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A freshly-mopped floor is slippery, but a wet shirt is super-clingy... so what's the deal? Why can water make some things slick and other things sticky?

Hosted by: Stefan Chin

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Those 'Caution: Wet Floor' signs are everywhere, warning us about slippery surfaces, but if you've ever been caught in a downpour without an umbrella, you know that while wet floors might make you slip, wet clothes tend to stick, so what's the deal?  How can water make some things slick and other things clingy?  

It all comes down to empty space.  Walking only works because there's friction between your shoes and the floor.  It keeps your foot in place as you shift your weight around, but most floor and road surfaces aren't very porous.  They don't have a lot of holes near the surface, so if they get wet, water fills any holes they do have pretty quickly, and then at least a little water tends to sit on top of them instead of sinking further in right away, and when you step in one of those wet spots, water gets sandwiched between your shoe and the floor, and there's practically no friction between the water in your shoe or between water and the floor, so there's nothing keeping your foot in place as you put more weight on it.

So it starts sliding.  If you're lucky, you might stumble before regaining your balance by stepping somewhere drier, where there's more friction, but if you're a little less lucky, your butt ends up on the floor where your shoes should have been.  That's why people in kitchens and other slippery environments usually wear slip-resistant shoes to keep from falling.  They have channels in the bottom that let water escape from under your foot, putting more of your shoe in contact with the floor itself and increasing the amount of friction keeping you in place.

When your clothes get wet, though, water doesn't pool on top of them.  It fills in all the little pores and cavities between fibers in the fabric, which stops outside air from getting through those cavities and into any space that opens up between your skin and the clothes, and if air can't get in to fill those gaps, trying to take off wet clothes involves creating a small vacuum between the clothes and your skin, which takes a lot more force than not creating a vacuum.  This is why it can help to take wet clothes off little by little, giving air the extra time to fill in each small gap you make, and to make matters worse, wet clothes make your skin wet, too.

The water on your skin attracts the water in your clothes through cohesion, the force that pulls similar molecules together.  To avoid these consequences, rain-proof clothes like rain jackets don't have the same kinds of big gaps, so water can't sneak in.  Instead, rain pools on top, just like it would on the floor, which leaves you and your clothes nice and dry inside while the outside of your jacket is slippery, just like any floor would be.

Thinking so much about slippery floors made me wonder about other science happening in my house. has a course on physics that you encounter in your own home.  I'm gonna work on this first quiz about how refrigerators work and I interact with fridges multiple times a day, so I hope I know how they work.  Let's see how I do.

So we're looking at the refrigerator quiz.  It seems like they're starting us off real easy.  What is the basic idea behind a refrigerator?  And my answer is, it's to keep this delicious looking cake nice and cold, of course, but then when you look over here at the answers and now I'm not sure what it is.  I don't think it sucks cold air in from the room, but does it make cold air and inject it into the food compartment or does it remove the heat from the compartment and pump it outside?  I kind of think it removes the heat from the compartment and shoves it outside, 'cause when you stick your hand around the back of a refrigerator, it's kinda warm back there.  And I got it!  And look, only 66% of people got it right.  It's harder than you think.  But the quiz goes on to explain how a refrigerator uses the compression and expansion of air to actually remove the heat from the compartment.  It's fairly simple physics, but it's a little more complicated than I had realized, so I had a lot of fun going through the quiz and figuring it out.

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