What is always present but never visible? ENERGY! Energy is a difficult concept to understand because it is not a concrete object that you can see or touch. Much like mass or volume, energy is a property of an object. It's just that energy is more abstract than some other properties. Although energy itself isn't visible, you can detect evidence of energy.
To understand what energy is, you can begin by first understanding what it does. That is, although energy isn't visible you can detect evidence of energy. Jumping, moving a wheelchair, eating, and singing all require energy. Nonliving things also use energy - a car, vacuum cleaner, and mechanical toys all require energy to move.
Energy is needed to move an object from one place to another, which involves doing work on the object. Because of the relationship between energy and work, energy is often defined as the ability to do work. This does not mean that energy is work, but that energy is needed for work to take place. We often think of work to mean getting something done or accomplishing something useful, which includes moving objects from one place to another among other things. In science, the term work has a very specific meaning, as we shall see later, in Unit 2: Energy Rules! Section B. Energy Transfer.
Not every example of energy is associated with doing the work of moving objects from one place to another. Energy is associated with matter in many ways that don't always show a connection to work. For example, a warm cup of tea has energy in it, but does this energy have the ability to do work? Does a football flying through the air have the ability to do work? Does the energy contained in the ocean have the ability to do work? The answer may be a "yes" under certain and restricted circumstances, but describing energy in this way calls for a broader definition: energy is the ability to transform, organize, or change a system that contains matter.
Since energy is a property of a system that contains matter, scientists have discovered ways to measure and quantify energy. Measuring energy helps to understand how it is used, how it changes forms, and how it may be used efficiently.
In this unit, you will learn more about the various forms of energy and how energy is measured and quantified.