Energy is a fundamental concept in physics that describes the ability of a system or object to perform work. It is a scalar quantity meaning it has magnitude but no direction. Energy is usually expressed using an and usually has the units of joules (J).

Conservation of Energy

One of the most important principles in physics is the conservation of energy, which states that energy cannot be created or destroyed. Energy can only be transformed from one form to another. The total energy in a closed system remains constant.

Where is the initial energy and is the final energy, the sum describes how all the forms energy before an "interaction" sum to the same energy as after an "interaction".

Example

For example when throwing a baseball directly upwards energy is converted from the kinetic energy of the ball moving into gravitational potential energy, which is then converted back to the exact same amount of kinetic energy as the ball falls back down to earth.