Answer:
Since F = M a force and acceleration both have maximum and minimum values at the same point in their motion.
Also, F = - k x where x is the extension of the displacement that creates the acceleration.
Since, there is no force acting on the object at the midpoint of the motion, then F = - k x = 0 and there is no acceleration.
Likewise, at maximum extension F = - k x is a maximum and both the force and acceleration have their maximum values at maximum extension.
