The Environment of the Earth’s Surface (Southard)

Energy

First you need to feel comfortable about the nature of energy. We have to be concerned with three forms of energy: energy of motion (called kinetic energy), energy of position (called potential energy), and energy of thermal motions of atoms of molecules of a material (called thermal energy or heat energy). Kinetic energy and potential energy together are often referred to as mechanical energy.

If you are not already familiar with these concepts of energy, here’s a little demonstration that should serve the purpose of giving you the basic ideas. You will need a lump of modeling clay or putty. Throw the lump straight up in the air. When it leaves your hand, it has a certain speed and therefore a certain kinetic energy. It’s slowed by the downward force of gravity as it rises, so it loses kinetic energy, but that lost kinetic energy is stored as energy of position, by virtue of being located higher up in the gravity field of the Earth. When it reaches the top of its trajectory, all of its kinetic energy has been stored as potential energy. Then on the way down, the stored potential energy is converted back into kinetic energy. If you were doing this in a vacuum in some enormous evacuated bell jar or on the surface of the Moon, the putty would have the same speed when it lands back in your hand as when you threw it upward. The mechanical energy of the lump, kinetic plus potential, is the same all along the trajectory; the mechanical energy is said to be conserved.

On the real surface of the Earth, of course, the lump is bathed in air, and its motion is retarded by friction both on the way up and on the way down. That friction warms the lump (and the air it passes through), slightly, converting some of the mechanical energy into thermal energy.

Another thing you can do with the lump is drop it on the floor from some high place. It has some kinetic energy as it lands on the floor, but then it comes to a stop upon impact, without even rebounding. What happened to all of its kinetic energy? It was converted entirely to thermal energy, via the friction involved in the deformation of the lump as it made impact. If you had the right equipment you could actually verify for yourself that the lump was warmer after it landed than before.