By Newtons third law, in Eq(7-15) the vector sum of all the internal forces is cancelled and Eq(7-15)reduces to F= Ma ext (7-16) Eq(7-16)is just the Newtons second law for the system of n particles treated as a single particle of mass M located at the center of mass( rem experiencing acm cm c M nnBy Newton’s third law, in Eq(7-15) the vector sum of all the internal forces is cancelled, and Eq(7-15) reduces to (7-16) Fext Macm    = acm  Eq(7-16) is just the Newton’s second law for the system of N particles treated as a single particle of mass M located at the center of mass( ), experiencing . cm r  → → cm = mn an M a 1 → → cm =  n n m r M r 1