Short Answer:

Energy is defined as the capacity of a body to do work. It means that a body possesses energy if it can apply force to move another object or produce some change in its surroundings. Energy is one of the most fundamental concepts in mechanics and physics.

In simple words, energy enables any physical system to perform work. It exists in various forms such as mechanical, thermal, electrical, chemical, and potential energy. The SI unit of energy is Joule (J), which is the same as that of work, since energy and work are closely related physical quantities.

Detailed Explanation:

Energy

In mechanical engineering, energy plays a very important role in understanding how systems perform work, move, or transfer power. Every physical or mechanical process involves energy in some form — either being used, stored, or transformed.

Energy is the measure of a system’s ability to perform work. It can exist in different forms such as potential energy (energy due to position), kinetic energy (energy due to motion), or thermal energy (due to temperature). Although energy can change from one form to another, the total energy of a closed system always remains constant, which is known as the law of conservation of energy.

Definition of Energy

Energy is defined as the capacity of a body or system to do work. When a body is capable of applying force and causing displacement, it is said to possess energy.

Mathematically,

Since work and energy are equivalent, their SI unit is also the same:

This means that when a force of 1 newton displaces a body by 1 meter in the direction of the force, 1 joule of energy is expended or transferred.

Forms of Energy

Energy can exist in several forms depending on the situation and type of system. The main forms of energy relevant to mechanical engineering are as follows:

1. Mechanical Energy

Mechanical energy is the energy possessed by a body due to its motion or position. It is further classified into two types:

(a) Kinetic Energy:
Kinetic energy is the energy possessed by a body due to its motion. If a body of mass  moves with velocity , then its kinetic energy is given by:

This energy depends on the mass of the body and the square of its velocity.

Examples:

• Moving vehicles on a road.
• Rotating machinery parts.
• Flowing water in turbines.

(b) Potential Energy:
Potential energy is the energy possessed by a body due to its position or configuration. For example, a raised object or compressed spring possesses potential energy because of its stored capacity to do work.
For a body of mass  raised to a height :

where  is the acceleration due to gravity.

Examples:

• A stone held at a height above the ground.
• A stretched rubber band.
• A compressed spring in a machine.

The total mechanical energy of a system is the sum of its kinetic and potential energies:

2. Thermal Energy

Thermal energy is the internal energy of a body due to the motion of its molecules. It is related to the temperature of the body — the higher the temperature, the greater the thermal energy.

Examples:

• Heat produced by friction in mechanical parts.
• Heat energy in steam engines.
• Energy generated in combustion engines.

3. Chemical Energy

Chemical energy is stored within the bonds of atoms and molecules. It is released during chemical reactions such as combustion or oxidation.

Examples:

• Energy in fuels like petrol, diesel, and coal.
• Energy stored in batteries.
• Food energy in biological systems.

4. Electrical Energy

Electrical energy is associated with the movement of electric charges. It is widely used in mechanical systems through motors, generators, and circuits.

Examples:

• Electric motors converting electrical energy into mechanical work.
• Lighting and heating systems.

5. Nuclear Energy

Nuclear energy is stored in the nucleus of atoms and released during nuclear reactions such as fission or fusion. It is a highly concentrated form of energy used in power plants and defense applications.

Examples:

• Energy generated in nuclear reactors.
• The sun’s energy produced by nuclear fusion.

Law of Conservation of Energy

The law of conservation of energy states that energy can neither be created nor destroyed; it can only be transformed from one form to another. The total amount of energy in a closed system remains constant.

Mathematically,

In mechanical systems, some of the energy may be converted into heat due to friction, but the overall energy in the system remains the same.

Examples:

• In a pendulum, potential energy is converted into kinetic energy and back again during oscillation.
• In an engine, chemical energy of fuel is converted into mechanical energy of motion.

Units and Dimensions of Energy

1. SI Unit: Joule (J)

1. CGS Unit: Erg

1. Other Units:
   • Kilojoule (kJ) = 10³ J
   • Megajoule (MJ) = 10⁶ J
   • Calorie (cal) = 4.186 J
   • Kilowatt-hour (kWh) = 3.6 × 10⁶ J
2. Dimensional Formula:

Applications of Energy in Mechanical Engineering

1. Power Generation:
   Energy is converted from thermal or chemical forms into mechanical energy to generate electricity.
2. Engines and Machines:
   In engines, chemical energy of fuel is converted into kinetic energy of moving parts.
3. Hydraulic and Pneumatic Systems:
   Potential energy of fluids is converted into mechanical energy for power transmission.
4. Structural Design:
   The study of energy helps in analyzing work done by loads and determining strain energy in structures.
5. Energy Conservation:
   In modern mechanical systems, efforts are made to reduce energy losses and improve efficiency.

Examples of Energy Transformation

1. Hydroelectric Power Plant:
   Potential energy of water → Kinetic energy → Electrical energy.
2. Internal Combustion Engine:
   Chemical energy of fuel → Thermal energy → Mechanical energy.
3. Electric Motor:
   Electrical energy → Mechanical energy.
4. Wind Turbine:
   Kinetic energy of air → Mechanical energy → Electrical energy.

Conclusion

In conclusion, energy is the capacity of a body or system to do work. It exists in various forms — mechanical, thermal, electrical, chemical, and nuclear — and can be transformed from one form to another. The total energy of a closed system remains constant, as stated by the law of conservation of energy. Understanding energy is essential in mechanical engineering because all machines, engines, and processes are based on the conversion and utilization of energy to perform useful work efficiently.