# walsh416

Members

53

1

## Blog Entries posted by walsh416

First seen as a relatively mature product in the 1910s, the typewriter quickly became an indispensable part of the 'modern' office and home.

An entirely mechanical, analogue device, the typewriter uses a complex series of metal levers to raise 'typebars' (longer levers with characters on the ends). These typebars strike an ink ribbon and press it against the page. This impulse transfers some of the ink from the ribbon onto the page. At the end of a line of text, a typist must press the carriage return; a bar which moves the paper over and up to prepare for the next line to be entered.

A truly classic example of mechanical advantage, typewriter keys only need to be depressed half an inch or so to cause one of the type bars to swing up many inches and strike the ink ribbon and paper. This conversion lessens the force the type bars can exert at the end of their radius, but means this point moves very quickly. Because momentum is equal to mass multiplied by velocity, the end of the typebar builds up a great deal of momentum. This momentum is then used as an impulse to compress the paper and ribbon against the character and actually 'type' something.
So we launched catapults on Friday, that was pretty intense. In theory, ours was utterly perfect. We optimized it mathematically, and built it with the strongest \$1.99 2x4s in all the land. What we didn't account for was wind. Not wind's effect on our projectile, but on the catapult itself.

When cocked, our catapult had 135 pounds roughly four feet in the air (about 700 joules of potential energy, for those keeping score). During one launch our catapult, well, fell on me. If it fell two feet before hitting me, it was moving at (super rough mental math) ~1.5m/s.

More interestingly, the piece of rebar I was hit with was half inch diameter, or .00051 square meters. This becomes an average of 690371 Joules/square meter of my flesh. In turn we can convert this measure to Langleys, we see I was hit with the equivalent of 16.51 ly of solar radiation, a dose surely considered hazardous in any reasonable scientific culture.
"The following is a recreation of the real world events of a late October day in two thousand eleven, anno dominae."

T-10sec: Timothy is riding along on his bicycle, and comes across a group of walkers blocking the roadway. Being the amiable gentleman he is, he decides to go around them, swerving onto the sidewalk.

T-1sec: Disaster seems ready to strike our hero, for as he prepares to dive back into the street he strikes a pedal on the driveway, lifting his rear wheel up and reducing its frictional force from relatively high to null in a matter of milliseconds.

T-0sec: Our hero hits the deck! Due to a sudden and catastrophic loss of the rear wheel's frictional force, the bike's forces are no longer balanced (centripetal force is no longer opposed by the static/rolling friction of the rear wheel) and the bicycle/cyclist system rotates about the z and y axes, throwing our hero to the ground.

T+.5sec: Our hero hit the pavement with a momentum in the z-plane roughly equal to 422 Newton seconds, and is now sliding along the asphalt.

T+1sec: After sliding on the asphalt for ~1 second, Timothy comes to rest. This decrease of speed was (pun alert!) forced by a force imbalance. Kinetic friction was retarding forward motion and no force was causing forward motion, so our hero slowed to a stop.

T+5sec: Timothy says a small prayer that no one he knows saw any of the previous six seconds, pulls himself to his feet, and rides off. He swears to never again strike a pedal (that promise lasted a depressingly short amount of time).
I just skimmed blog post, and it got me thinking: what are some different ways to think about electrostatics and magnetism? I always function better when I can picture things as basic, mechanical physics concepts. I make it a point to always try to draw connections between more advanced concepts in physics (say, anything and everything covered in E&M) and more simplistic concepts (you know, water and force of gravity and junk like that). Until finding a Wikipedia article on the hydraulic analogy, I had no idea that this was a common thing to do!

Essentially, voltage is the difference in pressure between two points within a pipe, and current is the amount of water that flows past a point within a given time. Beyond that, electric potential is the pressure at one point within a pipe, electric charge is simply a quantity of water, and resistance is a temporary narrowing of a pipe to restrict flow.

Beyond this hydraulic analogy, it's easy to draw comparisons between electric fields/magnetism and gravity. Yes, electric forces are much stronger, but they work in similar ways. They're both attractive forces that can never be seen; they are observed through their effects on other bodies.
Ever since my birth in a log cabin in Montana, I've made a hobby of moonlighting in all of the occupations listed here, proceeding through them alphabetically. Personally, I've found I have a real interest in Taxi and Exotic dancing, grioting, and mechanical/aerospace engineering. I cannot wait to learn about all of these within AP Physics C. My strengths include algebraic manipulation of numbers and a truly superior superior vena cava. I think I can certainly stand to improve my sink-throwing skills, as well as my ability to remember about blog posts before the night they're due. In terms of what I'd like to do with my life... the list is long and varied. Create earth boots and sell them on the moon, master Physics C, buy my teacher a silver porsche... all the usual suspects.

