# Conversation of Energy

## 1. Introduction

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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