Question

why acceleration is zero at mean position & maximum at extreme position?​

Answer 1

Hey mate here's your answer

Acceleration is zero because at that point, it is the mean position, which means it is the equilibrium position. Hence, the spring is not compressed (or extended) or the pendulum suffers no tangential force.

However, at that point, due to velocity being non-zero, the object crosses the mean position (law of intertia), and the acceleration again becomes non zero! This process continues and SHM takes place.

It is not that velocity is maximum, that's why the acceleration is zero. The velocity is maximum there because acceleration changes direction at that point, hence at all other points, the acceleration is decelerating the object.

Answer 2

Answer:

It is max at extremum and zero at mean point