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# Mechanical engineering

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## Q: What causes friction? (and some other friction questions)

Original author:
The Physicist

Physicist: Political conversations with family, for one.

“Friction” is a blanket term to cover all of the wide variety of effects that make it difficult for one surface to slide past another.

There a some chemical bonds (glue is an extreme example), there are electrical effects (like van der waals), and then there are effects from simple physical barriers.  A pair of rough surfaces will have more friction than a pair of smooth surfaces, because the “peaks” of one surface can fall into the “valleys” of the other, meaning that to keep moving either something needs to break, or the surfaces would need to push apart briefly.

This can be used in hand-wavy arguments for why friction is proportional to the normal force pressing surfaces together.  It’s not terribly intuitive why, but it turns out that the minimum amount of force, Ff, needed to push surfaces past each other (needed to overcome the “friction force”) is proportional to the force, N, pressing those surfaces together.  In fact this is how the coefficient of friction, μ, is defined: Ff = μN.

The force required to push this bump “up hill” is proportional to the normal force.  This is more or less the justification behind where the friction equation comes from.

The rougher the surfaces the more often “hills” will have to push over each other, and the steeper those hills will be.  For most practical purposes friction is caused by the physical roughness of the surfaces involved.  However, even if you make a surface perfectly smooth there’s still some friction.  If that weren’t the case, then very smooth things would feel kinda oily (some do actually).

Sheets of glass tend to be very nearly perfectly smooth (down to the level of molecules), and most of the friction to be found with glass comes from the subtle electrostatic properties of the glass and the surface that’s in contact with it.  But why is that friction force also proportional to the normal force?  Well… everything’s approximately linear over small enough forces/distances/times.  That’s how physics is done!

That may sound like an excuse, but that’s only because it is.

Q: It intuitively feels like the friction force should be directly proportional to the surface area between materials, yet this is never considered in any practical analysis or application.  What’s going on here?

A: The total lack of consideration of surface area is an artifact of the way friction is usually considered.  Greater surface area does mean greater friction, but it also means that the normal force is more spread out, and less force is going through any particular region of the surface.  These effects happen to balance out.

If you have one pillar the total friction is μN. If you have two pillars each supports half of the weight, and thus exert half the normal force, so the total friction is μN/2 + μN/2 = μN.

Pillars are just a cute way of talking about surface area in a controlled way.  The same argument applies to surfaces in general.

Q: If polishing surfaces decreases friction, then why does polishing metal surfaces make them fuse together?

A: Polishing two metal surfaces until they can fuse has to do with giving them both more opportunities to fuse (more of their surfaces can directly contact each other without “peaks and valleys” to deal with), and polishing also helps remove impurities and oxidized material.  For example, if you want to weld two old pieces of iron together you need to get all of the rust off first.  Pure iron can be welded together, but iron oxide (rust) can’t.  Gold is an extreme example of this.  Cleaned and polished gold doesn’t even need to be heated, you can just slap two pieces together and they’ll fuse together.

Inertia welders also need smooth surfaces so that the friction from point to point will be constant (you really don’t want anything to catch suddenly, or everyone nearby is in trouble).  This isn’t important to the question; it’s just that inertia welders are awesome.

Q: Why does friction convert kinetic energy into heat?

A: The very short answer is “entropy”.  Friction involves, at the lowest level, a bunch of atoms interacting and bumping into each other.  Unless that bumping somehow perfectly reverses itself, then one atom will bump into the next, which will bump into the next, which will bump into the next, etc.

And that’s essentially what heat is.  So the movement of one surface over another causes the atoms in each to get knocked about jiggle.  That loss of energy to heat is what causes the surfaces to slow down and stop.

## \$17,000 Linux-powered rifle brings “auto-aim” to the real world

CES is about technology of all kinds; while we're busy covering cameras, TVs, and CPUs, there's a huge number of products that fall outside our normal coverage. Austin-based startup TrackingPoint isn't typical Ars fare, but its use of technology to enable getting just the perfect shot was intriguing enough to get me to stop by and take a look at the company's products.

TrackingPoint makes "Precision Guided Firearms, or "PGFs," which are a series of three heavily customized hunting rifles, ranging from a .300 Winchester Magnum with a 22-inch barrel up to a .338 Lapua Magnum with 27-inch barrel, all fitted with advanced computerized scopes that look like something directly out of The Terminator. Indeed, the comparison to that movie is somewhat apt, because looking through the scope of a Precision Guided Firearm presents you with a collection of data points and numbers, all designed to get a bullet directly from point A to point B.

The view through the TrackingPoint's computerized optics. TrackingPoint

The PGF isn't just a fancy scope on top of a rifle. All together, the PGF is made up of a firearm, a modified trigger mechanism with variable weighting, the computerized digital tracking scope, and hand-loaded match grade rounds (which you need to purchase from TrackingPoint). This is a little like selling both the razor and the razor blades, but the rounds must be manufactured to tight tolerances since precise guidance of a round to a target by the rifle's computer requires that the round perform within known boundaries.

## For a Bit of Colored Ribbon

For the last year or so, I've been getting these two page energy assessment reports in the mail from Pacific Gas & Electric, our California utility company, comparing our household's energy use to those of the houses around us.

Here's the relevant excerpts from the latest report; click through for a full-page view of each page.

These poor results are particularly galling because I go far out of my way to Energy Star all the things, I use LED light bulbs just about everywhere, we set our thermostat appropriately, and we're still getting crushed. I have no particular reason to care about this stupid energy assessment report showing our household using 33% more energy than similar homes in our neighborhood. And yet… I must win this contest. I can't let it go.

