關於 how do currents work ，我們在網路上蒐集到這些相關的討論、資訊與評價

這個標誌是危險的宣傳，它提升了瀑布暗流的神話。 如圖所示，在此瀑布中，任何其他瀑布中，甚至在海灘上都不會出現暗流。 除少數極端情況外，唯一會在大壩和水力發電廠的排水管附近找到類似水流的地方。 由於在池底沒有出口流出，因此自然界中不容易發生這種現象。 在瀑布底部或附近漂浮的困難是由於與水中混合的空氣有關的浮力的變化，而不是向下的水流。

實際上，該瀑布的流入和流出都在水池的表面。 這意味著最強的水流在水面上，最弱的水流在底部。 在一年中的大部分時間裡，幾乎沒有地下暗流。 如果您沒有在頂部看到它們，它們就不會隱藏在底部。

當河流中有大量的水流過時，瀑布的力量會在水池表面形成一個小的凹陷，導致水向該方向流動。 多數反彈並產生從池中流出的水流。 有些也從瀑布流回到瀑布下的凹陷處。 這些僅表面水流。 由於水只能從較高的高度（例如，地面）向下流動，因此它們不會在池中深處延伸。

因此，要逃開這些水流的正確方法是在它們下面游泳。 表面水流將您反覆拉出來，而不是暗流。 因此，要逃脫，您需要向水底下游泳。 這是應該在標誌上顯示的消息。 逃脫離岸流方法相同，在這裡也適用。卡住在垂直的水流中游泳，不要跟它對抗。但要在瀑布前游泳，是有悖常理的。中心的水流將帶您離開瀑布跟水池。這些訊息應該要標示在告牌上。就像在海灘告示牌上一樣，有標示如何逃開離岸流，所以這裡也應該有標示如何逃開瀑布中的水流。 但相反地，這個標誌沒有提供任何有用的訊息，實際上還具有誤導性。 這是進行水安全教育的尷尬方式。

附註1：這僅適用於一個水池範圍。 在河流中，整個橫截面都在流動，由於摩擦因素使其底部水流動仍然較慢。 而表面的水流動得更快。

附註2：颱風過後，當河流太大而超過水池的承載能力時，以上這些規則將失效。 這並不重要，因為對於任何理智的人來說，顯然無論如何在颱風過後是不要進入溪河裡。

This sign is dangerous propaganda which promotes the myth of the waterfall undercurrent. Currents as depicted in this picture do not occur in this waterfall, any other waterfall, or even at the beach for that matter. With a few extreme exceptions, the only place you will find currents like that are near drainage pipes for dams and hydroelectric power plants. It's not a phenomenon that can occur easily in nature because there is no outflow at the bottom of the pool. Difficulties with staying afloat at or around the base of the waterfall is due to changes in buoyancy related to air mixed in the water... not downward currents.
In fact, the inflow and drainage for this waterfall are both on the surface of the pool. This means that the strongest currents on the surface of the water, and the weakest currents are on the bottom. For most of the year there are barely any subsurface currents at all. If you don't see them on top... they aren't hiding on the bottom.

When there is a lot of water flowing in the river, the force of the waterfall creates a small depression in the surface of the pool, causing water to flow in that direction. The majority bounces out and creates a current flowing out of the pool. While some also flows back to the depression under the waterfall, typically from the side. These surface-only currents. They do not extend deep in the pool as water can only flow downward from a higher elevation (eg, the surface).

Therefore, the proper way to escape these currents is to swim under them. Surface currents are pulling you out, not undercurrents. So to escape you need to swim down. The same method for escaping a ripcurrent will also work here. Swim perpendicular to the current you are stuck in, not against it. It's counterintuitive, but swim in front of the waterfall. The current in the center will take you away from the waterfall and out of the pool. This is the message that should be on the sign. Just like at the beach there are signs explaining how to escape a rip current, there can be signs explaining how to escape a back current in a waterfall. Instead this sign provides no useful information, and is in fact misleading. It’s an embarrassing way to conduct water safety.

Note: This only applies to pools. In a river the entire cross sectional area is flowing, although it's still slower at the bottom due to friction. Surface water moves faster.

Note 2: After a typhoon when the river is so large as to exceed the carrying capacity of the pool, these rules breakdown. It's not important because to any sane person, it's obvious not to go into the river after a typhoon anyway.

