What is Friction?

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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.

How Often Should You Poop?

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When it comes to No. 2, plenty of people aren’t really sure what’s normal. Are you supposed to go every day? What if you go 10 times a day? Is that a sign that you’re dying? What about once every three days? Short of asking everyone you know for their personal poop statistics, how do you know how often you’re supposed to hit the head?

Everyone’s system is a little different, and according to experts, regularity is more important than how often you do the deed. Though some lucky people might think of having a bowel movement as an integral part of their morning routine, most people don’t poop every day, as Lifehacker informs us. In fact, if you go anywhere between three times a day and three times a week, you’re within the normal range.

It’s when things change that you need to pay attention. If you typically go twice a day and you suddenly find yourself becoming a once-every-three-days person, something is wrong. The same thing goes if you normally go once every few days but suddenly start running to the toilet every day.

There are a number of factors that can influence how often you go, including your travel schedule, your medications, your exercise routine, your coffee habit, your stress levels, your hangover, and, of course, your diet. (You should be eating at least 25 to 30 grams of fiber a day, a goal that most Americans fall significantly short of.)

If you do experience a sudden change in how often you take a seat on the porcelain throne, you should probably see a doctor. It could be something serious, like celiac disease, cancer, or inflammatory bowel disease. Or perhaps you just need to eat a lot more kale. Only a doctor can tell you.

However, if you do have trouble going, please, don’t spend your whole day sitting on the toilet. It’s terrible for your butt. You shouldn’t spend more than 10 to 15 minutes on the toilet, as one expert told Men’s Health, or you’ll probably give yourself hemorrhoids.

But if you have a steady routine of pooping three times a day, by all means, keep doing what you’re doing. Just maybe get yourself a bidet.

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

What Would Happen If a Plane Flew Too High?

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Tom Farrier:

People have done this, and they have died doing it. For example, in October 2004, the crew of Pinnacle Airlines 3701 [PDF]  was taking their aircraft from one airport to another without passengers—a so-called "repositioning" flight.

They were supposed to fly at 33,000 feet, but instead requested and climbed to 41,000 feet, which was the maximum altitude at which the aircraft was supposed to be able to be flown. Both engines failed, the crew couldn't get them restarted, and the aircraft crashed and was destroyed.

The National Transportation Safety Board determined that the probable causes of this accident were: (1) the pilots’ unprofessional behavior, deviation from standard operating procedures, and poor airmanship, which resulted in an in-flight emergency from which they were unable to recover, in part because of the pilots’ inadequate training; (2) the pilots’ failure to prepare for an emergency landing in a timely manner, including communicating with air traffic controllers immediately after the emergency about the loss of both engines and the availability of landing sites; and (3) the pilots’ improper management of the double engine failure checklist, which allowed the engine cores to stop rotating and resulted in the core lock engine condition.

Contributing to this accident were: (1) the core lock engine condition, which prevented at least one engine from being restarted, and (2) the airplane flight manuals that did not communicate to pilots the importance of maintaining a minimum airspeed to keep the engine cores rotating.

Accidents also happen when the "density altitude"—a combination of the temperature and atmospheric pressure at a given location—is too high. At high altitude on a hot day, some types of aircraft simply can't climb. They might get off the ground after attempting a takeoff, but then they can't gain altitude and they crash because they run out of room in front of them or because they try to turn back to the airport and stall the aircraft in doing so. An example of this scenario is described in WPR12LA283.

There's a helicopter version of this problem as well. Helicopter crews calculate the "power available" at a given pressure altitude and temperature, and then compare that to the "power required" under those same conditions. The latter are different for hovering "in ground effect" (IGE, with the benefit of a level surface against which their rotor system can push) and "out of ground effect" (OGE, where the rotor system supports the full weight of the aircraft).

It's kind of unnerving to take off from, say, a helipad on top of a building and go from hovering in ground effect and moving forward to suddenly find yourself in an OGE situation, not having enough power to keep hovering as you slide out over the edge of the roof. This is why helicopter pilots always will establish a positive rate of climb from such environments as quickly as possible—when you get moving forward at around 15 to 20 knots, the movement of air through the rotor system provides some extra ("translational") lift.

It also feels ugly to drop below that translational lift airspeed too high above the surface and abruptly be in a power deficit situation—maybe you have IGE power, but you don't have OGE power. In such cases, you may not have enough power to cushion your landing as you don't so much fly as plummet. (Any Monty Python fans?)

Finally, for some insight into the pure aerodynamics at play when airplanes fly too high, I'd recommend reading the responses to "What happens to aircraft that depart controlled flight at the coffin corner?"

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

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