• Installed a Nest 2.0 learning thermostat.
• I made sure every last bulb in our house that gets any significant use is LED. Fortunately there are some pretty decent \$16 LED bulbs on Amazon now offering serviceable 60 watt equivalents at 9 watt, without too many early adopter LED quirks (color, dimming, size, weight, etc).
• I even put appliance LED bulbs in our refrigerator and freezer.
• Switched to a low-flow shower head.
• Upgraded to a high efficiency tankless water heater, the Noritz NCC1991-SV.
• Nearly killed myself trying to source LED candelabra bulbs for the fixture in our dining room which has 18 of the damn things, and is used quite a bit now with the twins in the house. Turns out, 18 times any number … is still kind of a large number. In cash.

(Most of this has not helped much on the report. The jury is still out on the Nest thermostat and the candelabra LED bulbs, as I haven't had them long enough to judge. I'm gonna defeat this thing, man!)

I'm ashamed to admit that it's only recently I realized that this technique – showing a set of metrics alongside your peers – is exactly the same thing we built at Stack Overflow and Stack Exchange. Notice any resemblance on the user profile page here?

You've tricked me into becoming obsessed with understanding and reducing my household energy consumption. Something that not only benefits me, but also benefits the greater community and, more broadly, benefits the entire world. You've beaten me at my own game. Well played, Pacific Gas & Electric. Well played.

This peer motivation stuff, call it gamification if you must, really works. That's why we do it. But these systems are like firearms: so powerful they're kind of dangerous if you don't know what you're doing. If you don't think deeply about what you're incentivizing, why you're incentivizing it, and the full ramifications of all emergent behaviors in your system, you may end up with … something darker. A lot darker.

The key lesson for me is that our members became very thoroughly obsessed with those numbers. Even though points on Consumating were redeemable for absolutely nothing, not even a gold star, our members had an unquenchable desire for them. What we saw as our membership scrabbled over valueless points was that there didn't actually need to be any sort of material reward other than the points themselves. We didn't need to allow them to trade the points in for benefits, virtual or otherwise. It was enough of a reward for most people just to see their points wobble upwards. If only we had been able to channel that obsession towards something with actual value!

Since I left Stack Exchange, I've had a difficult time explaining what exactly it is I do, if anything, to people. I finally settled on this: what I do, what I'm best at, what I love to do more than anything else in the world, is design massively multiplayer games for people who like to type paragraphs to each other. I channel their obsessions – and mine – into something positive, something that they can learn from, something that creates wonderful reusable artifacts for the whole world. And that's what I still hope to do, because I have an endless well of obsession left.

[advertisement] What's your next career move? Stack Overflow Careers has the best job listings from great companies, whether you're looking for opportunities at a startup or Fortune 500. You can search our job listings or create a profile and let employers find you.

## TEDxBYU - Christopher Mattson - Design For The Developing World

TEDxBYU - Christopher Mattson - Design For The Developing World

Engineer, Christopher Mattson describes what made the difference between his team's successful and unsuccessful designs for the developing world. Christopher A. Mattson is an assistant professor of mechanical engineering at Brigham Young University. He earned his PhD from the Department of Mechanical and Aerospace Engineering at Rensselaer Polytechnic Institute. He is the recipient of a National Science Foundation CAREER award focusing on designing for the developing world. Before working at BYU, Mattson was global director of engineering design and research at ATL Technology. While at ATL Technology, he led the design of products that have been used by more than 15 million people around the world. He established and managed ATL Technology's Silicon Valley office, and ATL technology's twenty-five person Engineering Design Center in mainland China. In thespirit of ideas worth spreading, TEDx is a program of local, self-organized events that bring people together to share a TED-like experience. At a TEDx event, TEDTalks video and live speakers combine to spark deep discussion and connection in a small group. These local, self-organized events are branded TEDx, where x = independently organized TED event. The TED Conference provides general guidance for the TEDx program, but individual TEDx events are self-organized.* (*Subject to certain rules and regulations)
From:
TEDxTalks
Views:
78

5
ratings
Time:
15:04
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## Spanish Court Rules In Favor of P2P Engineer

Sir Mal Fet writes "In line with previous rulings discussed here, a judge in Spain has ruled that P2P technologies are "completely neutral" (original in Spanish ; Google translation ), thus dismissing a lawsuit originated in 2008 from the Spanish Association of Musical Producers (Promusicae), Warner, EMI, and Sony suing Pablo Soto, a Spanish man who created the Blubster, MP2P y Piolet programs to share files. The labels demanded 13 million euros in damages arguing that the mere existence and distribution of P2P technologies violated copyright, but the ruling stated the technology itself was neutral, so the creator could not be held responsible for how the software was used, and demanded that they pay for legal expenses. Promusicae said it was going to appeal the ruling."

Read more of this story at Slashdot.

## UnityCar 1.0 Released. The most complete and accurate vehicle simulation on Unity3D

UnityCar is a realistic and complete vehicle simulation package for Unity3D Game Engine. With UnityCar you can easly integrate (without scripting) any kind of vehicle in your game, from a simple city car to a powerful Formula 1, from an offroad to an heavy truck.
You can obtain any level of simulation accuracy, from an arcade behavior to a almost hardcore simulation.

## Skynet: It began in Vegas

The State of Nevada has become the first to legalize driverless cars.

## Genius of Einstein, Fourier key to new computer vision that mimics how humans perceive 3-D shapes

Two new techniques for computer-vision technology mimic how humans perceive three-dimensional shapes by instantly recognizing objects no matter how they are twisted or bent, an advance that could help machines see more like people.