現在是夏天，每個人都在瀑布裡游泳，我開始看到一連串的留言警告，警告人們不要在瀑布中形成隱藏的，看不見的水流下游泳。這是一個非常常見的留言，用來嚇唬沒有經驗的游泳者或不熟悉瀑布的人，以免他們掉入水中。它常出現在FB和YT的留言中，並在新聞文章中發表，甚至溯溪教練有時也會重複（即使他們知道這是錯誤的）。在每個深瀑布池中，水下的水流比表面的流得慢。
😣
這是深水池的物理屬性，沒有例外。頂部放快水，底部放慢水。發生這種情況是因為水在水池的頂部進入和流出，所以水流在整個表面上做移動，而且還因為更深的水與其他水分子和水池底部之間的摩擦更大，從而減慢了水的速度（動能為轉化為熱量）。告訴人們深水池中存在隱性水流，這不僅不準確，而且很危險。
😊
向人們提供有關安全的虛假信息是危險的。試圖通過散佈煽動性的恐懼心理來恐嚇某人遠離水域，這只會適得其反。如果您了解水流的工作原理，那麼您應該在教這些知識，而不是虛構知識。
💦
知道不是看不見的水下水流非常重要，原因有兩個：
🌊
1）當水流過大而您不能逆流游泳時，最好的措施是在水底下游泳。沒錯！您可以簡單地更深入地潛水並在水流底下游泳，而完全避開它。在下面的影片中，表面水流過強而無法游泳抵抗。在水底下游泳卻很容易。在1-2公尺深處，水幾乎沒有移動。 （觀看我們輕鬆逆流游泳）
☀
2）告訴人們水下存在看不見的水流，而忽略了表面上存在非常可見且可預測的水流這一事實。只要通過查看水池，我們就可以對水池的流體動力學有很多了解。通過視覺觀察，我們可以輕鬆地看到流動的速度和方向。如果要使水流方向更清晰，也可以拋出棍子或樹葉之類的物體。

因此，你無法看到水中有什麼時，請不要告訴別人那裡很危險。這不是事實。您只需看一下就可以看到發生了什麼。在您看到它之後，它不再是未知的危險。一旦知道了，您就可以進行研究並製定相應的計劃。水流可能會成為繞行的障礙，或者是幫助您到達目的地的工具。這不是一個令人恐懼的隱藏妖怪惡靈，而是河中自然而可預測的一部分。知道自己的極限，學習如何閱讀河流，並做好相應的準備。這樣，您每次都會安全。教育永遠比嚇阻來的有效！
🙏
EDIT:一些評論認為這對不懂游泳的人沒有幫助。 這是不正確的。 如果您不知道如何游泳，那麼此信息對您而言甚至更為重要。 因為您需要潛水才能在強流下游泳，所以這意味著，如果您穿著救生衣，將會被卡住而無法返回。 這意味著，如果您不會潛水或不會游泳，那麼在這種情況下您不應入水。 即使有救生衣。 知識總是使您更安全。Some comments suggest this is not helpful for people that don't know how to swim. This is incorrect. If you don't know how to swim, this information is even more important for you. Because you need to dive to swim against a strong current, it means that if you are wearing a life jacket, you will be stuck and unable to return. What this means is that if you can't dive, or can't swim, you should not go in the water in that situation. Even with a life jacket. Knowledge always makes you safer.
,🤙
Wutan Waterfal (Dongxi Waterfall)
Pingtung County, Taiwu Township
武潭瀑布 (東西瀑布)
屏東縣泰武鄉
Waterfall and Parking: 22.585093, 120.641308
💪
Now that it's summer and everyone is out swimming in waterfalls, I'm starting to see a string of comments warning people against hidden, unseen underwater currents that can form in waterfalls. It's a very common phrase which is used to scare away inexperienced swimmers or people unfamiliar with waterfalls, so that they don't go in the water. It shows up in comments all over FB and YT, it's published in news articles, and even river tracing coaches repeat it sometimes (even though they know it's false). In every single deep waterfall pool, the current is slower under the water than it is on the surface.

This is the physical property of a deep pool, there are no exceptions to it. Fast water on top, slow water on the bottom. This happens because water is entering and exiting at the top of the pool, so the flow is moving across the surface, and also because deeper water has more friction against other water molecules and the pool bottom, which slows down the water (kinetic energy is converted to heat). Telling people that there are hidden currents in deep pools is not only inaccurate, but it's also dangerous.

