Nicky and Mark are picnicking on a moonlight beach.

　　M: Look. The tide's coming in again.

　　N: I don't understand why we have high tides twice daily --- the moon only revolves around the earth once a day.

　　M: Well, it's because the earth and moon rotate around a common center of gravity that's located somewhere inside the earth.

　　N: Is that because the earth is more massive and so has greater gravitational pull?

　　M: That's right. Now, if you imagine a bowl of soup rotating around a point somewhere near its edge, what happens to the soup?

　　N: Centrifugal force pushes it toward the point furthest, away from the point of rotation.

　　M: Exactly! So the moon's gravity pulls the sea toward it on one side of the earth, creating one high tide.

　　N: Oh, and on the opposite side, the other high tide is created by centrifugal force.

　　M: Right. Just like the bowl of soup analogy. And we pass both high tides during each twenty-four-hour rotation of the earth on its axis.

　　N: Now I understand. You're so smart, Mark. Could gravity attract you to someone like me?

　　M: No. You're not big enough. Your mass is insignificant compared with the earth's.

　　N: Well, in that case, maybe I'll eat some more sandwiches!

　　---by Sean McCormack




科普知识：为什么一天有两次涨潮？
　　尼基和马克正在月光下的海滩上野餐。

　　马克：你看。又涨潮了。

　　尼基：我不懂为什么一天会有两次涨潮──月球一天只绕地球一周。

　　马克：嗯，那是因为地球与月球绕着同一个重心旋转，这个重心位于地球内部某处。

　　尼基：是不是因为地球比较重，地心引力也就比较大？

　　马克：对。现在，想象一碗汤绕着碗边某一点转，汤会怎么样？

　　尼基：离心力会把汤甩向最远的一点，远离旋转点。

　　马克：完全正确！所以月球的引力把地球一侧的海水拉向它，造成一次涨潮。

　　尼基：哦，在另一边，离心力造成另一次涨潮。

　　马克：没错。就像汤碗的比喻一样。地球自转24小时期间，我们就会经历两次的涨潮。

　　尼基：现在我懂了。你真聪明，马克。那引力会不会把你吸引到像我这样的人身边来呀？

　　马克：不会。你的引力还不够大。你的重量跟地球比起来简直微不足道。

　　尼基：嗯，既然这样，也许我会再多吃点三明治！