It automatically follows from Eq. ( ) that Ė = 0. A solution that satisfies Eq. ( ) is: where n (β) ν is an arbitrary vector defining the direction of f(β) ν,l . While any direction for n (β) ν in Eq. ( ) guarantees energy conservation, we choose n (β) ν as the instantaneous mechanical momentum of the trajectory n ν = M ν Ṙ(β) ν , with the reasoning that the accumulated force, Eq. ( ), represents a momentum along the trajectory