Introduction:
Energy is a fundamental concept in physics that refers to the ability to do work or cause change. It comes in various forms such as mechanical, thermal, chemical, electrical, and more. The unit used to measure energy is the joule (J).

Joules (J):
- The joule (J) is the base unit of energy in the International System of Units (SI). It is named after the English physicist James Prescott Joule, who made significant contributions to the study of energy.
- The joule is defined as the amount of energy transferred when a force of one newton acts over a distance of one meter in the direction of the force. Mathematically, it can be expressed as J = N * m, where N represents newtons and m represents meters.
- The joule is a versatile unit that can be used to measure various types of energy. For example, it can quantify the amount of work done by a force, the amount of heat energy transferred, or the energy stored in an object.
- Energy values can be expressed in multiples of joules, such as kilojoules (kJ) or megajoules (MJ), for larger amounts of energy. Similarly, smaller amounts of energy can be measured in millijoules (mJ) or microjoules (μJ).

Other Units:
- Newtons (N): Newtons are units of force, not energy. They are used to measure the strength or magnitude of a force.
- Watts (W): Watts are units of power, not energy. Power refers to the rate at which energy is transferred or converted. It is measured in joules per second (J/s) or watts.
- Kilograms (kg): Kilograms are units of mass, not energy. Mass is a measure of the amount of matter in an object.

Conclusion:
In summary, the unit used to measure energy is the joule (J). It is a versatile unit that can quantify various forms of energy. Other units such as newtons, watts, and kilograms are used to measure force, power, and mass respectively, and are not appropriate for measuring energy.