# AlphaGeek

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## Blog Entries posted by AlphaGeek

SO, because everyone else decided to take a break, my brain also decided to go off on holiday for a while. In that case, I've decided to make this blog post 100% fun (yet still on topic), and what's more fun and physics related than comics? Hope these tickle your funny bone, have a great break everyone!!!

...And finally, a comic that Mr. Fullerton would enjoy:

Have a great break! Make sure to relax (and pretend that midterms aren't coming up)!

--Alphageek
As Goalkeeper0 and Mr. Fullerton suggested, I decided to give the soap-and-water approach to understanding flux a shot. If you'd like to try this experiment but can't find one of those ancient metal coat hangers, here's a different approach:
>>>Credit to Goalkeeper0<<<

I bent a coat hanger into a solenoid with 5(ish) loops and filled the dish basin in our sink with soapy water. I'm not sure if it's because here wasn't enough soap in the water or a different factor, but the darn soap layer kept popping before I could pull the hanger completely out of the basin. Here's low quality evidence of my findings;

...Again, you really can't see much in the picture. I recommend doing the experiment yourself. To give you a better idea of what the soap spiral looked like, it resembled the shape of fusilli pasta. EX:

Yum. Love the stuff

Again, this shape is meant to demonstrate how the number of N turns in a solenoid effect the flux. The more loops in the coat hanger, the more bubble-surface there is in our solenoid. This helps reinforce the equation Mag. Flux = BANcos, with B= Mag. field, A= area and N= number of turns in the solenoid.

--Alphageek
A friend returned from training for the air force and told me about some of the crazy things that he and his comrades would do to pass the time. Some of these included taping each other to the ceiling and human-drawn chariot races, but one of the most messy and fun instances he described was a hallway slip n' slide. True story: he and his hall mates put towels to the bottom of their sleeping quarter doorways, filled the hall with soap and water and proceeded to slide down the hallway as though it were a seabreeze water slide.

Let's say the hallway is 15 meters long, and each cadet gets a 8 meter running start before diving down for a slide. If one guy sprints and reaches a speed of 4 m/s before flopping into the suds (which has a coeff. of friction of .2), does the cadet slam into the other side of the hallway or get soapy bragging rights?

...In terms of the cookie question, Charlie: 2; entire rest of the class: 0. Come on, guys! (Nice job Charlie)

Good luck! --Alpha Geek

DISCLAIMER: It's unlikely that this exact situation with such distance in the slide will occur. The coefficient of friction with the floor and the speed of the person are strictly for cookie-question fun, not actual values. In other words, don't try this in your living room!
I'll set the scene: It's a dark night and the fog is thick as soup. You drive along in your pink jeep, hoping to get home in time for dinner (your favorite!), when a white mass appears in the road.

A COW!:eek:

You thrust the break pedal to the ground, and your wrangler just stops short of the bovine J-walker. What is the only thing that came between you and a pile of ground beef? Physics is the hero of this story-- specifically friction.

A car's breaking system is usually one of two types: a disk break or a drum break. The disc break system is composed of a rotor (or break disk), a caliper, and break pads. The rotor turns with the wheel, and the break pads apply pressure to the sides of the rotor in order to slow the car down. Disk breaks are commonly used in smaller vehicles like cars and minivans, as they produce less heat and are easier to change. If you need to "change your breaks," it's more likely that you have to swap out the worn-down break pads than the rotor.

Disk break system

In a drum-break system, the curved break pads, or "break shoes" push up into the sides of a dish-like cylinder, called the break drum. This system of breaks is used in larger vehicles, like semi trucks. While the drum break system produces a larger amount of heat energy than the disk breaks, it is much more effective. For very large vehicles (ie. busses) air breaks are used, but we won't get into that.

Drum break system

So here's the pure physics of it all: break pads are made of steel with ceramic or another friction-inducing substance. When pressing up against the rotor/drum, the pads convert kinetic energy into heat energy due to the high pressure and friction of the interaction. The larger surface area of the break shoe for the drum model causes there to be more heat released in the drum model than the disk model, which is why the disk model is more common in cars.

Thank you, consumer auto! I miss Mr. M as our homeroom teacher... Shout out to room 1071!

