CBSE Class 9 Answered

1.When the bob of the pendulum is taken to one side : 
The metabolic Energy → Potential Energy of the bob.
2. When you leave it  : 
   The potential Energy gets converted into Kinetic Energy while its moving towards the mean.The Potential Energy gets completely converted into Kinetic energy at mean.
3. When the bob overshoots the mean and starts moving towards the other end ,the Kinetic Energy again gets converted into Potential Energy.

This process continues.The bob when moving does work against the air resistance(friction due to air) and the friction in the mechanism(The point where the strings is tied or hook may lead to some friction).Hence the Energy is slowly lost in doing this Work.Eventually the bob comes to rest when the entire energy is lost.Its not violating the Law of Conservation of Energy because the Energy is lost in doing work against friction.