Here's why:

* Potential energy is stored energy due to an object's position or configuration.

* Kinetic energy is the energy of motion.

Mechanical energy is the total energy of an object due to its motion and position. It's the sum of its potential and kinetic energies.

Important Note: This is true for conservative systems, where energy is only transferred between potential and kinetic forms. In real-world situations, some energy might be lost due to friction or other non-conservative forces.