Relativity Lite

50 | Relativity Lite visualizing a marble rolling from one rim of a bowl down to the bottom, across that and up to the opposite rim, as in figure 6. This diagram is purely Newtonian, with no “bending of spacetime” to it. The marble is momentarily motionless at the rim so it has no kinetic energy at that point. Newton’s “gravitational force” pulls it downward, and it gains speed. It moves its fastest across the bottom of the bowl—where it has zero potential to fall further and, thus, zero po- tential energy—so we say that it has pure kinetic energy during that passage. It loses speed as it climbs the far wall of the bowl, converting its kinetic energy into potential energy, so that when it reaches the opposite rim, it has no kinetic energy and maximal potential energy—perhaps understood as the potential to release energy back into kinetic energy as it rolls back down. Figure 6. The transformation of potential energy into kinetic energy is demonstrated in Newtonian terms by a marble rolling in a bowl. In the initial position, the marble is temporarily still on the upper rim of the bowl on the right. Newton’s “gravitational force” pulls it downward, and it gains speed, rolling downward and transforming its potential to move into actual motion. This kinetic energy is greatest as it rolls across the bottom of the bowl, where its potential energy is zero. As it rolls up the left-hand wall, it loses kinetic energy and gains potential energy until it temporarily comes to rest on the opposite rim, where its potential energy is greatest. This process is an expression of the law of conservation of energy : energy is neither cre- ated nor destroyed but simply converted from one form to another. Another example is the conversion of the stored chemical energy of gasoline into motion in a car upon combustion. Unfortunately, one cannot back up the car to get that energy back, nor will doing so retrieve the carbon the process releases into the atmosphere.