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walsh416

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  1. walsh416
    Welcome to our humble abode. Today, we shall perform a brief, directed discourse on the workings of the zipper.

    Near the turn of the twentieth century, man kind was confronted with a conundrum: how the dickens would they close the flies on their snazzy new Goldrush-era Levis? Already the button fly was becoming associated with a rebel, skater boy type of crowd, and the people were clamoring for something new!

    Luckily, someone came up with the zipper. Consisting of two rows of teeth, or "keys," and joined by a y-shaped slider, the zipper is a highly functional fastener. By applying a net force on the slider parallel to the keys, the slider can be moved up or down along the keys, bringing them together or pushing them apart.

    The interlocking nature of the keys allows them to resist tensive and shear forces. Since each key is surrounded by other keys for more than 180 degrees, any directional force can be transferred to the next successive key, and so forth. Essentially, no single key is ever subjected to an unbalanced force from stresses placed on the zipper, and as such they can stay in place.
  2. walsh416
    Analog mice are markedly simple devices, essentially motion sensors that perform vector addition to calculate a change in position. At the heart of a mouse you'll find ventricles and aortas... (sorry, wrong kind of mouse). At the heart of a computer mouse a rubber ball rubs against two or more rolling bars.

    This ball is designed to be rather heavy, and have a high coefficient of static friction. These two attributes combine to form a ball that rolls instead of slides (heaviness increases normal force, and high coefficient of static friction increases the value for mu, multiply them together and one ends up with a high maximum static friction force). When the mouse is moved along a horizontal surface, the ball will roll underneath it.

    This rolling is registered by two or more small cylinders within the mouse that are in contact with the ball. Because the radius of the cylinders and the radius of the ball is known, it is easy to figure out how far each roller has moved (essentially, how far the mouse has moved in each axis). The mouse then performs some form of vector addition of of this information (if there are just two rollers at ninety degrees to each other, it would be a simple Pythagorean identity, if there are three or more things get tougher).

    The mouse then transmits these vectors to the computer via USB, and the computer moves the cursor on screen accordingly.
  3. walsh416
    In short, friction. Specifically, its function within screw mechanisms designed to hold things in place.

    For me, the most apparent example of this is on dumbbells where one has to put on weight plates and then screw a ring in place to hold the plates. If the screw were "ideal," it would have no friction, which would be great except for the fact that it would no longer be a functional fastener.

    By screwing the ring tight against the weight plates, a force is applied pushing the ring "out." In the absence of friction, this would push the ring out and be converted to a torque by the threads, spinning the ring away from the plates. Luckily, the friction between the plates and the ring mean the ring can't spin, and the friction between the threads on the ring hold it laterally in place.

    More generally, screws in their traditional sense would be utterly useless without strong frictional forces holding them in place. They would lose their holding power and simply unscrew themselves as soon as a force was introduced.
  4. walsh416

    Turning a bike!

    By walsh416,

    Golly gee biking (cycling) is hard. Perhaps the hardest part of all is mastering high speed cornering. You see it all the time in the Tour de France; pros carving graceful arcs as they fly down mountainsides at 100kph. How do they do it? By maintaining an incredible awareness of where their center of mass is relative to their bike at all times, and adjusting it so that they can achieve the right angle of cornering.

    By far the most common mistake any new cyclist will make is to turn their handlebars in the direction they wish to go. At low speeds this works to steer the bicycle, but at anything above a walking pace, all this does is cause one to eat asphalt. Instead, one must "counter steer," especially when beginning a corner. Counter steering is the act of pushing the handlebars in the direction opposite the one you want to go. This causes the bike to lean into the corner, moving your center of gravity lower and towards the inside of the corner.
  5. walsh416
    So we were touring an unnamed university's engineering department this weekend. It was pretty cool, the vibe was positive, the people were well adjusted and friendly, and all seemed well.

    Things took a turn for the dire as we stepped into the acoustics laboratory. Tragically, our tour guide uttered an utterly unforgivable sentence... "If I stood back to back with you inside this acoustically silent room and talked, you couldn't hear me because the sound-proofing is so great!"

    My first thought? "That's so cool!!"

    My second thought? "Wait... that's completely false, even in the most acoustically silent room around," and here's why....

    His premise for the statement was that the walls reflected no sound waves, and thus by standing directly behind someone you couldn't hear them speak. False. If you stand back to back with someone in the middle of a football field and say something, they will hear you immediately. There's nothing for sound waves to reflect off of (anything that might reflect them is far enough away that the time it takes sound to travel there and back would create a noticeable lag), yet one can still hear.

    So basically, no, nothing will reflect off of the walls, but sound will still travel throughout the room. This is because, unlike light, sound is not line-of-sight reception dependent. One vibrating air molecule will vibrate all those around it, not just the ones in the direction of projection, and so on.

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