I'm taking AP Physics C cause physics is cool!! I plan to become an engineer of some sort (in the occupation list between elementary child discipliner and exuberant english examiner), so I figure there's a chance I'll have to know some physics anyways.

I hope to walk away from AP Physics C with a greater understanding of real-world physics. No more of this "constant acceleration" crock they feed you in Physics B, no "negligible resistance" BS, let's do fizziks!!

I'm most excited about learning how physics uses calculus, but also most anxious about it.

That's all, folks!
So I went to see Gravity this weekend. Overall, it was pretty good! Unfortunately, Sandra Bullock was truly brain dead throughout much of the film.

In one memorable scene, she's trapped inside the International Space Station, alone, and fighting a massive fire. Panicking, she grabs a fire extinguisher off the wall and aims it. Now, before we get to what happened, it is important to understand a few things. The character she is portraying is an MD Ph.D, with years upon years of education. Unfortunately, it appears she never learned about a dead guy named Isaac Newton, who taught us that "any action will have an equal and opposite reaction."

Poor Sandy believed that she would be able to float in zero gravity, firing a fire extinguisher, and remain in the same position. That was stupid. As it happened, she flew backwards and smashed her head on a metal pipe, something that definitely wasn't helpful as she tried to navigate her way back to earth.
Lumberjacks are big dudes. Because they are more massive than the average human, they also weigh more. This bigger weight (or, more properly, increased gravitational force) means that the must do more work to travel the same distance.

Since work is directly related to force in the direction of travel, much greater forces are required. As force the second derivative of kinetic energy and the first derivative of momentum, when one increases the rate of the others will as well.

As seen in the diagrams below, lumberjacks mean business. So does Ryan Gosling.

Lately, it seems as if everyone and their mother has a "Lifeproof" case for their iPhone. Seriously. If my mom had an iPhone she would absolutely buy one of these.

The logic behind it seems to be that the average life is wet, rough, and crazy (enough said on that subject...) and that one should always use protection. For your phone. C'mon, people.

But how do they work? Typically, they contain a series of seals made of a relatively malleable type of rubber. When compressed and then held in place by clasps or screws, the rubber conforms to the other surfaces, forming a watertight seal.

For those of you looking to waterproof anything homebrew style (you know, waterproof a sink or a cat or a little sister just for fun), the trick is in the type of rubber, the surface it's conforming to, and the way it is held in place. The rubber must be flexible enough to meet up with the surface, but not so flexible as to deform and let water in. The surface has to be flat and straight, something it's easy to seal against. And finally, the rubber must be kept under essentially as much pressure as possible to maintain a seal.

So that's it!! Post pictures of any waterproofed cats/sisters in the comments below!
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.
Here, we'll be looking at "over center devices," for lack of a better term. Traditionally, this term is applied to the weird little hinged things in the arms of dentist chairs, where a pivot goes over the center of the mechanism, so applying more force doesn't affect the position, instead one must physically move the pivot.

Instead, we'll be referring to over center devices as anything which requires a large force to get over a "hump," before remaining locked in place (think about calculator covers and how they slide on, for instance). These work by using some sort of flexible material that, with enough force, can be deformed to fit over something else. By pushing up on a calculator cover, the angled "humps" on either side transfer that into a normal force against the calculator, pushing out the sides of the cover and slipping it over, before locking it into place.

Naturally, a mechanism like this is susceptible to fatigue. Typically, this takes the form of permanent deformation of the plastic or outright breakage. The first is caused by repeated use; squish the plastic around enough and eventually it begins to wear. Breakage is more rare, and typically only results from excessive force, or underclassmen.
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.
A bicycle's drivetrain includes the pedals, cranks, bottom bracket, gear rings, chain, sprockets, freewheel, and derailleur. With all of those parts working in harmony, it takes the power provided by a human pedalling at 90rpm and uses it to turn a 27.75" diameter wheel at 20mph, all with an efficiency upwards of 95%.