--Alphageek
Hi everyone! I thought this would be applicable since we're in the electricity and magnetism portion of the year

In electric fish, such as an eel or a ray, there is a body part called an "electric organ." This mass of muscle and/or nerve cells produce an electric current when the fish sees fit. It is used for protection, navigation, communication and sometimes (but not often) against prey. The organ itself consists of a group of connected electrocytes, through which the current passes through.

An electric catfish AKA a strongly electric fish. He might look like he wants a kiss but believe me, he doesn't.

In weakly electric fish, the organ is used for navigation as the electricity produced is too little to do harm. However, in strongly electric fish, a discharge of electricity is strong enough to be used for defense. Something interesting to note is the difference in the structures of freshwater electric fish and saltwater electric fish (this difference is also mentioned in pg. 795 of the text). Freshwater has a higher resistivity than salt water, and as a result freshwater fish release a higher amount of voltage than salt water fish in order to be effective. Another cool fact: in order to achieve this difference, the fresh water fish's electrocytes are connected in series, while the saltwater fish's electrocytes are connected in parallel. Awesome, no?

'Geek out!
More electricity-themed blog posts!

Neurons are cells in the nervous system. This cell transfers information via chemical and electrical signals. The long, stem-like part of a nerve cell is called the axon. In the human body, the axons that run from your spinal chord to your feet can be over a meter long. Electrical pulses are transferred through the axon down to the neurotransmitter molecules. The membrane potential of the average neuron cell is between -60 and -80 mV when the cell is not transmitting signals.

The electrical signal is converted into a chemical one once it reaches the synapse. The synaptic vesicles (containing ligands called neurotransmitters) release small molecules, which flow over to the receptor molecules on the adjacent nerve cell, and the message travels through a net of these cells until it reaches its destination.

Some interesting facts about the nervous systems of various species:

-- The electric eel is equip with 8,400 neurons, which can potentially crank out a painful 600 V.
-- It is estimated that the human brain contains roughly 100 billion nerve cells.
-- All animals except sponges have a type of nervous system.
-- The contraction and expansion of a Hydra is controlled by a nerve net, a web-like system of neurons that span the organism's body.

Shocking, eh? :einstein)

--Alpha Geek
Family Guy isn't exactly school appropriate in most cases, but it is, however, physics appropriate. In one episode, Brian (the dog) educates Peter (the tubby man) on his weight issue. Brian claims that Peter has his own gravitational pull, and continues to demonstrate this by placing an apple nearby his stomach. The fruit then assumes orbit directed around Peter's abdomen.

...For those of you who are not familliar with the episode, here is a not-so-legally posted, poor quality youtube video featuring our obese friend. :apple:

Lets take a closer look at the physics of this cartoon.

Assuming that a feasible weight for roughly 44 year old Peter is 100 kg (220 pounds) and the average apple weights .15 kg, as well as the radius of the orbit being roughly 1 meter from Peter's center, here are the technicalities of the situation.

The force of gravity on the apple = (GmM)/r2 = ((6.67E-11)(100*.15)/12 = 1E-9 N

The acceleration due to gravity on planet Peter is = ((6.67E-11)(100))/12 = 6.67E-9 m/s2

And finally, for the apple to escape its orbit around Peter, it would have to be going a grand total of [(2GM)/r]1/2
, or 1.15E-4 m/s.

Note how these values are extremely small. For one thing, this situation is impossible in the first place. To emphasize this, the gravitational force is so small that it likely could not even pick up the apple. Even if it were magically strong enough to do so, the speed of the apple was far greater in the video than its small escape velocity, and would fling out of orbit before Brian even turned on the Three Stooges. Sorry, Seth McFarlane. Physics disagrees with you. Guess Family Guy isn't such intelligent programming after all...

--Alpha Geek
So far, no other particle has been able to move at the speed of light. However, human beings are capable of seeing light move. Ramesh Raskar and his team at MIT have developed a camera capable of capturing light at 1 trillion frames per second. This method, called fempto photography, can take slow motion videos of light in motion. Watch the video for a better explanation but for those of you in a rush below is a summary of MIT's amazing research.

As shown in the video, Raskar uses a laser to send a packet of photons through an object. Using fempto photography, the MIT team created videos of light traveling through a coca cola bottle and washing over a tomato.

The group presents promising applications of their technology, such as finding survivors in unsafe conditions or hiding beings as well as exploring inner organs by seeing around corners with light.

