Dynamics of Mechanical Systems
A small compact physical structure, such as a marble, can be thought of as simply a mass. It will move in response to an external force applied to it, and Newton’s laws of motion will govern its movement. Simply put, Newton's laws dictate that if the marble is at rest, it will remain at rest unless acted on by an external force, and if in motion it will continue in motion unless acted on by an external force. If it is subjected to an external force, its acceleration will be proportional to that force.
Most mechanical systems are more complex than a simple mass, and they do not necessarily move as a whole when subjected to a force. Mechanical systems, such as rotating machines, are not infinitely rigid, and have varying degrees of flexibility at different frequencies. As we will see, their motion in response to an external force is dependent on the nature of that force and the dynamic characteristics of their mechanical structure, and is often difficult to predict. The disciplines of Finite Element Modeling (FEM) and Modal Analysis are dedicated to predicting how a structure will respond to a known force. We will not discuss these fields further, for they are very complex, but it is instructive to look into how forces and structures interact if we are to understand the usefulness of vibration analysis of machines.
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