What is Resonance?


Resonance occurs when the frequency of the applied periodic forced is equal to one of the natural frequencies of vibration of the forced or driven harmonic oscillator.


resonanceAssociated with the motion of a driven harmonic oscillator, there is a very striking phenomenon, known as resonance.It arises if the external driving force is periodic with a period comparable to the natural period of the oscillator.

In a resonance situation, the driving force may be feeble, the amplitude of the motion may become extra ordinarily large. In the case of oscillating simple pendulum, if we blow to push the pendulum whenever it comes in front of our mouth, it is found that the amplitude steadily increases.

To demonstrate this resonance effect, an apparatus is shown in Fig. A horizontal rod AB is supported by two strings S1 and S2. Three pairs of pendulums aa’, bb’and cc’ are suspended to this rod. The length of each pair is the same but is different for different pairs. If one of these pendulums, say c, is displaced in a direction perpendicular to the plane of the paper, then its resultant oscillatory motion causes in rod AB a very slight disturbing motion, whose period is the same as that of c’. Due to this slight motion of the rod, each of the remaining pendulums (aa’ bb’.and cc’) under go a slight periodic motion. This causes the pendulum c’, whose length and, hence, period is exactly the same as that of c, to oscillate back and forth with steadily increasing amplitude. However, the amplitudes of the other pendulums remain small through out the subsequent motions of c and c’, because their natural periods are not the same as that of the disturbing force due to rod AB. The energy of the oscillation comes from the driving source. At resonance ,the transfer of energy is maximum.

Advantages And Disadvantages of Resonance:

  • We come across many examples of resonance in every day life. A swing is a good example of mechanical resonance. It is like a pendulum with a single natural frequency depending on its length. If a series of regular pushes are given to the swing, its motion can be built up enormously. If pushes are given irregularly, the swing will hardly vibrate. The column of soldiers, while marching on a bridge of long span are advised to break their steps. Their rhythmic march might set up oscillations of dangerously large amplitude in the bridge structure.
  • Tuning a radio is the best example of electrical resonance. When we turn the knob of a radio, to tune a station, we are changing the natural frequency of the electric circuit of the receiver, to make it equal to the transmission frequency of the radio station. When the two frequencies match, energy absorption is maximum and this is the only station we hear.
  • Another good example of resonance is the heating and cooking of food very efficiently and evenly by microwave oven. The waves produced in this type of oven have a wavelength of 12 cm at a frequency of 2450 MHz. At this frequency the waves are absorbed due to resonance by water and fat molecules in the food, heating them up and so cooking the food.
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