The drivetrain is, essentially, a system of levers and adjustable pulleys, working together to convert torques and forces. A typical crank is 175mm, measured from the center of the bottom bracket hole to the center of the pedal hole. This means that the downward force supplied to the pedals by the rider is instantly converted into a torque .175 times as great. This torque is altered again by whichever chainring is selected, and then travels via the chain to the back of the bike. Here the torque is again changed by the selected sprocket (the series of smaller gears at the back of the bike), where it is then transferred along the rigid spokes to the outside of the rear wheel. Here it is finally appropriate to talk about it in terms of a force again (instead of a torque, as it was throughout the drive train),at least when one is thinking about it relative to the road surface.

In all, the torque/force relationship is repeatedly altered throughout the drivetrain, all to allow the most efficient input of human power for a given output of force by the rear wheel.
2013. San Francisco Bay. The 34th America's Cup. Race 8.

Emirates Team New Zealand is neck and neck with Oracle Team USA, headed upwind (the slow point of sail; the 72 foot catamarans might only hit 30mph). In a typical race with typical boats, this would be a run of the mill maneuver. Do your best to stay in front of the other guys, don't lose ground, and find the perfect balance between outright speed and sailing as close as possible to the wind. However, nothing about racing the AC72 is typical.

One of the biggest (pun intended) reasons behind their ability to attain mind-boggling speeds is the 132 foot wing. Most sailboats have, well, a sail. When writing the rules for the 34th America's Cup, Larry Ellison decided, in all his infinite (eg nonexistent) wisdom that the boats would have wings rather than sails. This means that instead of a piece of cloth propelling the boat, a rigid, adjustable wing is used, almost like taking a wing off of an airplane and mounting it vertically. Each time the boat turns, the wing's trailing edge must be adjusted, as must the angle of the curve in its midsection. These adjustments are made with hydraulic pumps, pressurized by the grinders onboard the boat.

Now, back to race 8. New Zealand tacks (turns, pointing the bow [the front of the boat] into the wind as it does so) to avoid Oracle, but seems to lack the hydraulic pressure required to "pop" its wing. The boat has now turned but the wing remains convex to the wind, when it should be concave. This causes their boat to lean near 45 degrees for a heart-stopping 10 seconds before it returns to a normal angle with the water.

In the end, always remember Bernoulli's Principle! If the pressure has not been provided initially, there is no ability to create a pressure differential to properly execute the action.
Let's just get this out of the way first: if it doesn't have three pedals and require both hands to drive, it's simply not a car. It's a sad, sad excuse for transportation.

A manual transmission lets the driver select which gear they would like to be in, and control how they shift. By depressing the clutch (operated by one's left foot), the engine is disconnected from the wheels. One can now use the gear shift lever to select an appropriate gear. The gear shift lever typically operates a set of selector forks in the transmission. These forks engage different gears, depending on the position of the gear shift lever.

One a gear is selected, one slowly lets the clutch out while increasing the amount of gas given to the engine. The clutch is essentially a plate connected to the engine and a plate connected to the wheels that mesh together. When the clutch pedal is to the floor, the plates are completely separated. By letting the clutch out, one slowly brings the plates together. It is incredibly important to let the clutch out slowly: when the clutch is in, the plates are likely rotating at different speeds. By bringing them into contact slowly, one allows slippage between them before full engagement takes over, which means that they can accelerate each other until they are rotating at the same speed.

Increasing the amount of gas given to the engine while letting the clutch out makes one less likely to stall. Stalling occurs when the engine isn't producing enough power to continue to rotate, and so it simply seizes. This is deeply embarrassing, especially when it occurs at a busy red light (ask me how I know...). In letting the clutch out one is engaging a gear, and therefore increasing the load on the engine. Pushing the gas pedal in more lets the engine produce the power the transmission is now calling for.
When it comes down to it, your basic mechanical pencil really is a remarkably simple device.

A plunger is used to depress the entire inner sleeve. A spring keeps the plunger under tension and draws it back after it has been pushed. The inner sleeve contains a lead holder with the end split into two 'pincers' that are held together by a collar. This collar is moved by depressing the plunger, which pushes the inner sleeve down while keeping the collar in the same place.

Perhaps the most critical part of the apparatus is the 'exit tube', located on the outer sleeve. This tube is the last thing the lead comes into contact with before being used, and functions as a way to retain the lead as it is pushed out by the pincers. Because the tube is functionally the same diameter as the lead, they are in complete contact and therefore exert a normal force on each other. Because neither of them are frictionless, this normal force combines with their coefficient of friction and forms a frictional force. It is this frictional force that holds the lead in place when one presses the plunger and opens the pincers.

