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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 whiz 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.

Hey, Vsauce, Michael here, and bananas are fantastic. They're actually one of the most

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Is The 5-Second Rule True?

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    盧怡靜 posted on 2013/07/06
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