Energy and its forms

Something that can do work has energy. A ball thrown by a fielder can knock down the wickets. A flowerpot on a high ledge could damage something if it fell.

"Where did that energy come from?" From work done on them: the fielder did work on the ball; someone did work raising the pot. When positive work is done on an object, its energy increases. It can then pass that energy on by doing work on something else, like a striker hitting carrom coins.

This is the work-energy theorem: the work done on an object = the change in its energy. It also works for groups of objects, and for forces that change as they act. A simple picture: you eat food giving you 100 J; after running an errand you have 20 J left, so you did 80 J of work.

Energy is measured in joules, named after James Prescott Joule. He studied how movement energy turns into heat: churning water made it warmer.

Doing work is one way to pass energy on, but not the only one. Energy also moves:

• as heat, from hotter to colder things;
• as radiation, like the Sun's energy reaching us;
• through electric circuits;
• as sound waves;
• in nuclear reactions.

Energy comes in many forms: mechanical, electrical, thermal (heat), light, sound, chemical (in fuels and food), nuclear. It keeps changing from one form to another:

In this chapter we focus on mechanical energy: kinetic (moving) and potential (stored).