Giving people false information regarding safety is dangerous. Trying to scare someone into staying out of the water by spreading sensationalist fear-mongering is counter productive. If you have knowledge about how currents work, you should be teaching that knowledge instead of making stuff up.

It's so important to know that aren't invisible underwater currents for two reasons:

1) When the current is too strong, and you can't swim against it, the best action to take is to swim under it. That's right. You can simply dive deeper and swim under the current, avoiding it entirely. In the video below the surface current is too powerful to swim against. It's very easy to swim under. At 1-2 meters depth, the water is practically unmoving. (Watch Nicky swim directly against the current with ease)

2) Telling people that there are invisible currents underwater ignores the fact that there are very visible and predictable currents on the surface. We can understand a lot about the fluid dynamics of a pool just by looking at it. We can easily see the speed and direction of the flow by visual observation. Objects such as sticks or leaves can also be thrown if to make it more clear.

So stop telling people that there is a danger you can't see in the water. It's not true. You can see what happens just by looking at it. And after you see it, it's not an unknown danger anymore. Once it's known to you, you can study it and plan accordingly. The current can be an obstacle to get around, or a tool to help you get where you are going. It's not a hidden boogie man to be feared, but a natural and predictable part of the river. Know your limits and learn how to read the river, and prepare accordingly. Then you will be safe, every single time.

Apple CEO tells college graduates: ‘We’ve failed you’
蘋果CEO給大學畢業生的致詞

蘋果執行長庫克(Tim Cook)於5月18日應邀至杜蘭大學(Tulane Univeristy)做畢業典禮演講(Commencement Speech)，內容是鼓勵畢業生處理困難的問題，有勇氣嘗試找出解決問題的方法，並以20年前的親身經驗告訴年輕學子，為何當年從前途似錦的科技業巨擘康柏公司(Compaq)，投入前途黯淡的蘋果公司。

杜蘭大學是位於紐奧良的研究型私立大學，有「南方常春藤」之稱，以下摘錄庫克的演講內容：

∎ Life will always find lots of ways to tell you no, that you can't, that you shouldn't, that you'd be better off if you didn't try. But New Orleans teaches us there is nothing more beautiful or more worthwhile than trying.
人生總會用很多方式告訴你，這個不可以、那個做不到、你不應該這麼做，或是你最好連試都別試。但紐奧良教導我們，沒什麼比嘗試更美妙，更有價值。

∎ For me, it was that search for greater purpose that brought me to Apple in the first place. I had a comfortable job at a company called Compaq that at the time looked like it was going to be on top forever.
對我來說，當初就是為了尋找更大的目的，才讓我來到蘋果。我原本在康柏的工作很舒服，而且那時康柏看來將永遠處於顛峰。

∎ As it turns out, most of you are probably too young to even remember its name. But in 1998, Steve Jobs convinced me to leave Compaq behind to join a company that was on the verge of bankruptcy.
你們大多數人可能都太年輕，不記得康柏的名字，但在1998年，賈伯斯說服了我離開康柏，加入一家處於破產邊緣的公司。

∎ They made computers, but at that moment at least, people weren't interested in buying them. Steve had a plan to change things. And I wanted to be a part of it.
他們生產電腦，但至少那時大家沒什麼興趣買電腦。賈伯斯想要改變這個局面，而我想參與其中。

∎ It wasn't just about the iMac, or the iPod, or everything that came after. It was about the values that brought these inventions to life.
這不只攸關iMac或iPod，或之後問世的所有東西，而是關於把這些創新真正做出來的價值。

∎ The idea that putting powerful tools in the hands of everyday people helps unleash creativity and move humanity forward. That we can build things that help us imagine a better world and then make it real.
這個想法是將強大工具放到一般人的手中，釋放出創造力，推動人類前進；也就是我們可以打造的東西，能讓我們想像出更美好的世界，再實現這個夢想。

∎ Try something. You may succeed. You may fail. But make it your life's work to remake the world because there is nothing more beautiful or more worthwhile than working to leave something better for humanity.
去多多嘗試，你可能成功，也可能失敗，但要把改造世界變成你的人生目標，努力留下任何東西讓人類更好，沒有什麼比這麼做更美妙、更值得。

以下是演講內容全文：
Hello Tulane! Thank you, President Fitts, Provost Forman, distinguished ( ) faculty ( ), other faculty [laughs], and the entire Tulane family, including the workers, ushers ( ), [and] volunteers who prepared this beautiful space. And I feel duty-bound ( ) to also recognize the hard-working bartenders at The Boot. Though they’re not here with us this morning, I’m sure some of you are reflecting on their contributions as well. [The Boot is a popular college bar right next to Tulane’s campus which has been around for decades.]

