What is work? Work is a measure of how much total energy is transferred into or out of an object. In this lesson we'll see that by calculating the work done on an object (how much energy is transferred into or out of it), we can predict the object's future state of motion.
Work is energy transfer. But what happens when work is done on an object such that its height above the ground doesn't change? The answer is that only kinetic energy is transferred into or out of the object. And since work is the total amount of energy transfered into or out of the object, it therefore follows that the work is just equal to the object's change in kinetic energy.
What happens if the only effect of the work done on an object is to change its height without changing its speed? The answer is that the work done on the object only changes in potential energy. In this lesson, we'll consider the work done by Earth's gravity on an object whose height above the ground changes but whose speed stays the same.