Everyone knows that the best way to set a child’s swing in motion is to give it small pushes in time with the natural period of the swing. This is an example of a general principle in physics which is called resonance.
Resonance is said to occur whenever a particular body or system is set in oscillation at its own natural frequency as a result of impulses received from some other system which is vibrating with the same frequency.
set up. Cases have been known in the past where suspension bridges have been damaged by the resonant vibrations caused by marching military columns. This can happen if the rate of marching happens to bear a simple numerical relationship to the natural frequency of oscillation of the bridge. Nowadays, in order to guard against accidents, soldiers are always given the order to break step when crossing a bridge.
When steel bodies were first introduced for motor-cars it was often noticed that a drumming sound made its appearance at certain speeds. This was caused by resonance between the body panelling and the engine vibrations. It was soon discovered that it could be prevented by coating the inside of the panels with a layer of plastic material. This reduced the natural period of vibration of the panels and also damped out the vibrations.
As well as mechanical examples of resonance, we find applications of the same principle in other branches of physics. The action of tuning a radio set is to adjust the value of the capacitance in a circuit until it has the same natural period of oscillation for electricity as that of the incoming signal. The small alternating e.m.f. set up in the aerial is then able to build up similar e.m.f. of large amplitude in the tuned circuit.
The use of resonance in tuning a sonometer wire to have the same frequency as a fork has already been mentioned on page 331. We shall now discuss the production of resonance between a tuning fork and a column of air in a closed tube.