What is Friction?

iStock
iStock

What is friction?

Akshat Mahajan:

THE MOST COMMON ORIGIN OF FRICTION

Friction exists because most surfaces really look like this under a microscope.

Note the hills, the crests, and the valleys. These are called asperities, or material deformations. They occur on rough surfaces, and even flat surfaces might have minor irregularities on them.

Now imagine trying to rub them against each other. Feel, for a moment, in your mind’s eye the opposition you are likely to encounter as they grind against each other.

That grinding is the most common origin for friction, both static (when your object is still and you’re trying to get it to move) and kinetic (when your object is already moving, and you need to keep it moving).

It is why the friction that wheels encounter while rolling (called rolling friction) is much lower than other types of friction, and why tires have deep grooves in them. It is also why friction tends to decrease the flatter and more uniform your surface gets.

FRICTION FOR FLAT SURFACES

But friction can still exist for perfectly flat surfaces, where asperities of the form above are no longer present.

Perfectly flat surfaces bond to each other—in other words, they try to stick to each other. This is called adhesion, and can be caused by any number of reasons:

  • Van der Waals forcesa class of forces where attraction between electric dipoles on the surface of the materials cause them to be attracted. In such cases, the materials are said have been dispersively adhered.
  • Chemical bonding, where the atoms of the two materials exchange electrons in some way and become chemically attached to each other. For instance, hydrogen in one material may form tenuous bonds with strongly electronegative elements in the other.

These forces are typically weak enough that they are easily broken once you lift the object—but not so weak that overcoming the attraction by dragging the object doesn’t pose a problem.

FINALLY, A DEFINITION

Friction, then, is not so much a force as it is the sum of very tiny forces and irregularities along the edge of surfaces.

It is possible to model it as a true force—and indeed, most high school textbooks do just that—but it is important to recognize that that is just a convenient abstraction. To put it another way: Friction is to forces what Santa Claus is to a jolly man in a fat suit—window dressing on top of a very complex reality.

This post originally appeared on Quora. Click here to view.

Why Do Dogs Lick?

iStock/MichaelSvoboda
iStock/MichaelSvoboda

​One of the more slightly annoying things our dogs do (or most adorable, depending on who you ask) involves their tongue obsessively licking every crevice of every spot possible in pretty much the whole world. From our faces to our furniture to themselves, some dogs are absolutely in love with licking anything and everything. Although it can be cute at first, it quickly gets pretty gross. So why do they do it?

According to ​Vetstreet, your pup's incessant licking is mostly their way of trying to show affection. When we pick up our dogs or give them attention, chances are we kiss or pat their heads, along with petting their fur. Their way to show love back to us is by licking.

However, there are other reasons your dog might be obsessively licking—including as a way to get attention. Licking can be a learned behavior for dogs, as they see that when they lick their owner, they get more attention. The behavior can seem like something humans want which, to an extent, it is.

Licking is also a sensory tool, so if your dog is licking random objects or areas of your home, they're probably just exploring. It's easier to get a feel for their surroundings if they can taste everything. But licking objects like your rug or furniture can also be indicative of anxiety or boredom (which can often lead to destructive behavior), and a recent study linked excessive licking of surfaces to certain gastrointestinal disorders.

Another reason for licking is your dog wanting to clean themselves and/or spots around them. They've seen it since they were born; animals lick things ritualistically for cleaning and care. If your dog seems to be obsessed with licking themselves or one particular thing, they probably are. (Yes, dogs can have OCD, too.)

As Vetstreet points out, "excessive" dog licking often only seems excessive to the dog's owner, not the pooch itself. But if it's bothersome enough to you, a trainer can often help curb your dog's enthusiasm for giving wet, sloppy kisses. And while strange behavior is not rare for pets, if your dog's licking seems odd or in any way concerning, there's no harm in taking your pet to the vet to check it out—even if it's just for peace of mind.

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How Are Balloons Chosen for the Macy's Thanksgiving Day Parade?

iStock/RoBeDeRo
iStock/RoBeDeRo

The balloons for this year's Macy’s Thanksgiving Day Parade range from the classics like Charlie Brown to more modern characters who have debuted in the past few years, including The Elf On The Shelf. New to the parade this year are the Teenage Mutant Ninja Turtles. But how does the retail giant choose which characters will appear in the lineup?

Balloon characters are chosen in different ways. For example, in 2011, Macy’s requested B. Boy after parade organizers saw the Tim Burton retrospective at the Museum of Modern Art. (The company had been adding a series of art balloons to the parade lineup since 2005, which it called the Blue Sky Gallery.) When it comes to commercial balloons, though, it appears to be all about the Benjamins.

First-time balloons cost at least $190,000—this covers admission into the parade and the cost of balloon construction. After the initial year, companies can expect to pay Macy’s about $90,000 to get a character into the parade lineup. If you consider that the balloons are out for only an hour or so, that’s about $1500 a minute.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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