Energy occurs in many forms: electricity, nuclear power, gasoline, and our own muscles all provide energy. But, these are only sources of energy, and not energy itself.
Energy is defined as the ability, or potential, to perform work. Science defines work as one physical system working on a second system to produce force applied over distance. Put simply, energy equals the force needed to move an object from one point to another.
Movement is an integral part of energy. Even heat involves movement. Water boils in a kettle because heat from the kettle causes water molecules to move faster. Without that movement, the water would never boil.
Science recognizes two types of energy: kinetic and potential. Kinetic energy is energy associated with actual movement of some kind. A speeding car and a sprinter both possess kinetic energy. The heavier and faster an object, the greater its kinetic energy.
The moment an object stops moving, it loses kinetic energy and gains potential energy. The energy exists, but is stored for later use. Usually an object’s potential energy is in relation to another object. For instance, imagine holding a rock in your hand, above the floor. The rock has potential energy: if you drop the rock, it falls, turning potential energy into kinetic energy. Once the rock hits the ground and stops actually moving, its kinetic energy returns to potential energy.
The Law of Conservation of Matter and Energy dictates that the sum total of matter and energy in the universe remains constant. This means energy isn’t consumed only transferred from one object to another. For instance, throwing a paper plane transfers energy from your body to the plane without diminishing the total amount of energy in the universe.