时差N小时:物理学冲浪

A sailboat is powered by wind, a motorboat bygasoline, and a rowboat by whatever the rower hadfor breakfast. There's one water craft, however, thatgets all its horizontal push from the downward pullof gravity alone. Is this some futuristic new type ofboat? Not at all. It's the humble surfboard, and we'lllearn about the physics of surfing on today's Momentof Science.

帆船靠风行驶，摩托车靠汽油发动，而划艇的动力则来自于浆手的早餐。然而有一种水上运动完全依靠向下的重力获得水平方向的推力。这是具有前瞻性的新型船只吗?不，根本不是。那就是不起眼的冲浪板，在今日科学时刻，我们将要学习物理冲浪。

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

冲浪与乘滑板从山上冲下非常相似，只不过多了一项有利条件——“山”会跟随你一块前行!当一个滑板运动员滑向山下时，重力将她往下推。她的水平方向的运动也部分来自于重力作用。一旦到达山底，她就不能继续滑行了，除非她再次爬上山。

Surfers don't need to climb their watery hills. They simply wait on their boards for a wave torise up beneath them. When they get near the top, they stand on their surfboard, then letgravity start pulling them down the watery hill of the wave.

冲浪者不必爬上浪形成的山。它们只需在冲浪板上等待浪花从底下翻涌上去便可。当他们接近顶部时，就站在冲浪板上任重力将他们推到浪花形成的水山下。

The key to successful surfing is to slide downhill at exactly the same speed the wave moves upbeneath you. If you slide down too fast, you'll reach the bottom of the wave quickly and yourride will be over. If you slide down too slowly, the wave will outrun you.

成功的关键在于，冲浪的速率要与浪花在你身下翻涌的速度一致。如果你滑行的速度过快，很快你就会到达浪底，行程自然就结束了。如果你滑的太慢，浪花就会将你淹没。

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 themole is pushing the dirt up, she'll stay at the same height above the ground--and be coastingdownhill the whole time!

再次设想一下，将正常的山换成一只巨大的，有几百英尺高的鼹鼠，它从地底破土而出。而滑板运动员刚好站在鼹鼠之上，她以鼹鼠推开泥土的速度滑向山下，一直保持在地面之上同样的高度，并且自始至终一直滑行!