Free-body diagrams In order to use Newton's second law of motion it is necessary to determine the forces acting on a body so that the resultant force (F) can be found. Thus a system of objects such as springs and masses should be divided up into its separate components There are equal and opposite forces (F) at the ends of the spring. The force F is given by F = k x the extension.. Newton's third law states that for every force there is an equal and opposite reaction. Thus there is a force F also acting on the mass as shown. The free body diagram of the mass m1 is thus, If we now apply Newton's second law we obtain the equation of motion of the mass m1 as, and the free body diagram of the mass m2 is, and the equation of motion of the mass m1 is,