Perhaps the most interesting aspect of this video is featured in 9:20 - 10:04, in which time appears to be moving in reverse according to the camera's images. How is this possible? Watch to find out! Weird things happen when humans try to go faster than the speed of light

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So. I was thinking of what to carve on my pumpkin earlier and thought, "What's something that'll scare the pants off of anyone, even high schoolers?" Bingo, air resistance. Many of us were shaking in our boots when Mr. Fullerton derived a few drag-related equations, but looking back they're not too bad right?

Here's a little review. That long page really boils down to a few key equations:

Air resistance = Fdrag = bv = cv2 , where b and c are constants
VT= (mg)/b
V = VT ( 1 - e(-b/m) )
V = (mg/b)( 1 - e(-b/m) )
a = g e(-b/m)t

Some equations may contain the variable tau, which looks like a backwards J or a T with a tail. I will use T to represent tau. It is a time constant that can be substituted into the equations above. Tau = T = m/b.

I hope that helped a little. Sleep with the lights on tonight, folks. 'Tis the season!
HAPPY NEW (school) YEAR EVERYBODY!!! I'm super excited for some serious Physics C. Just set up my account! I found this comic online and thought it would be a great way to break the ice:

Hee hee. And of course I'll site my oh-so-credible source: http://memebase.cheezburger.com
...Although they did spell cheeseburger wrong. ^-^; And look, I found a fencing smiley face!:fight)

--Geek out!
How to become an autodidact (defn: self-directed learner) :einstein)

Monday, we were given a few packets of work, some written directions and a "finish this before the test next week." Weird. A class with no teacher? A few groups popped up to grab a computer, others buried their noses in the textbook, and some started chatting leisurely with friends. It's not that we don't have a teacher, it's just that for the next few days, we're our own teachers.

For a few of us (including myself), this whole learning-on-our-own thing is a little bit intimidating. I've scoured the inner depths of google to put together an E2K countdown of ways to survive without teacher supervision. Try reading them over to pick up a helpful hint or two:

3.) Having trouble? All you need is a little help from your friends. Try forming small groups for support, review, and advice. Not only will confusion be dismissed, but it turns out that people retain information longer after they've taught a peer. It's a win-win situation. However, make sure to refrain from copying or relying on others to teach you. The point of self-directed learning is to learn independently, not to leech off of others.

2.) Use your resources. Between the textbook, aplusphysics.com, and the unit summary packets from class, you're bound to find an answer to your problem. READ THE TEXTBOOK. Key chapters are listed on the board, but if the answer still isn't there try skimming the glossary for key terms. Google and other internet searches are okay, but sites like Wikipedia often over complicate or give false information. Try the aplusphysics site. The video sections, tutorials, and course notes are great tools to reinforce concepts if the textbook was unclear, or even for review before the test. Don't forget about asking others (and making friends-- you can friend people on aplus, you know ).

1.) The number one suggestion for learning on one's own is manage your time. Procrastination is our number one enemy when it comes to self-guided learning. In order to avoid doing the web assign, worksheet, problem packet and 8 blog posts the night before they're due, take a few steps of precaution. Try making your own due dates. These are most functional if they're BEFORE the one Mr. Fullerton assigned. Making a calendar is a little over the top, but writing down goal dates to finish certain sections of the work is helpful. At least have mental due dates-- no one should be staying up 'til 1 the night before the test.

A few of you may ask, "Why is God's name did Mr. Fullerton do this to us? Does he hate us? Was it something we said?" This way of learning will benefit us greatly. For one thing, it's preparing us for college next year. Not all classes will have 20-30 kids. A large intro/lecture class could contain anywhere from 50 to 500 students. In otherwords, the teacher can't tend to hundreds of kids at once, making an independent learning style an essential survival tool. Even if the classes are small, there's no guarantee that the teacher is good at,well, teaching. In the event your teacher is a story teller, a newbie, or just plain unhelpful, knowing how to consult a textbook or another resource means the difference between passing and failing.

