Question 1: Give two applications in which resonance plays an important role.

 Resonance: If a body is oscillating with its natural frequency and a force having the same period and frequency is applied, there is a remarkable increase in the amplitude of the oscillating body. This phenomena is called resonance.

However, we obviously, want to listen to one of them. The tuning system of the radio distinguishes the desired frequency for us. The tuning system consists of electric circuit in the radio set, usually a knob on the radio body and a calibrated part showing different readings.
Now, when we are turning the knob on the radio set, we are in fact changing the frequency of the receiving circuit. When this frequency equals the transmission frequency of the station we want to listen, it absorbs maximum energy and we hear that frequency clearly.

Swing

Another good example is of mechanical resonance, the swinging of a swing.
The swing acts like a pendulum. It goes back and forth with a certain natural frequency of its own. Suppose the daughter is sitting in the swing and the father is helping her swinging by applying pushes to the swing. The father is applying a periodic force on the swing and increasing the amplitude of the swing with each push. If the periodic force applied by the father also has the same frequency as that of the natural frequency of the swing, the swing will maintain its maximum amplitude. However, if the periodic push of the father is not equal (or not an integral multiple) of the swing resonance frequency, the swing will be stopped!

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