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
科普知識：為什么一天有兩次漲潮？
尼基和馬克正在月光下的海灘上野餐。
馬克：你看。又漲潮了。
尼基：我不懂為什么一天會有兩次漲潮──月球一天只繞地球一周。
馬克：嗯，那是因為地球與月球繞著同一個重心旋轉(zhuǎn)，這個重心位于地球內(nèi)部某處。
尼基：是不是因為地球比較重，地心引力也就比較大？
馬克：對?，F(xiàn)在，想象一碗湯繞著碗邊某一點轉(zhuǎn)，湯會怎么樣？
尼基：離心力會把湯甩向最遠的一點，遠離旋轉(zhuǎn)點。
馬克：完全正確！所以月球的引力把地球一側(cè)的海水拉向它，造成一次漲潮。
尼基：哦，在另一邊，離心力造成另一次漲潮。
馬克：沒錯。就像湯碗的比喻一樣。地球自轉(zhuǎn)24小時期間，我們就會經(jīng)歷兩次的漲潮。
尼基：現(xiàn)在我懂了。你真聰明，馬克。那引力會不會把你吸引到像我這樣的人身邊來呀？
馬克：不會。你的引力還不夠大。你的重量跟地球比起來簡直微不足道。
尼基：嗯，既然這樣，也許我會再多吃點三明治！