The principle states that the sum of the differences between the forces acting on a system of massive particles and the time derivatives of the momenta of the system itself projected onto any virtual displacement consistent with the constraints of the system is zero. Thus, in mathematical notation, d'Alembert's principle is written as follows, ∑ ∑ i ( F i − − m i v ˙ ˙ i − − m ˙ ˙ i v i ) ⋅ ⋅ δ δ r i = 0 ,