When the elements on the surface of the shaft move along the circumference of the shaft, the vibrations are called:

When the elements on the surface of the shaft move along the circumference of the shaft, the vibrations are called: Correct Answer torsional vibrations

Explanation:

Vibration:

Vibration is a periodic motion of small magnitude. But for sake of simplicity, we can assume it as a simple harmonic motion of small amplitude.

Torsional vibrations:

• 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 elongated and shortened alternately and thus the tensile and compressive stresses are induced alternately in the shaft.

Transverse vibrations:

• 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.
• Rotating shafts tend to vibrate violently in the Transverse direction at critical speeds.

Longitudinal vibrations:

• When the particles of the shaft or disc move parallel to the axis of the shaft, then the vibrations are known as longitudinal vibrations.
• In this case, the shaft is elongated and shortened alternately and thus the tensile and compressive stresses are induced alternately in the shaft.

Damped Vibrations: 

• When the energy of a vibrating system is gradually dissipated by friction and other resistance the vibrations are said to be damped vibrations.