In other words, take a deep breath, plan briefly, and get to work. Have confidence in yourself as a student and don't be afraid to take autodidacticism for a test spin!
Do you find your blogs boring, drab and in need of fanciness? Do you think that int(x^2) is an acceptable substitute for ? Because the APlusphysics site has undergone improvements, I think that our blogs' equation quality should improve as well. ;D

A little birdie (Mr. Fullerton) told me about this great tool called a latex editor. One site to go to is http://www.codecogs.com/latex/eqneditor.php , which you don't have to download and it's not blocked by the school. It's a site where you can choose the symbols that you want in an equation, like sigma or pi, and it spits back a code.
When you paste that code into your blog post, put $$before it and$$ after it, then preview the blog, the symbol you chose will be in its place.
I had to put the information above into the code box or the computer would've read it as part of a code. For example, if I choose the pi button and the latex editor spits back CHERRY, I would write [ tex] CHERRY [ /tex] and the symbol for pi would come up.
(the code is actually \pi, so if I surround that with $$and$$ it looks like: $$/pi$$

I hope thats helpful! If you have any questions, Mr. Fullerton or I would be glad to help
Infrasound is sound with a frequency lower than 20 Hz. Human hearing registers sounds from roughly 20 to Hz 20,000, though under certain circumstances the body will hear/feel sound at a lower frequency than 20Hz. Though the human ear does not normally register infrasound, these inaudible waves effect our everyday lives.

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In Nature:

Infrasound is produced naturally by severe weather and other forms of nature. Storms, thunder, volcanic activity, earthquakes, avalanches, tsunamis, waves, wind, and even the aurora borealis emit these sound waves. (1) Because many animals can detect sounds of lower frequencies than humans, they are able to sense oncoming natural phenomenon. This explains why many animals move to higher ground or act oddly before a storm.

Animals also use infrasound to communicate. Whales, hippopotamuses, giraffes, elephants and alligators are among the group that do so. These animals are able to send messages over thousands of miles in this manner. Homing pidgeons and other birds are also able to sense these sounds. They use infrasound in combination with magnetic fields to create a mental map of their surroundings.

Human encouters:

Infrasound is utilized by humans to locate earthquakes, specific types of rock, petroleum, and even artillery/nuclear weaponry. Low frequencies are also created by man-made objects. These noises are theorized to create disturbances for creatures that can register infrasound. The sounds may cause the animals to become disoriented, causing whales to beach themselves or pidgeons to lose their bearings. (2) Infrasound also causes irritation to human beings. In studies such as that of Professor Richard Wiseman, Richard Lord (acoustic scientist), and Vic Tandy, human exposure to infrasound may increase paranoia, physical discomfort and even nausia. (3)(4)
In Tandy's situation, halucinations and supernatural "sightings" were later found to be linked to low frequency readings in the area. Infrasound is also the factor behind mysterious humming, called "The Hum," in certain areas that are audible to some people yet nonexistent to others. The hums are often emitted by man-made or natural sources. For example in Kokomo, Indiana, the humming was later pinpointed in a local factory. A fan producing a 10 Hz noise was the source of the scare.

***Authors note: This post took a little bit more research than usual ^^; It was interesting, though! Enjoy

Sources:
1.) http://www-dase.cea.fr/public/dossiers_thematiques/infrasons/description_en.html
2.) http://web.archive.org/web/20041102004955/http://pao.gsfc.nasa.gov/gsfc/educ/science/2000/04-07-00.htm
3.) http://www.msnbc.msn.com/id/3077192/#.UJV9R2-unBY
4.) http://www.coventrytelegraph.net/news/coventry-news/page.cfm?objectid=12722447&method=full&siteid=50003
5.) http://www.theindychannel.com/news/possible-source-found-for-kokomo-hum
You are a Kerbal physicist for Kow Jumped Over the Mun, a company that excels in anything spacey or astronaut-y. Your co-worker, Kirby McKerbin, is arguing with the desk clerk Moony Muni. Because you clearly have nothing better to do until next launch, you decide to listen in. Kirby is absolutely convinced that if he were to oscillate a pendulum on Kerbin, move it to Mun, and repeat the same occilation, the period of the pendulum on Mun would be roughly 3.5 times that of the period of the pendulum on Kerbin. The clerk argues that the period on mun would be 2.5 times that of the period on Kerbin. Settle this mess! Who's right and who should be working at McKerbals (Kerbin's largest fast food restaurant chain)?

****HINT: the surface gravity of Mun and Kerbin can be found on this site >> http://wiki.kerbalspaceprogram.com ******

...Scary but true. They can't tell the females from the males in our species, either.

