Equation (8) is labeled “self-energizing” because the moment of the friction force ( fNa) assists the applied force (F) in applying the brake. For counter-clockwise drum rotation, the direction of the friction force would be reversed. This would cause it to oppose the application of the brake, making the brake self-de energizing.
The direction of the magnetic moment m of a compass needle is from the end marked S for south to the one marked N for north. The lowest energy occurs for ϑ = 0, when m and B are aligned. In a typical situation, the compass needle comes to rest after a few oscillations and points along the B field in the direction called north.