1Prof. Phoenix, 10/21/09 Forced Mechanical Vibrations: A Summary (An Alternate View of E&P Section 3.6) Figure 1. A periodically forced mass-spring-damper system where motion is vertical (left). What’s left of the brand new Tacoma Narrows Bridge that collapsed in heavy driving winds in 1940 caused by periodic forces stemming from the shedding of vortices (right). Many engineering systems, when subjected to “shaking forces”, have oscillation behavior that can be adequately modeled by a mass-spring-damper system subject to a harmonic forcing function. Such a system is shown in Figure 1, where the applied force, 0cosFt, causes the mass to move up and down relative to its equilibrium position in the absence of a force. We will study this system and two variations (applications) that are almost the same mathematically. The system itself can be relevant to periodic wind forces as come from “vortex shedding” as caused the spectacular collapse in 1940 of the then new Tacoma Narrows Bridge. A variation we will study (see Figure 2 given later) is rotating imbalance in a machine (such as a refrigerator) or in an automobile tire and the annoying shaking that can result. The second variation is the effect of ground disturbances on a building during an earthquake or the effect of a bumpy “washboard” road on an automobile (see Figure 3 given later). All of these situations can result in very serious engineering problems with catastrophic consequences if not understood and dealt with in design. So we shall try to explain what goes on from a mathematical point of view. One important aspect is that the most important behavior from an engineering perspective results mainly from the particular solution, pxt, whereas the vibrations considered in E&P Section 3.4 were free vibrations under an initial displacement 0xand velocity 0vat time 0t, and drew on the complementary solution cxt. In most cases of interest in this section, these initial values are zero (though paradoxically this does not mean there is no transient component when the unknown constants of the complementary solution are evaluated). The general solution combining the two does play a role in the response here too (especially in homework problems) but only early in time since the complementary solution, now called the transient solution, dies out fairly rapidly.
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