In vibration analysis ‘Beat’ frequency is

A.The frequency at which vibrations of a system are damped
B.The frequency when a system exceeds the vibrations frequency from the critical value
C.The frequency below which vibrations do not cause appreciable damage to equipment
D.Said to occur when two vibrations of almost equal frequency are present
E.One at which the vibrations of a system act exactly opposite to the other thereby causing damping

Creep in belts is due to

A.Elasticity of belt material
B.Plasticity of belt material
C.Elongation of belt due to tension
D.Differential elougation of belt due to difference in tension on two sides of a pulley
E.Plastic deformation of belt material due to constant use