Energy can be defined, for our purposes, as the "ability to do work." There are two types of energy we are concerned with: kinetic energy and potential energy. Kinetic energy is the energy of an object in motion and potential energy is the "stored" energy of an object while it is at rest. The two are not mutually exclusive: an object can have both kinetic and potential energy at certain points in its movement, but the two must always be conserved. This means that, no matter what, the total amount of energy in a system must remain constant.
Energy is measured in units called joules, J, which are Newtons ÷ meter (N ÷ m). A Newton, just for knowledge sake, is defined as a kilogram × meter ÷ second squared.
These laws of energy have important applications in physics, as we shall see later.
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