Question

The rearview mirror of a motorbike starts vibrating violently at some particular speed of the motorbike. Why does it happen?

Answer 1

When a bike runs at a particular speed it produces some vibrations. This vibration puts pressure on the bike and causes violent vibrations in the bike. Also, then the frequency of vibrations of the engine becomes equal to the natural frequency of vibration of the rearview mirror. Here, the amplitude of vibration becomes maximum.

Complete step by step answer:

The reason behind the rearview mirror of a motorbike that starts vibrating violently at some particular speed is resonance. When a bike runs at a particular speed it produces some vibrations, which comes from the engine of the bike. Then this vibration puts pressure on the bike that causes the violent vibrations in the bike. Also, then the frequency of vibrations of the engine of the motorbike becomes equal to the natural frequency of vibration of the rearview mirror and the amplitude of vibration of the mirror becomes maximum, and this maximum amplitude of vibration results in vibrating violently.
Now, what is this resonance? Usually, when a body executes a forced vibration, its amplitude and velocity remain small. In most cases, the frequency of the applied periodic force does not come close to the natural frequency of the vibrating body. However, when the frequency of the applied periodic force becomes equal to the natural frequency of the body, the amplitude and the velocity of the body become very large. This phenomenon is called resonance.
Now, resonant vibration is of two types. One is amplitude resonance and another one is velocity resonance. In the case of amplitude resonance, the amplitude of vibration becomes maximum.

Note:
It is important to keep in mind that some objects are very resonant at a particular frequency while others barely resonate. For example, a temple bell will only ring at a fixed frequency, which is its resonant frequency; but a lump of jelly will vibrate at many different frequencies but will not resonate at all.