Problem

You want to get a part to rotate in a direction.

Solution

Use the BodyAngularVelocity object.

local b = Instance.new('BodyAngularVelocity')
b.maxTorque = Vector3.new(math.huge, math.huge, math.huge)
b.angularvelocity = Vector3.new(0, 10, 0)
b.Parent = Workspace.Part

Discussion

The BodyAngularVelocity object uses its AngularVelocity property to determine speed and rotation. This time instead of using the maxForce property, we used maxTorque which is rotational power.