It is recalled the fact that due to active F j and reactive (constraint) F c j forces being applied, with resultant R = ∑ ∑ j = 1 n F j + F c j , a system will experience an acceleration r ¨ ¨ = ∑ ∑ j = 1 n F j m j + F c j m j = ∑ ∑ j = 1 n a j + a c j .