Incidentally, the work one on the plunger by the operator is converted into both spring potential energy that is released when the plunger is released, and heat due to the sliding of the lead through the 'exit tube.' As such, Al Gore has suggested that we all "be kind to the great mother earth, and stop global warming by using fewer mechanical pencils." He always was an odd duck...
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.
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.
As we learned from Professor Goho, friction due to air resistance is equal to .5c[ROW]Av^2. In bicycle racing, one's goal is to minimize this value as much as possible. Manufacturers spend millions of dollars each year lowering frontal area of their components and increasing making them more aerodynamic.

As an individual cyclist, one lowers aerodynamic drag by drafting; the practice of following as closely behind another rider as possible to catch their slipstream, essentially lowering the local density of the surrounding air. This is done by riding down the road at 25mph behind someone you've never met, keeping your front wheel no more than half an inch to an inch behind their rear wheel, and praying they don't swerve or stop. Fun, right?

By following so closely behind, you are taking advantage of the zone of low density air behind them and lowering your friction due to air resistance. The practical effect of this is absolutely huge; in the middle of a well-developed peloton (large clump of riders), your effort can decrease by 25% or more. That is to say that if the leading rider is outputting 500W to ride at 30mph, you need only 375W to stay in the pack.

In a race, this creates a curious paradox: you want to remain close to the front of the group to avoid the “yo-yo” effect caused by the reaction times of individuals braking to avoid collisions, but leading the group is the very last thing you want to do, as you are burning energy you'll need later to keep up with the breaks or contest the final sprint.
First, let us bow our heads and give brief thanks:
"We thank thee, creator of roads and corners. Bringer of snow and handbrakes. You alone make driving fun."

And now, lets talk about driving in the snow! Effectively, snow lowers the coefficient of static friction between the tires and the road surface. This means that for the same weight of car and contact patch with the road, The road can provide less "push" against the tires, so they're more likely to spin instead of roll.

This effect is increased in corners, due to the added force applied to the tires. Rather than simply being asked to provide force in the plane of the wheel, they're now also being asked to provide force in a plane perpendicular to the wheel. Vector addition tells us that if we add two perpendicular vectors, the resultant is the square root of the individual vectors squares (Pythagorean theorem).

This spinning instead of rolling is also 'helped along' by using the car's brakes. Fortunately for safety, but unfortunately for someone who actually enjoys skidding, modern cars are equipped with antilock brakes (ABS). This means that if a wheel starts to slide under braking rather than continue to roll, the car automatically pulses the breaks, working to find the point where maximum breaking force is applied to a wheel without causing it to skid. As evidence that there really is a divine being, most cars are equipped with a hand-operated emergency brake. This brake has a direct connection to the wheels, and is therefore unaffected by the antilock braking system.

In the end, maximum slide is achieved by going around a corner and then pulling the handbrake. This ensures that the tires are under maximum load, and then knocks them loose by locking them up.

As always, be safe, don't wreck any lawns or mailboxes, past results do not guarantee future success, don't drink and derive, and don't share needles.
Honking the hooter. Emptying the schnozz. Launching the snot rocket. Everybody blows their nose, but how exactly does it work?

First, you create positive pressure in your lungs by tensing your diaphragm. Since initial PV/T must equal final PV/T, when a larger pressure is created it will result in an increase in the velocity of air exiting the lungs. This fast moving air is then guided up to the nose and out the nostrils.

As the air passes through your nostrils, it must pick up the snot clinging there and carry it out. It does this through drag due to air resistance. As soon as the force on a snot particle is greater than the maximum adhesive force holding it to your nose, the piece of snot will be blown off and carried away.

Thus, it follows that there must be a 'terminal blowing velocity' -- a velocity at which blowing faster will not result in the purging of any additional snot. This author encourages all readers to grab a pack of kleenex, a wind speed gauge, a family member with the flu, and experimentally determine said terminal blowing velocity!
Ahh, the keyboard. Platter of choices. Pallet of Arabic characters. Based on the QWERTY layout introduced in 1867, the keyboard is our portal to the world. But just how do they work?

First, it is important to note that this article centers on modern, computer keyboards. Check back later for the physics of a typewriter. Modern computer keyboards feature upwards of 70 keys (I just counted, mine has 84) which take a consistent and relatively small magnitude of force to depress and register as a keystroke. On my keyboard, this consistent force is provided by a small synthetic spring and series of two levers.

