A sailboat is powered by wind, a motorboat by gasoline, and a rowboat by whatever the rower had for breakfast.

There's one water craft, however, that gets all its horizontal push from the downward pull of gravity alone. Is this some futuristic new type of boat? Not at all. It's the humble surfboard; we'll learn about the physics of surfing on today's Moment of Science.

Surfing is a lot like riding a skateboard down a hill–with the added advantage that the hill is moving along with you! When a skateboarder rolls down a hill, she's letting gravity pull her downward. She gets all her horizontal motion as part of this gravitational bargain. Once she reaches the bottom of the hill, she can't coast anymore unless she climbs the hill again.

Surfers don't need to climb their watery hills. They simply wait on their boards for a wave to rise up beneath them. When they get near the top, they stand on their surfboard, then let gravity start pulling them down the watery hill of the wave. The key to successful surfing is to slide downhill at exactly the same speed the wave moves up beneath you. If you slide down too fast, you'll reach the bottom of the wave quickly and your ride will be over. If you slide down too slowly, the wave will outrun you.

Imagine our skateboarder again, but instead of a regular hill there's a monstrous hundred-foot-tall mole pushing up the ground beneath her wheels. If she rolls downhill at the same rate the mole is pushing the dirt up, she'll stay at the same height above the ground-and be coasting downhill the whole time!