Coulumb's Damper:

The dry sliding friction acts in this kind of damper. This is almost a constant force however direction is opposite to the sliding velocity. So, direction of friction changes in direction.

The Excitation Force

This is a source of constant supply of energy to the vibratory system. This is an external periodic force that acts on the vibratory system.

It is significant to study the single degree freedom system for an apparent understanding of fundamental features of a vibration problem.