What is buoyancy?

Have you ever wondered how a massive metal cruise ship weighing 100,000 tons can float, but a tiny pebble drops straight to the bottom of the ocean? It seems completely backwards! Get ready to dive into the invisible, magical battle happening in the water every single day: buoyancy! Join Mira and Finn on Science is Not Boring as they uncover the wild secrets of why things float and why they sink. You will travel back in time to Ancient Greece to meet a brilliant scientist named Archimedes, who solved a royal mystery in his bathtub and ran dripping wet through the streets shouting 'Eureka!' Discover the mind-blowing science of displacement, and learn the golden magic rule of floating. Plus, we will explore some epic animal fun facts! Find out how the Blue Whale—the biggest animal on Earth with a tongue as heavy as an elephant—glides through the sea, and why swimming in the super-salty Dead Sea is like sitting in an invisible floating chair! Tune in for this splashy adventure, and don't forget to stick around for our trivia quiz at the end!

Transcript

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Welcome to Science Is Not Boring by Kidopoly.com. I'm Mira. And I'm Finn. Okay Finn, picture this. One of the world's largest cruise ships, the icon of the seas, weighs around a hundred thousand tonnes and is 1,198 feet long. That's like stacking thousands and thousands of elephants on top of each other. Thousands of elephants? That's unbelievably heavy. That is definitely going to sink straight to the bottom of the ocean. Nope. It floats perfectly on the ocean waves, carrying thousands of passengers and massive swimming pools. But if you drop a tiny little pebble that weighs less than a piece of candy into the exact same water, what happens? It goes straight to the bottom. Plop. Not even a contest. Exactly. It seems totally backwards, right? A massive metal skyscraper floats, but a tiny rock sinks. Today we are exploring the wild, invisible science of floating. Wait, so why does a heavy ship float when a tiny rock sinks? I need to know how the water decides. We'll find out exactly how. We are going to discover how enormously heavy blue whales glide through the sea, why swimming in super salty water feels like magic, and how human engineering keeps giant metal ships above the waves. Let's dive in. It all comes down to an invisible battle happening in the water right now. Every single time you drop something in a pool, lake, or ocean, gravity is pulling it down toward the centre of the earth. Right. Gravity wants everything to fall. We experience that every time we drop a toy. So what stops the heavy stuff from falling all the way down? The water pushes back up. This upward force is called buoyancy. When any object goes into water, it has to push some of the water out of the way to make room for itself. Like when I get into a really full bathtub and the water level rises up the sides. Yes, exactly like that. That scientific process is called displacement. The object displaces, or pushes aside, the water. And here is the golden magic rule of floating. Ooh, I love magic rules. What is it? Tell me. If the object pushes away an amount of water that weighs the exact same as the object itself, it floats. The upward push of the water is strong enough to hold the object's weight. Wait, so the upward push from the water actually holds its weight up like an invisible hand? Yes, but a tiny pebble is very dense. It's really heavy for its small size, so it only pushes a tiny bit of water out of the way. That tiny bit of displaced water doesn't weigh enough to push back and hold the pebble up. So the pebble sinks. But the giant ship, it pushes a lot of water out of the way. You got it! To understand how humans first figured this out, we have to travel back in time to ancient Greece, around the 3rd century BC. Whoa, that's over 2,000 years ago. Who are we visiting in ancient Greece? A super smart scientist and mathematician named Archimedes. And he was living in a city called Syracuse, where the king had a very stressful, very expensive problem. Uh-oh. What was the king's problem? Was it a dragon? Not a dragon. King Hiero II had given a goldsmith some pure gold to make a fancy crown. But when the crown was finished, the king suspected the goldsmith had secretly swapped some gold for cheaper silver. The ultimate royal scam! But how could they prove it without melting the beautiful crown down? That's exactly what the king asked Archimedes to figure out. Archimedes was stumped. He knew silver was lighter than gold, but he didn't know how to measure the crown's exact volume because of its weird, bumpy shape. Yeah, it's not like measuring a perfectly square box. What did he do? Well, he decided to take a break and go to the public baths. As he stepped into the tub, he watched the water spill over the edges. Just like my bathtub. Displacement. Exactly. He realised that the volume of water spilling over was exactly equal to the volume of his body going in. He could use water to measure the crown. Did he run back to the king right away? Yes. He was so excited that he leaped straight out of the bath and ran all the way home, still dripping wet, shouting, Eureka! Haha, no way! Dripping wet through the streets. What does Eureka even mean? It means, I have found it, in ancient Greek. He used his water trick to prove the crown displaced more water than pure gold wood, meaning it had lighter, cheaper silver mixed in. Busted. So that's Archimedes' principle. But how does that make a huge metal ship float? It's