Question

Explain change in energy when bob of pendulum moves from extreme position to mean position?

Answer 1

PE=mgh, just as, I assume, your Science or Physics teacher told you.

Don’t confuse yourself with anything other than mass, local gravitation acceleration, & difference in height between its position when held suspended & when at the bottom of its swing. Work in SI/metric or watch your g carefully to convert into correct units.

If by “extreme position” you mean level with its pivot, h is then simply the length of the swing arm, though I suspect in this simplified example it’s the elusive mass-less un-stretchable string. I’ve only found one shop that carries them, usually shelved between the frictionless pulleys & the perfectly elastic bouncing balls.