This video covers the basics and fundamentals of biomechanics in orthodontics including force, moments and couples. There is a useful hint on torquing springs.
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For Previous videos on Biomechanics in Orthodontics:
1. Headgear
2. Intermaxillary elastics
The reference is The Biomechanical Foundation of Clinical Orthodontics. It is a valuable read and you can purchase your copy from amazon: