Accordingly, why doesn't the principle of mechanical energy conservation hold in situations when frictional forces are present?
There actually are simple Conservative forces preserve total energy. You can move something up and down on a roller coaster and the total energy stays constant, ignoring friction, because gravity is a conservative force, and the path does not affect the changes in kinetic/potential energy.
Beside above, what is the principle of conservation of mechanical energy? Law of Conservation of Mechanical Energy: The total amount of mechanical energy, in a closed system in the absence of dissipative forces (e.g. friction, air resistance), remains constant. This means that potential energy can become kinetic energy, or vice versa, but energy cannot “disappear”.
In this way, is conservation of mechanical energy likely to hold in these situations?
Yes. In the absence of frictional force, the total mechanical energy is conserved.
What if energy was not conserved?
When some of energy is lost in form of heat due to frictional forces energy is not conserved obviously. Indirectly, frictional loss of energy is conserved in the form of heat. Even though there is always loss of energy due to friction, frictional forces are much needed in daily life.
