Kinetic and potential energy

Energy comes in stored and moving forms. Potential energy is the stored kind, held by an object's position, shape or condition. A lifted book has gravitational potential energy: raising it took work, and it can give the energy back by falling. A stretched spring and a raised coaster car both hold potential energy, waiting to be spent. Height is savings; the savings are the potential energy.

Kinetic energy is the energy of motion. Anything that moves has it: a rolling ball, a speeding car, or a molecule shaking. It depends on both mass and speed, and speed matters enormously because it is squared: a car at 60 mph carries four times the kinetic energy of the same car at 30 mph. When an object slows, its kinetic energy goes somewhere; when it speeds up, something paid for it.

The roller coaster is the classic demonstration. The engine hauls the car up the track; at the top, the car holds maximum potential energy and almost no kinetic energy if it is nearly stopped. Gravity pulls it down, height drops, and the potential energy converts into kinetic energy. At the bottom of the drop the car is fastest, with maximum kinetic energy and minimum potential. Height and speed trade places; the energy is conserved, only transformed.

Nothing in that ride was destroyed; energy only changes form. Friction steals some of it, turning motion energy into heat and sound along the track, which is why a real coaster never climbs as high as its first hill. The same conversion runs through everything: a falling apple, a swinging pendulum, a trampoline's stretch and bounce. Track where the energy sits and it is easy to say what happens next.