You are a Kerbal physicist for Kow Jumperd Over the Mun, a company that excels in anything spacey or astronaut-y. After ending an argument between two colleagues, you decide to take a lunch break. The cafeteria guy, Louie Eeloo, has a thing for riddles, which started out amusing and grew annoying as the years passed. You were hoping that the line would be long and Louie would be busy, seeing as how its Flungus day in the small cafe (who doesn't love a heaping plate of Flungus?). No such luck. The cafe is a ghost town. You turn to avoid Louie and order out, but he's already spotted his next victim.
Louie offers Ooglie Cookies to all riddle solvers. Because you have nothing better to do until the next launch, you decide to humor him.

Louie says, "I have four friends, we run 'round Jool happy as can be. A 50 kg Kerbal on Kerbin weighs 3 kg on me. What am I?"

Your stumach grumbles. You could really go for an Ooglie right now. What's the answer to Mr. Eeloo's riddle?

It's actually not the day of the dead. I just like this photo. Have a feliz day anyway

--Alpahgeek
Anybody else having trouble with orbiting other planets? Docking? Space planes even? I was on youtube the other night and came across a user who developed a number of Kerbal Space Program tutorials. They're long, but are thorough and walk you through processes step by step. He trouble shoots often, so you can clearn from his mistakes to address your own issues. Plus, he has an accent. Strangely enough that makes him fun to listen to.

Here's the link to his Kerbal Space Program Playlist:

There are a few intro videos, but he also has at least three for docking, a handful for orbiting various planets, and a number on space planes. I watched a few on the planes (just for kicks) and jeez they were complicated! The docking videos were loooong but helpful. If Kailzah makes it to the Mun before school ends, I'd really like to give docking another shot.

Has anyone else found resources/tutorials that they've found helpful? Only three days left! It's Mun or bust for Kalizah ]

...can all be found at a fencing tournament! It's about time that fencing found it's way onto this forum. Fencing is an Olympic sport consisting of three weapons, epee, sabre and foil. In foil and epee, the opponent must hit their opponent's target area with their tip in order to score a touch. In sabre, the fencer may hit with the tip and/or the side of the blade to score a touch.

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I stumbled upon these fencing related physics applications by Ann McBain Ezzell, an MIT alumini. GIve the questions a shot, but if nothing else, read through them as they are quite humorous.

A few comments on the question's content:
1. Fencers scream/yell during bouts. Odd, but true. A fencer may do this to celebrate a touch, frighten their opponent or convince the referee that they scored a touch. (Some sound like howls [Div 1 men's foil], others like pterodactyls [youth 12 women's epee]. My favorite yell is "YAZEE!," used frequently by a fencer at the University of Rochester).
2. Fencers sometimes thow their equipment when they are angry. If they do, the referee will likely black card the fencer and they are removed from the tournament (I've seen it happen, it's both frightening and comical).
3. Most of the terms used below are actual names of fencers, equiptment, etc. For example, Peter Westbrook is the founder of the Peter Westbrook Foundation in New York City, an organization allowing people to fence who normally would not be able to afford it.

Here is Ann's mock exam. I hope you have as much fun with this as I did!

FENCING PHYSICS FINAL
27 April 1989 - updated 11 December 1994

[Disclaimer: All similarities between real fencers and characters in this exam are purely intentional and completely without malice.]

Instructions: Answer all questions. Be sure to show your work (including, where appropriate, free body diagrams). Don't screw up the math. Except as noted, you may neglect air resistance and friction.

1. A 2m tall Italian epee fencer loses his last repechage bout by being pushed off the end of the strip (standard 14m length). He knocks his mask straight into the air and simultaneously kicks his reel, which had been positioned at the end line, towards the other end of the strip. The mask just touches the 6m high gym ceiling before starting its downward descent. The fencer sees the reel barely clear the head of the 1.75m tall referee, who is standing in front of the scoring table recording the result. Just as he is knocked unconscious by his plummeting mask, he sees the reel land at the feet of the chairman of the Directoire Technique, who had been watching the bout from the far end of the strip.
a) How long does it take the reel to reach the ground?
Calculate the initial magnitude and direction of the reel.
c) How long will it take after the fencer regains consciousness until he is expelled from the competition?

