Mechanical energy is the energy present in an object due to its state of rest or of motion. It is the sum of potential and kinetic energy possessed by an object and is used to do some work; or we can also say that potential energy and kinetic energy are two types of mechanical energy.
Mechanical energy is often confused with kinetic and potential energy. We will take an example to understand the concept of mechanical energy and know the difference. But before that, let us understand what “work” means.
We say that work is done whenever an object is subjected to some force and due to this force, it moves, changes its shape, displaces, or undergoes any other physical change. For example, if a child kicks a football placed on the ground, it will start moving forward. We will say that some amount of work is done on football (which causes it to move). But what is this force which enabled the football to move? Now, this is the point where the concept of mechanical energy comes in.
Initially, the foot of the child had some amount of chemical potential energy (the energy stored in his body). The child is able to kick the ball by swinging because of the kinetic energy in his leg. The mechanical force that caused work to be done, that is, causing the ball to move forward. For work to be done, it is essential that one object apply some force for another object to be displaced.
Therefore, mechanical energy is simply the force contained in an object that does work due to its motion (Kinetic Energy) or stored energy (Potential Energy), or both. An object possessing mechanical energy is capable of doing some work. Or simply, an object requires mechanical energy so as to do some work; for example, to lift the book placed on the shelf, for a rower to row the boat, for a weightlifter to lift the weights, etc. Mechanical energy is required to do all such kinds of work.
The formula for mechanical energy is;
Mechanical energy = kinetic energy + Potential energy
Law Of Conservation Of Mechanical Energy
It says that the mechanical energy of an object in a closed system remains constant if it is not under the influence of any dissipative force (for example friction, air resistance), except for the gravitational force.
Let us try to understand the concept of mechanical energy more plainly by taking a few examples from everyday life.
1. Wrecking Ball
A wrecking ball is a large round structure that is used for destructing buildings. When the ball is held at a height, it contains some amount of mechanical energy as potential energy (stored energy) and as soon as it falls, it gains some amount of kinetic energy too. When the wrecking ball hits the building to be demolished, it applies the force (in the form of mechanical energy) which causes the work to be done, i.e., demolishing buildings and other structures.
In simple words, we can say that a wrecking ball contains some mechanical energy in the form of potential or kinetic energy.
Whenever we use a hammer to, let’s say, hit a nail and drive it into the wall, we are simply applying some force on the nail with the help of the hammer which is causing some work to be done. At rest, a hammer does not contain any kinetic energy but only some mechanical energy in the form of stored energy (i.e. potential energy). When we swing a hammer up to some distance from the nail before hitting it, there appears some amount of kinetic energy due to its motion.
The combination of kinetic and potential energy in the hammer, called mechanical energy, will cause the driving of the nail into the wall. The hammer, possessing mechanical energy, was able to do work. Or, we can say that the force applied by the hammer to do work on the nail is the mechanical energy which is the sum of the potential and kinetic energy.
3. Dart Gun
A dart gun forms another good example of mechanical energy observable in everyday life. A dart gun works on the principle of elastic potential energy. The spring used in the dart guns consists of stored elastic potential energy.
When a dart gun is loaded, it causes the spring to compress. At that moment, the dart gun consists of mechanical energy in the form of elastic potential energy. Due to this mechanical energy, the spring is able to apply force on the dart and does work, that is, causes the displacement of the dart.
The mechanical energy in an object is its ability to do some work on an object. In this case, the mechanical energy present in the compressed spring (due to elastic potential energy) caused the displacement of the dart, hence, resulting in work done by the spring on the dart.
4. Wind Mill
Windmills are the structures which convert wind energy into the electrical energy and this energy is then supplied to our homes. But from where does this energy in the wind comes and moves the large blades of a windmill?
Windmills run on the principle of mechanical energy and work. Moving air (wind) possesses some amount of mechanical energy in the form of kinetic energy (due to motion). This mechanical energy gives the air the ability to do work on the blades of the fan.
The moving air applies force, present in the form of mechanical energy, on the blades and enables the work to be done, hence, resulting in their rotation. Therefore, the mechanical energy gave wind the ability to do work on the blades of the fan.
5. Bowling Ball
This interesting target sport illustrates another good example of mechanical energy and the work done by the object possessing it.
The bowling ball consists of some amount of mechanical energy in the form of kinetic energy as soon as it starts rolling towards the target. Due to this mechanical energy, the ball has the ability to do work on the pins. When the ball hits those target pins, it (ball) applies force (in the form of mechanical energy) and displaces them, hence, causing work to be done.
Therefore, here also, we can say that mechanical energy gave the bowling ball the ability to do work on the pins which caused their displacement.
6. Hydropower Plant
In hydropower plants, electricity is generated with the help of running water. The hydropower plants display an excellent example of the use of mechanical energy to do work.
In a hydropower plant, we often get to see the view of water running down the slope at an enormous speed. The running water is thrown from a great height just to gain a good amount of mechanical energy which is present in the form of gravitational potential energy (due to height) and kinetic energy (due to motion).
The water falling down the slope then hits the blades of the turbines which are installed at the bottom of the waterfall. The mechanical energy of the water enables it to do work on the blades resulting in their rotation. Once the blades move, the turbine converts the mechanical energy of the water into electrical energy. Hence, mechanical energy gave water the ability to do work on the blades of the turbine.
A person riding a bicycle possesses some amount of mechanical energy in the form of chemical potential energy. This mechanical energy is utilised by the cyclist to do work on the paddles of the bicycle by applying some force and enabling the bicycle to move forward.
Moon is earth’s only natural satellite and it revolves around the earth just like earth and other seven planets revolve around the sun. Moon possesses potential energy due to its position with respect to the earth as well as kinetic energy, since it orbits around the earth. Therefore, we can say that the moon exhibits high mechanical energy in the form of kinetic and potential energy due to its position and motion respectively.
The mechanical energy of the Earth-Moon system remains constant due to the law of conservation of mechanical energy. As explained earlier, this law says that the mechanical energy of a closed system remains constant if there is no external force applied to it except for gravitational force.
Since there is no friction or air resistance in space, therefore, the mechanical energy of the Earth-Moon system remains constant with the rhythmic interchange between kinetic and potential energy at different times of the month.
9. Electric Motor
Electric motors are present in most of the household gadgets like vacuum cleaners, blenders, washing machines, fans, A.C., etc. Electric motors convert electrical energy into the mechanical energy displaying the example of mechanical energy being utilised in our everyday life.
For example, when we switch on the fan, the electric motor starts converting the electrical energy into mechanical energy. The mechanical energy then gives the fan blades the ability to do work and hence, they start rotating. Therefore, we can say that the mechanical energy, converted by the electric motor, was responsible for work done on the blades.
10. Bow & Arrow
A bow and an arrow form another day-to-day example of mechanical energy. When an arrow is drawn, it possesses mechanical energy in the form of elastic potential energy and when it is released, the bow renders kinetic energy, due to pulling, to the arrow which propels it towards the target. Both these energies, when combined, give the arrow the mechanical energy to move and hit the target.
Hence, the mechanical energy of the arrow did the work on the target by changing its state.