Energy is the capacity to do work or produce change. It comes in many forms: kinetic, potential, thermal, chemical, electrical, nuclear. It can change from one form to another, but the total amount in a closed system stays the same. That is the conservation of energy, one of the most fundamental laws of physics. A ball at the top of a hill has potential energy. As it rolls down, that energy becomes kinetic energy. When it hits the bottom, some of it becomes heat and sound.

Energy is measured in joules. One joule is the energy needed to lift an apple one meter. A calorie is about 4.2 joules, and a food Calorie is 1,000 calories. A kilowatt-hour, used on electricity bills, is 3.6 million joules. Energy is not the same as power. Power is the rate at which energy is transferred. A 100-watt bulb uses 100 joules per second. Energy is also not the same as force. Force is a push or pull. Energy is what allows a force to move something. Energy is one of the most abstract concepts in physics, but it is also one of the most useful.

Forms of energy

• Kinetic. Energy of motion.
• Potential. Stored energy due to position or state.
• Thermal. Energy of random particle motion.
• Chemical. Energy stored in bonds.
• Nuclear. Energy stored in atomic nuclei.

Energy is not the same as matter. Matter has mass and occupies space. Energy does not, though the two are related by Einstein's equation E = mc².