Mechanical energy is a combination of the potential and kinetic energy. Since potential energy is associated with the position of an object and the kinetic energy is associated with the motion of the object, mechanical energy is associated with both the position and the motion of an object.
Mathematically the mechanical energy can be represented as: E = U + K
Where U is the potential energy and it depends on the position of an object that is exposed to a conservative force, giving the object the ability to do work.
K, on the other hand is the kinetic energy and it depends on the speed of an object. The moving object possess an ability to perform work when it collides with another body/object.
Principle of Conservation of Mechanical Energy
This principle states that if a body or system is subjected to conservative forces only, the mechanical energy of that body or system remains constant. As stated in the principle, the force has to be conservative. As stated during our Physics tuition classes on the topic of Work Energy Power, conservative force does work that is independent of the path moved by the object as the object is being moved from one point to another. However, if the force is non-conservative, the work done will be dependent on the path followed by the object as it is being moved by the non-conservative force.
When an object is moved in a direction opposite to a conservative net force, the potential energy, U, will increase. Also, if the speed of the object is varied, the kinetic energy of the object varies as well. In all systems, it is not possible to get rid of the non-conservative forces like friction. However, these non-conservative forces are sometimes very negligible and the mechanical energy can be approximated to a constant.
Another scenario where the mechanical energy is conserved is in the elastic collisions. Elastic collision is where the objects/bodies colliding remains is motion still although it is in opposite directions. An example of an elastic collision is the collision between a tennis ball and a wall. The tennis ball bounces back. In most systems also, some of the mechanical energy is lost in terms of heat (increase in temperature).
Just as energy can neither be created nor destroyed in an isolated system, the energy can be converted from one form of energy to another. That is the reason as to why there are devices that are made to convert the mechanical energy into other forms of energy. Examples of such devices are the steam engines and the electric motors.