I animate pendulum swing by observing the principle of drag, follow through and overlapping action that makes it flexible. I also analyze how does it affect the timing and travel distance of each back and forth swing until it comes to a complete stop. 

In the video, I animate the pendulum animation by: 

a) Animate the main object going from left to right.
b) Animate a pendulum that is attached to the main object  and make it look flexible.