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.
The energy of the pendulum is dissipated in the surrounding molecules of the air.
Similarly, there is also friction at the top point where it is tied. The pendulum continuously does work against this friction and its energy is dissipate against doing this work.
Consequently, the pendulum gradually loses all its energy and comes to a stop.