And just as many of you have New Orleans in your veins ( ), and perhaps your livers, some of us at Apple have New Orleans in our blood as well. When I was a student at Auburn, the Big Easy was our favorite getaway ( ). It’s amazing how quickly those 363 miles fly by when you’re driving toward a weekend of beignets and beer. And how slowly they go in the opposite direction. Apple’s own Lisa Jackson is a proud Tulane alum ( ). Yes. She brought the Green Wave all the way to Cupertino where she heads our environment and public policy work. We’re thrilled to have her talent and leadership on our team.

OK, enough about us. Let’s talk about you. At moments like this, it always humbles me to watch a community come together to teach, mentor ( ), advise, and finally say with one voice, congratulations to the class of 2019!

Now there’s another very important group: your family and friends. The people who, more than anyone else, loved, supported, and even sacrificed ( ) greatly to help you reach this moment. Let’s give them a round of applause ( ). This will be my first piece of advice. You might not appreciate until much later in your life how much this moment means to them. Or how that bond of obligation ( ), love, and duty between you matters more than anything else.

In fact, that’s what I really want to talk to you about today. In a world where we obsessively ( ) document our own lives, most of us don’t pay nearly enough attention to what we owe one another. Now, this isn’t just about calling your parents more, although I’m sure they’d be grateful if you did that. It’s about recognizing that human civilization began when we realized that we could do more together. That the threats and danger outside the flickering firelight got smaller when we got bigger. And that we could create more — more prosperity ( ), more beauty, more wisdom, and a better life — when we acknowledge certain shared truths and acted collectively.

Maybe I’m biased ( ), but I’ve always thought the South, and the Gulf Coast in particular ( ), have hung on to ( ) this wisdom better than most. [Tim Cook grew up in Robertsdale, Alabama, which is about an hour from New Orleans and is similarly close to the Gulf of Mexico.] In this part of the country, your neighbors check up on you if they haven’t heard from you in a while. Good news travels fast because your victories are their victories too. And you can’t make it through someone’s front door before they offer you a home-cooked meal.

Maybe you haven’t thought about it very much, but these values have informed your Tulane education too. Just look at the motto ( ): not for one’s self, but for one’s own. You’ve been fortunate to live, learn, and grow in a city where human currents blend into ( ) something magical and unexpected. Where unmatched beauty, natural beauty, literary beauty, musical beauty, cultural beauty, seem to spring ( ) unexpectedly from the bayou. The people of New Orleans use two tools to build this city: the unlikely and the impossible. Wherever you go, don’t forget the lessons of this place. Life will always find lots of ways to tell you no, that you can’t, that you shouldn’t, that you’d be better off if you didn’t try. But New Orleans teaches us there is nothing more beautiful or more worthwhile than trying. Especially when we do it not in the service of one’s self, but one’s own.

For me, it was that search for greater purpose that brought me to Apple in the first place. I had a comfortable job at a company called Compaq that at the time looked like it was going to be on top forever. As it turns out, most of you are probably too young to even remember its name. But in 1998, Steve Jobs convinced me to leave Compaq behind to join a company that was on the verge of bankruptcy. They made computers, but at that moment at least, people weren’t interested in buying them. Steve had a plan to change things. And I wanted to be a part of it.

It wasn’t just about the iMac, or the iPod, or everything that came after. It was about the values that brought these inventions to life. The idea that putting powerful tools in the hands of everyday people helps unleash creativity and move humanity forward. That we can build things that help us imagine a better world and then make it real.

There’s a saying that if you do what you love, you’ll never work a day in your life. At Apple, I learned that’s a total crock ( ). You’ll work harder than you ever thought possible, but the tools will feel light in your hands. As you go out into the world, don’t waste time on problems that have been solved. Don’t get hung up on what other people say is practical. Instead, steer ( ) your ship into the choppy ( ) seas. Look for the rough spots, the problems that seem too big, the complexities ( ) that other people are content to work around. It’s in those places that you will find your purpose. It’s there that you can make your greatest contribution. Whatever you do, don’t make the mistake of being too cautious. Don’t assume that by staying put, the ground won’t move beneath your feet. The status quo ( ) simply won’t last. So get to work on building something better.

