In a similar way, the idea of {{CMD_20}}, is ``energy'' objects have from motion. It is defined by the formula {{MATH_21}} The SI unit of energy is a {{CMD_22}}, which is defined to be {{MATH_23}} To get a feel for the ``size'' of a joule, consider this: if an apple has a mass of {{MATH_24}} and it is dropped from a height of {{MATH_25}}, then approximately {{MATH_26}} joule of energy is released when it hits the ground. Let's see if we can explain why this is true.