The spring is formed through a molded piece of what appears to be silicone, with small "humps" where each of the keys are located. These humps compress in a manner where the first millimeter or two of displacement requires most of the total force, and the rest of the displacement requires very little force. Practically, this means that the keys can support quite a bit of force (say, the weight of one's fingers), while still being relatively easy to compress.

I believe that the two small sliding levers function only to keep the keys oriented correctly, as they are sprung but very, very lightly.

If one had far, far too much time on one's hands and wanted to the spring constant of a key over its displacement, one could set up a range finding system to measure the acceleration of the key when a constant force is applied. This graph could then be examined to determine the change of net force and, therefore, variation in spring constant across the displacement. Let's not do that though, because that's the sort of thing only done by thirty year old guys named Ted who live in their mothers' basements.
The earth is big and confusing, as is all the magma and magnetic and [insert another alliterative m-word here] magic that goes on inside it. To compensate for some of this magicky nature, the leap second was created.

To back up a moment, there are many different time-standards in use today, all of them slightly different, and each of them quite confusing. Two of the most common are the atomic-clock based UTC, and the solar based GMT standard.

As we have developed ever more accurate clocks, it's become clear that there aren't actually 24 hours (or 1440 minutes, or 86400 seconds) in each day. Though the difference is fractions of a millisecond, these inaccuracies snowball, and eventually the 'more accurate' UTC is no longer accurate and has strayed from GMT.

To compensate for this straying due to variations in the earth's rate of rotation, the leap second was created. Used only when deemed necessary by observation, it has no set implementation (unlike, say, a leap year which comes every four years, except every 100 years, except every 400 years). And that's the cool part. We have no way of modelling when a leap second will be needed. There's no predictive formula or simple rule. In essence, we have no idea how to predict variations in the rate of rotation of our planet.

And that's just freaking awesome.

And slightly concerning.
Ahhh. Nothing like that feeling of a thousand baby angels caressing your scalp.

First used more than 9,000 years ago, the pillow is a simple, soft device that works to add comfort to any situation. In bed? Use a pillow. Walking to school? Need a pillow. Fighting house fires? Pillow! Skidding around corners in the snow? Pillows for days.

Most simply, a pillow works to distribute a force over a larger area. Filled with soft materials, a pillow conforms to the shapes of the two surfaces it lies between and increases the contact patch each has. Practically, this means that instead of having ten square centimeters over which to distribute force, the object being pillowed might now have forty or fifty; a sizable reduction in pressure.

Nerve cells in human skin feel only warmth, extreme temperatures, pain, and pressure. By significantly reducing the pressure on one's skin (by, say, putting a pillow underneath one's head before lying down), one can achieve much greater comfort.

Also, I found this on Wikipedia while doing some pillow research. The Japanese have weird pillows....
Physics is.. so gosh darn great
I feel like it and I... are fate.
With a Newton here, or a Pascal there,
These SI units we love and share.
Whenst look for a potential mate,
All emotions to physics, they equate.

If the air in the room feels perhaps electric,
Just know that physics isn't eclectic.
A standard mix of fun and function,
Studying physics fills one with compunction.

Alas, alack, it is time to go,
I'll need to do work, that's fo' sho'.
As Bernoulli said, just go with the flow.
So shine like a lumen and simply glow.
Ahh, sleep. Slumber. God's gift to mortals.

Sleep is all about comfort. As many a mattress commercial has drilled into my head, not all mattresses are comfortable. There is a supposedly optimal point of squishiness and firmness and pillowness and sleepnumberness and weird-yoga-guy-meditating-on-a-mattress-for-no-apparent-reasonness. In short, different people like different mattresses.

The generic want for a mattress, however, is relatively universal. This is due to the way gravity acts on the human body. Since we are not simply a point mass, gravity pulls on our entire body. When laying down, as if to sleep, this pulls us towards the ground. Due to the natural contours of the human body, the actual contact patch with a flat floor is relatively small, leading to high pressure areas (since pressure is inversely related to area, with a smaller area a greater pressure is produced). It is this pressure that is considered uncomfortable.

A good mattress will conform to one's body, greatly increasing the size of the contact patch between human and bed. Speaking of beds and mattresses, hot damn one would be nice right about now...
×
×
• Create New...