What is Friction?

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iStock

What is friction?Akshat Mahajan

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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.

Take Advantage of Amazon's Early Black Friday Deals on Tech, Kitchen Appliances, and More

Amazon
Amazon

This article contains affiliate links to products selected by our editors. Mental Floss may receive a commission for purchases made through these links.

Even though Black Friday is still a few days away, Amazon is offering early deals on kitchen appliances, tech, video games, and plenty more. We will keep updating this page as sales come in, but for now, here are the best Amazon Black Friday sales to check out.

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Instant Pot/Amazon

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HadinEEon Milk Frother; $37 (save $33)

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Apple/Amazon

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How Much Is the Earth Worth?

The New York Public Library, Unsplash
The New York Public Library, Unsplash

Our home planet may be the most precious place we know, but it isn't priceless. The Earth's resources and the value it offers to humans add up to some unknown, tangible cost. The species may never have to worry about buying or selling the world, but thinking of it in terms of concrete numbers can help us better understand its value. Now, as Treehugger reports, one scientist has developed a special formula that allows us to do just that.

According to the calculations of Greg Laughlin, an assistant astronomy and astrophysics professor from the University of California, Santa Cruz, the Earth is worth roughly $5 quadrillion (or $5,000,000,000,000,000). He came up with that price after gauging the planet's mass, temperature, age, and other factors that directly correlate to its ability to sustain life.

To emphasize just how valuable the Earth is, Laughlin also estimated the worth of other planets in our solar system. Our nearest neighbor Mars costs about the same as a used car at $16,000. That's a fortune compared to Venus, which he appraised at the meager value of one cent.

Laughlin doesn't expect these numbers to have applications in the real world. Rather, he hopes they will inspire people to better appreciate the only home they know. He's not the first person to put a massive, hypothetical price tag on something just for fun. The cost of the Death Star from Star Wars has been calculated at $852 quadrillion—many times Laughlin's estimate for Earth.

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