all about the shape. If you take a heavy lump of steel and drop it in water, it sinks because it's too dense. It doesn't displace enough water. But if you flatten that steel and fold it into a big wide bowl. Yes, by making it hollow and wide, the ship takes up way more space. That means it displaces a massive amount of water. So the wide shape pushes away enough water to match its super heavy weight. Exactly. Take the icon of the seas. It's 1,198 feet long. That's almost four football fields. Because its hull is shaped like a giant hollow bowl, it easily displaces around a hundred thousand tons of water. Four football fields of hollow metal pushing the ocean out of the way. That is wild. What about animals? Do they use the same trick? They sure do. Think about the blue whale. It is the largest animal to ever live on Earth. Bigger than the biggest dinosaurs? How heavy are we talking? Way bigger. A blue whale can weigh up to 200 tons. That is the weight of 30 African elephants. And they can grow to be a hundred feet long. 200 tons? If a blue whale is that incredibly heavy, why doesn't it just sink like a stone? Because of displacement. A whale has a massive body that takes up a ton of space, pushing an enormous amount of water aside. Plus, they have a secret biological weapon. Is it something gross? Not gross, just squishy. Blubber. Whales have a thick layer of fat called blubber. Fat is actually less dense than water, meaning it's naturally buoyant. So their massive size pushes the water away, and their blubber acts like a giant built-in life jacket. You got it! And scientists also found that whales can expand their lungs to hold a massive amount of air, which helps them float even better when they are near the surface. Wait, if things float by pushing water out of the way, does the type of water matter? Like ocean water versus a regular swimming pool? Oh Finn, you just asked the perfect question. The type of water changes everything. Because ocean water has salt in it, it's actually heavier and denser than fresh water. So if the water itself is heavier, does that mean its upward push is stronger? Exactly. You will float much easier in the ocean than in a lake. And there is one place on Earth where this goes to the extreme. The Dead Sea. The Dead Sea? That sounds spooky. Why is it called that? It's a huge salt lake bordered by Jordan and Israel. It's called the Dead Sea because the water is so incredibly salty that fish and plants can't survive in it. How salty are we talking? Regular ocean water is about 3.5% salt, but the Dead Sea is 34% salt. It's almost 10 times saltier than the ocean. Whoa! So if the water is that salty, it must be super heavy. Yes, because the water is so dense, its upward push is incredibly strong. If you walk into the Dead Sea, you can't even sink if you try. You just bob to the top and can sit in the water, like you're sitting in a floating chair. That is the coolest thing ever. I want to read a book while floating in a lake. People do that all the time. Okay, ready for some rapid-fire fun facts before our quiz. Always! Hit me with the weirdest stuff you've got. Speaking of the massive blue whale, its tongue alone weighs two to four tons. That's the weight of a whole elephant, just in its mouth. Ew, an elephant-sized tongue? That's insane! And its heart is the size of a golf cart. Its major blood vessel is about nine inches wide, like a dinner plate. A golf cart-sized heart and an elephant tongue. Science is wild. Alright, Finn, it's time for the quiz. Are you ready to test your floating knowledge? Okay, I'm ready. I hope my brain floats to the top for this. Let's do this. First question. What was the name of the ancient Greek mathematician who shouted, Eureka? Next one, let's see if you remember this. What is the name of the huge modern cruise ship we talked about? Question three. Thinking about the animal kingdom, how heavy can a blue whale get? Here's number four, about our salty lakes. What percentage of the Dead Sea is made of salt? Last question, going back to our vocabulary. What does the ancient Greek word eureka actually mean? Great job! You made it through! Let's see how you did and go through the answers. The first answer is Archimedes! He solved the mystery of the king's golden crown. For question two, we talked about the massive icon of the seas. The answer to question three is up to 200 tonnes, which is the weight of 30 African elephants. For question four, the Dead Sea is 34% salt, making it incredibly easy to float in. And the final answer is, Eureka means I have found it. I can't believe how much we learnt today. From super heavy ships to giant whales and incredibly salty lakes. I know, right? Next time you see a giant ship, just remember, it's all about displacing water with that wide hollow shape. And I'll never look at my bathtub the same way again. If I ever spill water over the edge, I'm shouting, Eureka! Just make sure you grab a towel first. Hey, if you had fun learning with us today, please leave us a five-star review. Yeah, if you think a blue whale's golf cart-sized heart is awesome, just scroll down on your podcast app and tap those five stars. Oh, and don't forget to check out our site, kidopoly.com. We've got tons of fun learning games and activities waiting for you there. And if you want a shout out on the show, or just want to tell us your favourite science fact, send us an email at hello at kidopoly.com. We love hearing from you. Thanks for exploring the world afloating with us. See you next time on Science Is Not Boring. Bye everyone. Bye.