Idealized spring-mass systems play an important role in mechanical and
other engineering problems. Figure 12.11 shows such a system. After they are released, the
masses are pulled downward by the force of gravity. Notice that the resulting displacement
of each spring in Fig. 12.11b is measured along local coordinates referenced to its initial
position in Fig. 12.11a.
As introduced in Chap. 1, Newton’s second law can be employed in conjunction with
force balances to develop a mathematical model of the system. For each mass, the second
law can be expressed as