If two spring pendula are coupled together by an attached
mass (Fig. 3) and one of the two systems is set into
motion, the oscillation performed by that pendulum soon
also transfers over to the other pendulum. Provided that the
springs used possess the same stiffness i.e., the same spring
constant the first pendulum remains briefly motionless
after some time; its oscillation energy has been completely
transferred over to the second pendulum. From then on, the
energy shifts between the two pendula, which is why each of
the pendula makes a typical swing motion.