Question 5: A simple pendulum is set into vibration and then left untouched eventually stops. Why?
During it’s to and fro motion, the pendulum continuously does work against the resistance of the air. So, the energy of oscillation gradually decreases. It decreases the amplitude of oscillation and eventually it comes to rest. (Remember the energy of the wave is proportional to the square of the amplitude of the wave).
The energy of the pendulum is dissipated in the surrounding molecules of the air.