Conservation of Mechanical Energy:
Mechanical energy is the sum of kinetic energy and potential energy in a system. The principle of conservation of mechanical energy states that the total mechanical energy of a system remains constant if the only forces doing work on it are conservative forces.

Conservative Forces:
Conservative forces are those that do not dissipate energy and can be associated with a potential energy function. Examples of conservative forces include gravitational force, elastic force, and electrostatic force. When conservative forces do work on a system, they can convert potential energy into kinetic energy or vice versa, while the total mechanical energy remains constant.

Explanation:
The total mechanical energy of a system is conserved if the forces doing work on it are conservative. This means that if the only forces acting on a system are conservative forces, the sum of the kinetic energy and potential energy will remain constant over time.

When conservative forces do work on a system, they can transfer energy between kinetic and potential forms. For example, consider a ball rolling down a hill. The force of gravity is a conservative force, and as the ball rolls down the hill, the gravitational force does work on it, converting potential energy into kinetic energy. At the bottom of the hill, when the ball reaches its maximum kinetic energy, the potential energy is at its minimum. However, the total mechanical energy (kinetic energy + potential energy) of the system remains constant throughout the motion.

On the other hand, if non-conservative forces such as friction or air resistance are present and do work on the system, they dissipate energy in the form of heat or other forms of energy. This results in a decrease in the total mechanical energy of the system over time.

Conclusion:
In conclusion, the total mechanical energy of a system is conserved if the forces doing work on it are conservative. Conservative forces can transfer energy between kinetic and potential forms without dissipating energy. Non-conservative forces, on the other hand, dissipate energy and result in a decrease in the total mechanical energy of the system.