Classical Mechanics

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Oscillations: Effect of Damping


When a simple mass-spring system is excited by a piston that pushes and pulls on the spring in a sinusoidal manner, the oscillatory response of the of the mass-spring system can be described by the equation:


where $x(t)$ is the position of the mass, $b$ is the damping coefficient, $\omega_o$ is the natural frequency of oscillation and $X(t)$ is the position of the piston that drives the system. Let the motion of the piston be of the form:


The response of the mass-spring system to the driving force of the piston depends strongly on the frequency of oscillation, $\omega$, of the piston motion, and is also strongly affected by the value of the damping coefficient $b$.

The response can be characterized by a dimensionless parameter $\xi$ equal to the ratio of the amplitude of oscillation, $x_o$ to the amplitude of the piston motion, $X_o$:


How does the value of the damping coefficient, $b$, effect the magnitude of the response, particularly as seen the parameter $\xi$?

The oscillating piston drives the mass-spring system into oscillation. Created for Copyright 2016. All rights reserved.


Increasing the damping coefficient, $b$, significantly widens the resonance. So, the increase in amplitude is seen for a larger range of driving frequencies $\omega$.


The major effect of changing the damping coefficient, $b$ is to change the height of the resonance peak. If $b$ is increased the resonance occurs with a smaller amplitude. If $b$ is decreased the resonance occurs with a larger amplitude. The effect is seen only in the region near the resonance condition i.e. when the driving frequency equals the natural frequency, $\omega=\omega_o$.


Decreasing the damping coefficient, $b$, significantly narrows the range of frequencies where the amplitude increases.


Increasing the damping coefficient, $b$, will increase the frequency $\omega$ where resonance occurs.