In some important ways, my generation has failed you in this regard ( ). We spent too much time debating. We’ve been too focused on the fight and not focused enough on progress. And you don’t need to look far to find an example of that failure. Here today, in this very place, in an arena where thousands once found desperate shelter ( ) from a 100-year disaster, the kind that seem to be happening more and more frequently, I don’t think we can talk about who we are as people and what we owe to one another without talking about climate change.

[applause] Thank you. Thank you.

This problem doesn’t get any easier based on whose side wins or loses an election. It’s about who has won life’s lottery and has the luxury of ignoring this issue and who stands to lose everything. The coastal communities, including some right here in Louisiana, that are already making plans to leave behind the places they’ve called home for generations and head for higher ground. The fishermen whose nets come up empty. The wildlife preserves ( ) with less wildlife to preserve. The marginalized ( ), for whom a natural disaster can mean enduring poverty.

Just ask Tulane’s own Molly Keogh, who’s getting her Ph.D. this weekend. Her important new research shows that rising sea levels are devastating ( ) areas of Southern Louisiana more dramatically than anyone expected. Tulane graduates, these are people’s homes. Their livelihoods ( ). The land where their grandparents were born, lived, and died.

When we talk about climate change or any issue with human costs, and there are many, I challenge you to look for those who have the most to lose and find the real, true empathy ( ) that comes from something shared. That is really what we owe one another. When you do that, the political noise dies down, and you can feel your feet firmly planted on solid ground. After all, we don’t build monuments ( ) to trolls ( ), and we’re not going to start now.

If you find yourself spending more time fighting than getting to work, stop and ask yourself who benefits from all the chaos. There are some who would like you to believe that the only way that you can be strong is by bulldozing ( ) those who disagree or never giving them a chance to say their peace in the first place. That the only way you can build your own accomplishments is by tearing down ( ) the other side.

We forget sometimes that our preexisting beliefs have their own force of gravity ( ). Today, certain algorithms ( ) pull toward you the things you already know, believe, or like, and they push away everything else. Push back. It shouldn’t be this way. But in 2019, opening your eyes and seeing things in a new way can be a revolutionary act. Summon the courage not just to hear but to listen. Not just to act, but to act together.

It can sometimes feel like the odds ( ) are stacked ( ) against you, that it isn’t worth it, that the critics are too persistent and the problems are too great. But the solutions to our problems begin on a human scale with building a shared understanding of the work ahead and with undertaking it together. At the very least, we owe it to each other to try.

It’s worked before. In 1932, the American economy was in a free-fall ( ). Twelve million people were unemployed, and conventional ( ) wisdom said the only thing to do was to ride it out, wait, and hope that things would turn around ( ). But the governor ( ) of New York, a rising star named Franklin Roosevelt, refused to wait. He challenged the status quo and called for action ( ). He needed people to stop their rosy ( ) thinking, face the facts, pull together ( ), and help themselves out of a jam. He said: “The country demands bold, persistent experimentation. It is common sense to take a method and try it. If it fails, admit it and try another. But above all, try something.”
This was a speech to college students fearful ( ) about their future in an uncertain world. He said: “Yours is not the task of making your way in the world, but the task of remaking the world.” The audacious ( ) empathy of young people, the spirit that says we should live not just for ourselves, but for our own. That’s the way forward. From climate change to immigration, from criminal justice reform to economic opportunity, be motivated by your duty to build a better world. Young people have changed the course of history time and time again. And now it’s time to change it once more.

I know, I know the urgency of that truth is with you today. Feel big because no one can make you feel strong. Feel brave because the challenges we face are great but you are greater. And feel grateful because someone sacrificed to make this moment possible for you. You have clear eyes and a long life to use them. And here in this stadium, I can feel your courage.

Call upon your grit ( ). Try something. You may succeed. You may fail. But make it your life’s work to remake the world because there is nothing more beautiful or more worthwhile than working to leave something better for humanity.

Thank you very much, and congratulations class of 2019!

#高雄人 #學習英文 請找 #多益達人林立英文
#高中英文
#成人英文
#多益家教班
#商用英文