Fluids
Everything you can touch is either a solid, a liquid, or a gas. While solids are stubborn and hold their shape, liquids and gases love to move. They slide, they swirl, and they flow. Discover how these flowing substances shape our weather, allow heavy aeroplanes to fly, and keep the world from freezing solid.
The Invisible Ocean
Yes, you absolutely can! Because air is a fluid just like water, it flows and takes the shape of its container. Bill Nye the Science Guy demonstrates this by taking a glass full of air underwater and tipping it, allowing the air bubbles to flow upwards and be caught safely inside another glass.
By pumping fluid! Air is actually thick and heavy. The engines and wings of an aeroplane act like giant pumps. They force massive amounts of this fluid downwards and backwards, which pushes the heavy plane up and forwards through the sky.
Put Your Instincts to the Test
Think about what you already know about how liquids and gases behave. Pick an answer for each question, then see if your instincts were right.
The hot water stays floating on top. Warm fluids rise and cool fluids sink. Because the hot fluid is already at the top, it rests happily upon the heavier, colder fluid below without mixing!
To reduce fluid drag. Air is a fluid, and pushing through it creates friction. By crouching, the cyclist creates a smoother, more streamlined shape, cutting through the fluid with much less resistance.
A thick, slippery fluid called oil. Oil is a lubricant with high viscosity. It coats the metal parts, allowing the solid pieces to safely slide past each other without scraping or creating too much heat.
Understanding the Science
Tap each card to reveal the explanation.
Key Concepts
Fluid
Tap to learn moreA fluid is any substance that flows and takes the exact shape of its container. Both liquids, like water and milk, and gases, like oxygen and helium, are categorised as fluids. Their molecules are loose enough to slide past one another easily. Solid objects are not fluids because their molecules are locked tightly together, allowing them to hold their own distinct shape.
Air Resistance (Drag)
Tap to learn moreThis is the physical friction caused when a solid object attempts to move through a fluid. Even though we cannot always see the air, it is thick and heavy. Bill Nye the Science Guy demonstrates this inside a wind tunnel, showing that objects must be specially shaped to reduce drag. Otherwise, pushing through the fluid requires a massive amount of extra energy.
Convection
Tap to learn moreConvection is the process of heat transfer through a flowing fluid. When a fluid is heated, its molecules move faster and spread out. This expansion makes the warm fluid lighter, causing it to rise upwards. Meanwhile, the cooler, heavier fluid sinks to the bottom. This constant circulating movement is what creates ocean currents and wind across the globe.
Viscosity
Tap to learn moreViscosity is a scientific measurement of how thick a fluid is and how easily it flows. Water has a very low viscosity, meaning it pours and splashes quickly. Engine oil and honey have a high viscosity, meaning their long molecules tangle easily, causing them to pour slowly and stick to surfaces.
Aerodynamic Lift
Tap to learn moreThis is the upward force that allows heavy aeroplanes to fly. A wing gets pushed through a fluid, specifically the air. Because of its curved shape, the wing forces the heavy air molecules downwards. The combined weight of all the air being pushed down results in the aeroplane being pushed upwards, creating lift.
Lubrication
Tap to learn moreLubrication involves using a slippery fluid to reduce friction between solid objects. Bill Nye the Science Guy uses marbles to represent oil molecules separating two pieces of metal. The fluid creates a protective sliding barrier, allowing solid machine parts to move freely without grinding against each other and generating destructive heat.
Surface Tension
Tap to learn moreThis is a property of liquid fluids where the molecules on the outer surface hold tightly together, acting almost like a flexible, elastic skin. In the episode, footage from space shows how water floating in zero gravity pulls itself together into a perfect sphere. Surface tension is the force fighting to keep the liquid completely intact.
Eddies
Tap to learn moreAn eddy is a swirling, circular current of fluid created when water or air flows rapidly past a solid obstacle. In the episode, kayakers explain how rocks in a fast river create a vacuum behind them, causing the water to swirl backwards. Reading these unique fluid patterns is absolutely essential for navigating dangerous rapids safely.
Explore: The Convection Dye Tank
Observe the beautiful physics of thermal convection. Use the control panel below to add a heat source, drop in some ice, or release colourful tracing dye directly into the fluid to reveal the invisible currents.
Apply Your Knowledge
Let us see if you can correctly match these fluid dynamics concepts to their definitions.
Match the Concepts
Click an object to select it, then click the matching description to place it.
Real-World Challenge
Imagine you are an engineer tasked with designing a new deep sea research submarine. Based on what you have learned about fluid dynamics, what shape should the exterior hull be to move efficiently through the dense water? Furthermore, how could you use the principles of pumping fluids (like air and water) in and out of internal tanks to make the heavy metal submarine intentionally sink to the bottom and surface again?
What Has Changed Since This Episode Aired
This episode first aired in 1998. While the core science remains accurate, here are a few things that modern research has expanded upon.
Updated: Geologists now carefully emphasise that the Earth's mantle is actually composed of solid rock, not a liquid ocean of magma. However, because of extreme heat and pressure over millions of years, this solid rock behaves plastically. It flows extremely slowly, like incredibly thick putty, allowing the tectonic plates to ride on top of it through convection.
Updated: Since 1998, oceanographers have discovered that this massive global fluid conveyor belt (specifically the Atlantic Meridional Overturning Circulation) is actually slowing down. Massive amounts of cold, fresh water melting from ice sheets are disrupting the natural sinking of cold, salty fluids, which threatens to drastically alter global weather patterns.
Updated: Physicists have expanded our understanding of states of matter by studying "superfluids" in extreme laboratory conditions. When certain elements, like Helium-4, are cooled to almost absolute zero, they become an exotic fluid with exactly zero viscosity. This means they flow without any friction whatsoever and can even spontaneously climb up and over the walls of their containers!
Test Your Understanding
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Results
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Reflection
Consider the wind blowing outside or the ocean currents off the coast. Now that you know air and water are both fluids driven by the exact same rules of convection, how does this change the way you look at a simple weather forecast?
Episode Discussion
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