Mechanical energy is often confused with Kinetic and Potential Energy. We will try to make it very easy to understand and know the difference. Before that, we need to understand the word ‘Work’.
‘Work’ is done when a force acts on an object to cause it to move, change shape, displace, or do something physical. For, example, if I push a door open for my pet dog to walk in, work is done on the door (by causing it to open). But what kind of force caused the door to open? Here is where Mechanical Energy comes in.
Mechanical energy is the sum of kinetic and potential energy in an object that is used to do work. In other words, it is energy in an object due to its motion or position, or both.
In the 'open door' example notice what happens...
I possess potential chemical energy (energy stored in me), and by lifting my hands to push the door, my action also had kinetic energy (energy in the motion of my hands).
By pushing the door, my potential and kinetic energy was transferred into mechanical energy, which caused work to be done (door opened).
Here, the door gained mechanical energy, which caused the door to be displaced temporarily.
Note that for work to be done, an object has to supply a force for another object to be displaced.
Here is another example of a boy with an iron hammer and nail. In the illustration below…
The iron hammer on its own has no kinetic energy, but it has some potential energy (because of its weight).
To drive a nail into the piece of wood (which is work), he has to lift the iron hammer up, (this increases its potential energy because if its high position).
And force it to move at great speed downwards (now has kinetic energy) to hit the nail.
The sum of the potential and kinetic energy that the hammer acquired to drive in the nail is called the Mechanical energy, which resulted in the work done.