In common engineering applications the mass in a system remains constant allowing as simple algebraic form for the second law. By the definition of momentum, F = d p d t = d ( m v ) d t , where m is the mass and v is the velocity. If Newton's second law is applied to a system of constant mass, m may be moved outside the derivative operator. The equation then becomes F = m d v d t . By substituting the definition of acceleration, the algebraic version of Newton's second law is derived: F = m a .