# Different types of vibration are

1. Free vibration or natural vibration
1. Longitudinal vibration
2. Transverse vibration
3. Torsional vibration
2. Forced vibration
3. Damped vibration
1. Free vibration or natural vibration : when no external force acts on the body, after giving it an initial displacement, then the body is said to be under free vibration or natural vibration. The frequency of the free vibration is called free or natural frequency. Different types of free vibrations are as follows
1. Longitudinal vibration : When the particles of the shaft or disc move parallel to the axis of the shaft, then the vibrations are known as longitudinal vibration. In this case, the shaft is elongated and shortened alternately and thus the tensile and compressive stresses are induced alternately in the shaft.
2. Transverse vibration : When the particles of the shaft or disc move approximately perpendicular to the axis of the shaft, then the vibrations are known as transverse vibrations. In this case, the shaft is straight and bent alternately and bending stresses are induced in the shaft.
3. Torsional vibration : When the particles of the shaft or disc move in  a circle about the axis of the shaft, then the vibrations are known as torsional vibrations. In this case, the shaft is twisted and untwisted alternately and the torsional shear stresses are induced in the shaft.
2. Forced vibration : When the body vibrates under the influence of external force, then the body is said to be under forced vibrations. The external force applied to the body is a periodic disturbing force created by unbalance. The vibrations have the same frequency as the applied force.
3. Damped vibration : When there is a reduction in amplitude over every cycle of vibration, the motion is said to be damped vibration. This is due to the fact that a certain amount of energy possessed by the vibrating system is always dissipated in overcoming frictional resistance to the motion.
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