we're not going to figure out this problem because i don't think it's necessary it's
pretty obvious what's going to happen right the elephant's going to die but let me show you
something pretty cool about like how powerful our assuming things our spheres actually is
so much amazing science has been done like this so elephants right let's assume an elephant is a
sphere welcome back my friends so before we get into it you should come sign up for the
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take time so we're getting there all righty what happens if we throw an elephant from a skyscraper
i can't say i've ever thought about this but uh you know it's it's pretty standard to get
questions like this uh in early physics they love to spice it up anyway let's check it out
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 species maybe let's not do that i remember
reading a couple years ago that wwf basically said uh that african elephants would be extinct
they're facing extinction by 2040 if we don't do something to stop it like literally a couple
decades no more african elephants in the world in the wild you know it's a bit depressing so
let's uh keep watching but i just wanted to you know bring some awareness to this this
size size is the most underappreciated regulator of living things size determines everything about
our biology how we're 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 as far as we know dogs humans elephants and blue whales
every size lives in its own unique universe right next to just watching a documentary
last night called uh the loneliest whale the search for 52. uh if you've never watched this
or heard about this whale should go check it out it's amazing so basically i won't ruin the
end of the docker there's a whole story door but there's this whale and it communicates at 52 hertz
and there's no other whale in the ocean and that we've detected in history that communicates at
this you know frequency so we're worried it's like completely all alone and no one can hear it
should go watch it 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 this is the stock standard you know physics joke imagine a cow is a sphere
there's so much of this sort of thing in physics 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 10 times its skin will grow 100 times and its insides or its volume grows by 1 000 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 mess
so you can literally throw an ant from an aeroplane and it will not be seriously harmed
but while falling is irrelevant in the small world there are other forces that are 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
three grams of water sticking to it which is more than ten percent of its body weight
imagine having eight full water bottles 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 it surfaced tension too
hard for you to break and you drown i think we also learned about this in that movie bugs
such a good baby 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 underwater air
stays inside their fur and forms a coat of air water can't enter because the 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 other 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 no elephants the size of amoeba why
let's assume an elephant is a sphere so why don't elephants get any bigger than what they currently
are well we can answer this with physics you do a lot of biology with just uh with
some physics so let me show you so imagine we have two elephants right we've got one elephant
that's an elephant then you got a bigger elephant okay now the great thing about elephants ah
that they're so simple to estimate a few things because they're so simple they're spheres so
for example let's say the let's imagine the radius of this elephant is one and the radius
of this this elephant is two okay so now we just need to know some things so first
of all we have to remember uh what's the volume of the sphere now some of you're gonna you know
crack it and go and some of you will know it but you don't actually have to know anything
uh this is what's great about physics you know like take medicine or being a doctor right
you got to remember a lot of things physics don't actually have to remember anything and so what do
we know about volume what do we measure volume in right cubic something cubic meters cubic
something right and so that means uh it's the cube of something nice thing about a sphere is
only got one scale the radius so the volume of a sphere is proportional to r cube okay r cubed
what do we measure area square uh the the radius is r the the area is r squared proportional to
that's all you need to know my friends that's an elephant and what's the mass of this big elephant
the mass of course is going to be the amount of stuff and if they're made out of the same material
the amount of stuff is going to be proportional to the volume of the elephant right and so the
amount of stuff of the big elephant if it's twice the radius the mass is eight times as big okay
uh as this one down here with mass one this one has mass eight in some set of units and so what's
what's the surface area of the cow the surface area that goes as r squared so the surface area is
four times bigger than this one right all right so now let's think about what holds an elephant
together it's pressure the pressure of the skin is holding the elephant against the pressure
of the elephant standing there hanging over okay and so pressure is force per unit area
and while the force of the elephant is on the skin is the best right so the mass per unit area gives
you the pressure and now we see that eight times the mass and only four times the surface area so
the pressure on the skin is twice as big and if it's made of the same stuff you know if you make
an elephant that's too big it'll fall apart the skin just won't hold and so that's why
we know you don't have bigger elephants because eventually they fall apart which is embarrassing
much more like an elephant right a little sphere attached to a bigger sphere with a little rod in
between right you've got a head attached to a body right by neck and what's the strength of a rod
proportional right it's proportional to the cross-sectional area of the rod
and so we've done the necessary work we know if this rod is twice as twice the radius of this
rod then the cross-sectional area is four times as big as this right uh but we know the head is
eight times more massive and this is why the dinosaurs became extinct
it's actually really kind of simple you can't make large animals with big heads god says no
they have to be stupid because eventually uh the strength of the rod holding up the
neck holding up the head sorry i will not be able to hold up the head
and so dinosaurs you know they're huge animals massive right with a small little head
and that's you know like biology depends on the laws of physics and you know that's why dinosaurs
had small heads and with that just by thinking of yeah we've now discovered why the largest animals
with brains are not on lead they're they're in the water because the buoyancy can hold up your neck
and you know that's why whales and dolphins have the largest brains on earth
and look how much you can learn just from uh picturing your sphere right so
i just wanted to show you that little example of why it's powerful to imagine things as spheres the
whole point of all that is that uh you know if you do a more realistic picture of an elephant
it doesn't matter like everything's still the same so you can throw out the stuff
you don't need and no like the the details or the shape of the elephant right so you
isn't that cool so i remember seeing some new research last year uh that basically found that
elephants dilate their nostrils in order to create more space in their trunks right which allows
them to store up to like nine liters of water in there and they can also suck up like three liters
per second and that's a speed of 30 times faster than a human sneeze right which is like 150 meters
per second which is 330 miles per hour the study was basically trying to understand physics of how
elephants use their trunks to move and manipulate air food and like other objects right uh and they
were trying to learn uh if the mechanics could inspire you know more efficient robots right that
could use air motion to hold and move things so elephant's trunks are kind of sounding like the
swiss army knight biology this reminds me of a video from last year we made and we we were
talking about the physics of poo and how elephants uh like why it takes elephants the same amount of
time to poo to defecate as humans it's actually kind of interesting a unified theory of uh poop
i can't remember the video i think it was the my hero academia so we won't go over it again
you can go watch that if you want to hear about that and then we also talked about how penguins
are when they poo it kind of like explodes and goes everywhere and they brought a physicist
in to design like the barricades because they have to know how far penguins could poo because
people had aquariums and stuff for getting
anyway