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  • Let's start this video by throwing a mouse, a dog, and an elephant

  • from a skyscraper onto something soft.

  • Let's say, a stack of mattresses.

  • The mouse lands and is stunned for a moment,

  • before it shakes itself off,

  • and walks away pretty annoyed,

  • because that's a very rude thing to do.

  • The dog breaks all of its bones

  • and dies in an unspectacular way,

  • and the elephant explodes into a red puddle of bones and insides

  • and has no chance to be annoyed.

  • Why does the mouse survive,

  • but the elephant and dog don't?

  • The answer is size.

  • Size is the most underappreciated regulator of living things.

  • Size determines everything about our biology,

  • how we are built, how we experience the world, how we live and die.

  • It does so because the physical laws are different for different sized animals.

  • Life spans seven orders of magnitude, from invisible bacteria to mites, ants,

  • mice, dogs, humans, elephants and, blue whales. Every size lives in its own

  • unique universe right next to each other, each with its own rules, upsides, and

  • downsides. We'll explore these different worlds in a series of videos. Let's get

  • back to the initial question: Why did our mouse survive the fall? Because of how

  • scaling size changes everything; a principle that we'll meet over and

  • over again. Very small things, for example, are practically immune to falling from

  • great heights because the smaller you are the less you care about the effect

  • of gravity. Imagine a theoretical spherical animal

  • the size of a marble. It has three features: its length, its surface area,

  • (which is covered in skin) and its volume, or all the stuff inside it like organs,

  • muscles, hopes and dreams. If we make it ten times longer, say the size of a

  • basketball, the rest of its features don't just grow ten times. Its skin will

  • grow 100 times and it's inside (so it's volume) grows by 1000 times. The volume

  • determines the weight, or more accurately, mass of the animal. The more mass you

  • have, the higher your kinetic energy before you hit the ground and the

  • stronger the impact shock. The more surface area in relation to your volume

  • or mass you have, the more the impact gets distributed and softened, and also

  • the more air resistance will slow you down. An elephant is so big that it has

  • extremely little surface area in ratio to its volume. So a lot of kinetic energy

  • gets distributed over a small space and the air doesn't slow it down much at all.

  • That's why it's completely destroyed in an impressive explosion of goo when it

  • hits the ground. The other extreme, insects, have a huge surface area in

  • relation to their tiny mass so you can literally throw an ant from an airplane

  • and it will not be seriously harmed. But while falling is irrelevant in the small

  • world there are other forces for the harmless for us but extremely dangerous

  • for small beings. Like surface tension which turns water into a potentially

  • deadly substance for insects. How does it work? Water has the tendency to stick to

  • itself; its molecules are attracted to each other through a force called

  • cohesion which creates a tension on its surface that you can imagine as a sort

  • of invisible skin. For us this skin is so weak that we don't even notice it

  • normally. If you get wet about 800 grams of water or about one percent of your

  • body weight sticks to you. A wet mouse has about 3 grams of water sticking to

  • it, which is more than 10% of its body weight. Imagine having eight full water

  • bottle sticking to you when you leave the shower. But for an insect the force

  • of water surface tension is so strong that getting wet is a question of life

  • and death. If we were to shrink you to the size of

  • an ant and you touch water it would be like you were reaching into glue. It

  • would quickly engulf you, its surface tension too hard for you to break and

  • you'd drown. So insects evolved to be water repellent. For one their exoskeleton is

  • covered with a thin layer of wax just like a car. This makes their surface at

  • least partly water repellent because it can't stick to it very well. Many insects

  • are also covered with tiny hairs that serve as a barrier. They vastly increase

  • their surface area and prevent the droplets from touching their exoskeleton

  • and make it easier to get rid of droplets. To make use of surface tension

  • evolution cracked nanotechnology billions of years before us. Some insects

  • have evolved a surface covered by a short and extremely dense coat of water

  • repelling hair. Some have more than a million hairs per square millimeter when

  • the insect dives under water air stays inside their fur and forms a coat of air.

  • Water can't enter it because their hairs are too tiny to break its surface tension.

  • But it gets even better, as the oxygen of the air bubble runs out, new oxygen

  • diffuses into the bubble from the water around, it while the carbon dioxide

  • diffuses outwards into the water. And so the insect carries its own outside lung

  • around and can basically breathe underwater thanks to surface tension.

  • This is the same principle that enables pond skaters to walk on water by the way,

  • tiny anti-water hairs. The smaller you get the weirder the environment becomes. At

  • some point even air becomes more and more solid. Let's now zoom down to the

  • smallest insects known, about half the size of a grain of salt,

  • only 0.15 millimeters long: the Fairy Fly. They live in a world even weirder than

  • another insects. For them air itself is like thin jello, a syrup-like mass

  • surrounding them at all times. Movement through it is not easy. Flying

  • on this level is not like elegant gliding; they have to kind of grab and

  • hold onto air. So their wings look like big hairy arms rather than proper insect

  • wings. They literally swim through the air, like a tiny gross alien through

  • syrup. Things only become stranger from here on

  • as we explore more universes of different sizes. The physical rules are

  • so different for each size that evolution had to engineer around them

  • over and over as life grew in size in the last billion years. So why are there

  • no ants the size of horses? Why are no elephants the size of amoeba? Why?

  • We'll discuss this in the next part.

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Let's start this video by throwing a mouse, a dog, and an elephant

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B1 UK surface size surface tension water surface area tension

What Happens If We Throw an Elephant From a Skyscraper? Life & Size 1

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    汪摳 posted on 2017/08/14
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