An object with weight $ W $ is dragged along a horizontal plane by a force acting along a rope attached to the object. If the rope makes an angle $ \theta $ with a plane, then the magnitude of the force is
$$ F = \dfrac{\mu W}{\mu \sin \theta + \cos \theta} $$
where $ \mu $ is a constant called the coefficient of friction. For what value of $ \theta $ is $ F $ smallest?