Energy is a quantity that has the following basic properties:

• Energy is a scalar quantity.
  
  
• Energy is an additive quantity. This means that the total energy of a system is the sum of the energies of the parts of the system.
  
  
• Energy comes in different forms: mechanical, thermal, chemical, nuclear, etc.
  
  Mechanical energy is the sum of kinetic and potential energy. Potential energy is related to internal conservative forces of a system. It has different mathematical expressions, depending on what forces are involved.
  
  
• Energy of one form can be transformed into energy of another form, but energy cannot be created or destroyed.
  
  
• Energy can be 'moved' in or out of a system. Scientists say that energy can be 'transferred'.
  
  
• There are different kinds of energy transfer, e.g., work, heat, and radiation. Work is transfer of mechanical energy. Heat is transfer of thermal energy. Electromagnetic radiation is transfer of electromagnetic energy.

The SI-unit of energy is the joule (J).