The relative angular momentum of the body about a point fixed in the body is more easily determined and it can be applied to calculate the corresponding value for the absolute angular momentum. This chapter develops an expression for the relative angular momentum of a rigid body about a point which is fixed to the body. The output from the motor drives a gearbox via a friction clutch, and the mixer is driven from the gearbox output shaft. The motor, clutch, mixer and gears are supported in bearings in the casing C and form a single unit. The casing is bolted to ground at the points A and B. The clutch allows the motor to be disengaged from the gearbox. The input gear is driven via the clutch and drives the output gear via an idler gear so that the input and output shafts rotate in the same direction.