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Hey, Vsauce, Michael here, and bananas are
fantastic. They're actually one of the most
radioactive foods we regularly eat. Sometimes
they're difficult to peel from the top. One
of my favorite ways to avoid that is to simply
hold the banana and snap it in half. Boom,
now it peels itself. It's a wonderful little
trick, except when you drop it on the ground.
This is unfortunate, but, if it picked it
up quickly, would it be safe enough to eat?
I mean, how quickly do the bacteria on the
floor move onto the food? Some people call
it the "5-Second Rule." Others, the "10-Second
Rule." And still, others, the "2-Second Rule."
But, is there any truth to it, or is it just
something we say to allow us to eat food off
the floor?
In 2003, Jillian Clarke famously investigated
this question. She found that 50% of men use
the 5-Second Rule to eat food off the ground.
But, 70% of women do. More importantly, she
found that even brief contact with a contaminated
floor will contaminate food, wet or dry.
The Myth Busters found similar results. And
a paper published in the Journal of Applied
Microbiology got even more technical. The
researchers contaminated various floor surfaces
with salmonella, and they found that 5 seconds
is way too long to wait. Bacteria adhere to
dropped food almost immediately, but time
does matter- after 5 seconds, they found that
the food had acquired anywhere from 150-8000
bacteria. But, if left for a full minute,
the number they found was 10 times greater.
Now considering it only takes about 10 bacteria
of certain strains of salmonella to infect
you, you should probably think twice before
eating off the ground.
Oh, and don't complacently think that you're
safe because the floor looks clean, or the
food that fell on it does. Floors make great
homes...for bacteria. Also, floors come in
contact with the bottoms of our shoes, and
the University of Arizona has found that 93%
of our shoes are contaminated with fecal bacteria.
I've linked all of these studies down in the
description. They're great reads- very thorough.
But, is it true that food dropped on a dirty
floor becomes contaminated immediately? I
mean, sure, practically speaking, that makes
a lot of sense. But instantly? Is there a
limit for how quickly bacteria can move from
one object to another? Is there an amount
of time, short enough, that were I to pick
the food up within it, there would be no way
for what it touched to contaminate it.
Well, we're going to need to define "touch."
I've talked about this before on Vsauce, and
on Thinker, and MinutePhysics has a great
quantum perspective, but when we say "touch,"
we tend to think of two objects contacting
each other with no space in between. Unfortunately,
that's not really what happens. At a subatomic
level, atoms resist smashing together because
their electrons repel. Electrons can be modeled
as waves, waves that overlap and interact,
but they never touch. There's always space
between them. When I touch something, or someone,
I'm really just feeling their electrons react
to mine at a distance. A sub-atomically small
one, but a real one.
So, if that's the case, how come glue can
stick things together? How come lipstick sticks
to lips? And how can bacteria on a dirty floor
stick to food that's been dropped on it?
The positive charge from the protons in an
atom or molecule isn't always balanced perfectly
in space by the negative charge from electrons.
Sometimes a molecule is asymmetrical and this
imbalance is permanent, which gives the molecule
a constant dipole, so it's like a tiny little
magnet.
But even in symmetrical molecules, electrons
are mobile. At any one point in time they
might happen to find themselves more towards
one end of the molecule than the other, creating
rapidly fluctuating dipoles, causing the molecule
to act like a magnet.
If molecules have a lot of energy they will
simply *** by one another, and their dipoles
and those of their neighbors won't matter
much. But, if you cool them down, slow them
down, the dipoles will have more of an effect,
which is why molecules tend to stick together
as they cool- going from a gas, to a liquid,
to a solid.
That's a lot of information about molecules,
but it's incredibly important. It's the reason
things can get wet. In fact, when you get
out of the shower, you, on average, weigh
about one more pound more than you did before
you got wet. It's also the reason Capillary
Action happens. If you dip a napkin in a glass
of water, you can watch water climb up the napkin
against gravity. That's because the molecules
of water have very strong dipoles, and they're
attracted to each other (cohesion), and they're
attracted to molecules of the paper (adhesion),
more strongly than gravity pulls them down.
If you want to learn more about intermolecular
forces, I've put a bunch of links down in
the description. But here's my question: how
quickly do those forces act? Can two surfaces
come into contact, briefly enough, that their
molecules don't have time to be influenced
by intermolecular forces.
Well, for this, we're going to need Molecular
Dynamics.
Molecular Dynamics is the computer simulation
of the physical movements of atoms or molecules.
For instance, this is a simulation of a single
Copper atom depositing on a Copper surface.
Notice that the timer up there is counting
off hundredths of a picosecond. These simulations
need to have a narrow enough time-step to
account for the fastest molecular vibrations
of the material, including everything from
wagging, to scissoring.
Typically, time as brief as a quadrillionth
of a second is taken into account. So, the
5-second Rule may be true...If we rename it
the "1 Femtosecond Rule." Spend less time
on the floor than that, and it's unlikely
that room temperature molecules would have
time to be influenced by intermolecular forces.
Except, oops, I think we've been thinking
too small. Intermolecular forces are fun,
but objects can become entangled with each
other on a macroscopic level.
Glue, and makeup, and other sticky stuff often
take advantage of the tiny imperfections-
ridges, and nooks and crannies- on an objects
surface. Sticky things can seep inside and
hold on. Even surfaces that seem smooth to
us, when you really get down to it, aren't
smooth at all.
Two sheets of paper from a phonebook may slide
across each other quite easily, but multiply
that friction by the number of pages in two
phonebooks by interweaving the pages, and
you've got yourself a monster capable of lifting
an entire car.
I was recently lucky enough to see this happen
in real life. You can check out my adventure
over at Fast, Furious, and Funny. It's a new
channel, stay tuned because we lifted cars
with more than just phonebooks...
The adhesion between two objects caused by
the shapes of their surfaces contributes to
friction and is known as "mechanical adhesion."
It plays a pretty big role in getting floor
germs to sick to dropped food.
In fact, if two surfaces can mechanically
adhere, but aren't close enough to do so,
we wouldn't say they were touching. And so,
if by definition "touching" means that you
can mechanically adhere to other object's surface,
well, then, maybe the 5-Second Rule shouldn't
be known as the "1 Femtosecond Rule," because
if they're touching, it's already too late,
and, instead, the rule should be known as
the "Don't Touch Food That's Fallen on the
Floor...Rule."
But come to think of it, maybe we shouldn't
be eating food that's touched anything because
bacteria are everywhere, including on you
right now. There are more bacteria on your
body right now than there are people living
in America.
There are 40 million bacteria in one gram
of soil, and 5 Nonmillion bacteria on Earth.
It's been found that one out of every 10 bank
cards, and one out of every 7 bills, has fecal
bacteria on it.
Here's something fun: go down to the comment
section and type the letter "V." If you used
a mobile phone to do that, congratulations,
you just touched 6,281 bacteria. If you're
using a desktop keyboard, you're a little
safer. You probably only touched about 180.
Cellphones are actually one of the most bacteria-ridden
things we frequently encounter.
If you want to estimate about how many bacteria
are specifically on your phone, based on how
clean you are, The Oatmeal has a really great
quiz- you can find that in the description.
But honestly, what amazes me the most isn't
how dirty our world is, or how much bacteria
there is; instead, it's the fact that despite
those numbers, we don't get sick more often.
Our immune system is amazing, but it relies
on the same principles of adhesion that bacteria
do.
The forces that cause food dropped on the
floor to pick up germs are the same forces
that we need to fight them. They're same forces
that bring us closer together, whether we
like it or not.
So, keep adhering, and, as always, thanks
for watching.