14.11 Sharpness of Resonance
We have seen that at resonance, the amplitude of the oscillator becomes very large. The amplitude as well as its sharpness, both depend upon the damping. Smaller the damping, greater will be the amplitude and more sharp will be the resonance.
A heavily damped system has a fairly flat resonance curve as is shown in an amplitude frequency graph in Fig. 14.20.
The effect of damping can be observed by attaching a pendulum having light mass such as a pith ball as its bob and another of the same length carrying a heavy mass such as a lead bob of equal size, to a rod (Fig. 14.17). They are set into vibrations by a third pendulum of equal length, attached to the same rod. It is observed that amplitude of the lead bob is much greater than that of the pith-ball. The damping effect for the pith-ball due to air resistance is much greater than for the lead bob.
(Fig. 14.20: Damping system illustration)
Point to ponder!
lightbulbThe resonance effect is very important in the design of bridges and other civil engineering projects. On July, the newly constructed Tacoma Narrow Bridge (Washington) was opened for traffic. Only four months after this, a mild wind set up the bridge in resonant vibrations. In a few hours the amplitude became so large that the bridge could not stand the stress and a part broke off and went into the water below.