Recall the concept of mechanical energy and its unit of measurement from your mechanics class. Review the new material. Answer the questions.
We learned that the potential energy of a body (such as an apple) at above the ground is given by
When an apple starts to fall from the top of a tree, the potential energy of the apple decreases as the apple gets closer to the ground. However, the energy cannot simply disappear. As the falling apple increases speed, its potential energy becomes kinetic energy. Since the law of conservation of mechanical energy holds
the kinetic energy of an apple hitting the ground is
We also learned that energy is measured in joules (denoted by ).
So, for example, if an apple weighing falls to the ground from a height of , the energy of its motion just before it reaches the ground is
Energy in mechanics and thermodynamics
In both mechanics and thermodynamics, we measure energy in joules. In mechanics, for example, joules measure the energy of motion, whereas in thermodynamics, we use joules to measure the heat given to or taken from bodies (or heat flux, or heat energy). In everyday situations, all mechanical energy is converted into heat, and joule of kinetic energy becomes joule of heat, so the law of conservation of energy applies here too.