In simple harmonic motion, the restoring force at the extreme position is equal in magnitude to the initial force imparted, allowing the system to return to the mean position. The displacement on either side of the mean position remains the same due to the symmetry of the setup and the conservation of energy. As the body oscillates, kinetic energy is converted into potential energy, with the restoring force opposing motion until the body comes to rest. This process results in the body overshooting the mean position due to momentum, leading to a continuous cycle of oscillation. Overall, in an ideal system without damping, total mechanical energy remains constant throughout the motion.