為什麼潛水艇回到水面時,速度比潛入海底慢，（快）

我相信樓主所問之問題，是潛艇在水底時可航行40海浬時速，而在水面最高時速只有20海浬(美國之戰鬥核潛艇)。
這與水面航行時會接觸水面張力；亦要面對海面之風浪；船隻航行時會產生波浪，波浪就是損耗船長動力之主因。在水底航行就只有水之阻力，雖然接觸面大左，阻力亦大左；但還是遠比水面阻力細。

Assume other factors are the same, when a submarine dives, its gravitational potential energy decreases. The loss of potential energy is transformed into kinetic energy which drives the submarine to go faster.

When the submarines goes to surface, there is a gain in gravitational potential energy. The slows down the speed of the submarine.

The situation is similar to riding a bicycle uphill and downhill. The speed of the bicycle going uphill is generally lower than that going downhill, because of a gain/loss of potential energy.

這是作用力與反作用力所産生的效果。

2011-07-06 22:35:32 補充：
潛水艇潛入海底時其作用力向下，再加上潛水艇本身重量（地心吸力）產生加速力。而潛水艇回到水面時其作用力向上，而潛水艇本身重量仍然向下產生反作用力，另方面海洋中水的壓力同样的對上升的潛水艇產生反作用力。因此，潛水艇潛入海底時在作用力與加速力合力強而速度快，潛水艇回到水面時,其作用力在反作用力（潛水艇本身重量、海洋中水的壓力）的合力弱而速度慢。