpha is equivalent to a in F=ma. For a table of similar equivalents, see this website: http://hyperphysics.phy-astr.gsu.edu/hba...