# walsh416

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## Blog Entries posted by walsh416

"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).
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.
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.
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!
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.
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