Chapter 4

Work, Energy, and Power

It’s difficult to give a precise definition of energy. Loosely speaking, energy is a
quantity which gives an object or system the ability to accomplish something (what
we will define as work). There are different forms of energy partly because there
are different kinds of forces. There’s kinetic energy (a train zooming at high
speeds), gravitational energy (a meteor crashing into the earth), elastic energy (a
stretched rubber band), thermal energy (an oven), radiant energy (sunlight),
electrical energy (a lamp plugged into a wall socket), nuclear energy (nuclear

power plants), and mass energy (the heart of Einstein’s equation E = mc^2 ).

Energy can come into a system or leave it via various interactions that produce
changes. For the SAT Physics Subject Test, you should think of force as the agent of
change, energy as the measure of change, and work as the way of transferring
energy from one system to another. And one of the most important laws in physics
—the law of conservation of energy, equivalent to the first law of
thermodynamics—says that the total amount of energy in a given process will stay