Galileo Galilei - at his trial

DAMPED
OSCILLATIONS

- To date our discussion of SHM has assumed that the motion is frictionless, the total energy (kinetic plus potential) remains constant and the motion will continue forever. Of course in real world situations this is not the case, frictional forces are always present such that, without external intervention, oscillating systems will always come to rest. The frictional (damping) force is often proportional (but opposite in direction) to the velocity of the oscillating body such that

where b is the
damping constant.

- This differential equation has solutions

where
when the damping is small (small b).

Notice that this solution represents oscillatory motion with an exponentially decreasing amplitude

Notice that this solution represents oscillatory motion with an exponentially decreasing amplitude

- Suppose now that instead of allowing our system to oscillate in isolation we apply a "driving force". For example, in the case of the (vertical) mass on a spring the driving force might be applied by having an external force (F) move the support of the spring up and down. In this case the equation of motion of the mass is given by,

One common situation occurs when the driving force itself oscillates, in which case we may write

where is the (angular) frequency
of the driving
force.

- This equation has solutions of the form

where the
amplitude of these
oscillations, B, depends on the parameters of the
motion,

The amplitude, B, has a maximum value when . This is called the resonance condition. Note that at resonance, B, can become extremely large if b is small. (In the diagram at right is the natural frequency of the oscillations, , in the above analysis). In designing physical systems it is very important to identify the system's natural frequencies of vibration and provide sufficient damping in case of resonance. This clearly did not happen in the design of the Tacoma Narrows Bridge (Tacoma Narrows Newsreel) in 1940.

- There are many physical and engineering systems
where resonance
is very important e.g. shock absorbers, earthquakes,
loudspeakers, NMR,
microwave ovens etc. etc. A very important
subject which,
unfortunately, we do not have time to discuss in any
more detail.

*Dr. C. L. Davis*

*Physics Department*

*University of Louisville*

*email*: c.l.davis@louisville.edu