The general statement of d'Alembert's principle mentions "the time derivatives of the momenta of the system." By Newton's second law, the first time derivative of momentum is the force. The momentum of the i -th mass is the product of its mass and velocity: p i = m i v i and its time derivative is p ˙ ˙ i = m ˙ ˙ i v i + m i v ˙ ˙ i .