Vibration is a mechanical phenomenon where oscillations occur about an equilibrium point. The oscillations may be periodic, such as the motion of a pendulum, or random, such as the movement of a tire on a gravel road. Vibration can be desirable, such as the motion of a tuning fork, the reed in a woodwind instrument or harmonica, a mobile phone, or the cone of a loudspeaker. Vibrations fall into two categories: free and forced.
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Free vibration: This occurs when a mechanical system is set in motion with an initial input and allowed to vibrate freely. Examples of this type of vibration are pulling a child back on a swing and letting it go, or hitting a tuning fork and letting it ring. The mechanical system vibrates at one or more of its natural frequencies and damps down to motionlessness.
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Forced vibration: This occurs when a time-varying disturbance (load, displacement, velocity, or acceleration) is applied to a mechanical system. The disturbance can be periodic, such as a rotating imbalance, or random, such as a tire on a gravel road. An example of forced vibration is a childs swing that is pushed on each downswing. Of special interest are systems undergoing simple harmonic motion (SHM) and driven by sinusoidal forcing. This leads to the important phenomenon of resonance. Resonance occurs when the driving frequency approaches the natural frequency of free vibrations. The result is a rapid take-up of energy by the vibrating system, with an attendant growth of the vibration amplitude. Ultimately, the growth in amplitude is limited by the presence of damping, but the response can be significant.
A vibration is a repeated back-and-forth motion of a particle when displaced from its equilibrium position. It is characterized by repeated oscillations of particles about a fixed point. Vibration involves the rapid back-and-forth movement of physical particles as a reaction to different forces.
