時差N小時:物理學(xué)沖浪

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.

帆船靠風(fēng)行駛，摩托車靠汽油發(fā)動，而劃艇的動力則來自于漿手的早餐。然而有一種水上運動完全依靠向下的重力獲得水平方向的推力。這是具有前瞻性的新型船只嗎?不，根本不是。那就是不起眼的沖浪板，在今日科學(xué)時刻，我們將要學(xué)習(xí)物理沖浪。

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.

沖浪與乘滑板從山上沖下非常相似，只不過多了一項有利條件——“山”會跟隨你一塊前行!當一個滑板運動員滑向山下時，重力將她往下推。她的水平方向的運動也部分來自于重力作用。一旦到達山底，她就不能繼續(xù)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.

沖浪者不必爬上浪形成的山。它們只需在沖浪板上等待浪花從底下翻涌上去便可。當他們接近頂部時，就站在沖浪板上任重力將他們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.

成功的關(guān)鍵在于，沖浪的速率要與浪花在你身下翻涌的速度一致。如果你滑行的速度過快，很快你就會到達浪底，行程自然就結(jié)束了。如果你滑的太慢，浪花就會將你淹沒。

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!

再次設(shè)想一下，將正常的山換成一只巨大的，有幾百英尺高的鼴鼠，它從地底破土而出。而滑板運動員剛好站在鼴鼠之上，她以鼴鼠推開泥土的速度滑向山下，一直保持在地面之上同樣的高度，并且自始至終一直滑行!