Question

If Damped vibrations are vibrations of decreasing amplitude under the influence of an external periodic force, can all the vibrations taking place in earth be classified as either damped or forced vibrations?

Answer 1

All the vibration on earth are actually damped vibration. Because there is always friction and viscus forces (mainly from air or other fluid) which are main cause for damping. But all vibration can't be classified as forced vibration. Because for most of the vibration there is no externally applied periodic force.