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Nope, that's not that. I have to implement something that simulates an angular spring - the force is proportional to the angle. To make matters worse, both arms of this angular spring are themselves linear springs, and the whole thing moves around. The task is to model a red blood cell as a circle consisting of material points linked together by springs, with an additional "angular spring" between two adjacent linear springs (this has to simulate the surface tension, I hope somebody understands what I mean). I ended up using numeric differentiation to calculate the forces from the angular spring.I don't see an obvious way to use vector math here at the moment. Maybe I'll come up with something more spiffy later.