2. Claus Block is bouncing up and down two meters from his opponent's end of the strip. His reel has slipped to 1.5 meters in front of his end line, and the reel cord is attached to his waist 1m above the ground. The mass of the exposed portion of the reel cord is 500g. A standing wave of three loops is being produced in the reel cord.
a) If Claus hits the ground 10 times per second (it's the finals), what is the tension in the reel cord?
Assume that the tension in the reel remains as calculated in part (a). Where would Claus have to stand and bounce, relative to his initial position, to produce a standing wave with only two loops?

3. A brand-new Uhlmann epee point is constructed such that the total travel is exactly 1.5mm, and it just passes the 0.5mm shim test. When a test weight of 750g is gently dropped onto the tip, the scoring machine light comes on. After the machine resets, the light remains off. However, any further depression of the tip causes the light to come on.
a) Calculate the spring constant (k) for the point spring (you may neglect the mass of the tip).
A Russian point is dimensionally identical to the Uhlmann point, but friction in the point produces an extra 1N of resistive force. Since its owner cannot readily fix his weapons, the point spring must be strong enough to lift 2kg (as above), to ensure that his weapons will never fail on the strip.
Calculate the spring constant (k') required for this point spring.
The two weapons are fixed horizontally, tip to tip, then the retaining screws are removed to allow free movement of the tips. The two tips are displaced 0.5mm from their equilibrium position and then released.
c) Calculate the frequency of the resultant SHM. (Assume that the mass of 1 tip is 1g and that both tips move together.)

4. Yuri Rabinovich and his long-lost identical twin brother Pavel (each with mass 65 kg) are fencing sabre. With weapon arms half-extended, they launch simultaneous fleche attacks and lock bell guards in mid air. Just before impact, each is traveling at a speed of 5m/s. When their bodies pass, the centers of mass are 1m apart. The bell guards remain locked and their arms extend to full length (adding 1m to the distance between the centers of mass).
a) What is the angular momentum of the resultant tangle immediately following the collision?
When the arms are extended, what is their rotational frequency in revolutions per second?

5. In the midst of a team free-for-all, Frank MacKenzie (mass 90 kg) picks up Lara Tomasso (mass 65 kg) and attempts to hold her at arm's length (this would put her center of mass 1m from his center of mass). Frank has enough upper body strength to support a mass of 25kg in this manner.
a) Frank, being an engineer, starts to spin. After accelerating for 5 seconds at a constant rate, his arms are forming an angle of 5 degrees with the horizontal. Find his angular acceleration.
At this same acceleration, how long will it take until his arms are 2.5 degrees from the horizontal?
c) How long before his arms are perfectly horizontal?
d) How long will it be before Lara throws up?

6. a) The maximum length of a foil blade from tip to bell guard is 90cm. Taking the pivot point to be at the bell guard, calculate the torque produced by a force of 20N applied perpendicular to the blade at the following distances from the tip of the foil:
1) 85 cm
2) 50 cm
3) 10 cm
If you are able to produce a torque of 10Nm around your own bell guard, calculate the resultant torque around your opponent's bell guard if your blades are pushing at right angles to each other and the intersection point is 10 cm from your bell guard and 45 cm from your opponent's bell guard.

7. Assume that a foil blade (not including the tang) is a uniform rod of length 90cm, diameter 5mm and mass 150g. Your opponent beats your blade sharply 40cm from the tip, breaking the blade. She then immediately does a circle disengage and hits the free end of the broken piece with a 20N force for .01 second. Calculate the rotational frequency of the broken piece of blade as it spins off end over end. (The rotational inertia, I, for a uniform rod of length L is 1/12mL^2, with the axis of rotation at the center of the length of the rod.)

8. A golf ball of mass 46g hangs from an ideal string 1m in length. A diligent epee fencer practicing point control strikes the ball with sufficient force to cause the string to form an angle of 15 degrees with the vertical.
a) What is the velocity of the golf ball immediately following impact?
How long after impact will it take the ball to reach the point where it is closest to the fencer?

9. Peter Westbrook (mass 70kg), having temporarily forgotten the end-of-strip rules in the heat of the finals, retreats rapidly off the end of a raised piste 0.30m high. Fortunately for Peter, the regulation run-off incline of 2m has been included.
Unfortunately, he trips and ends up rolling ignominiously the entire length of the incline. Assume that Peter's body approximates a cylinder of 50cm diameter as he rolls without slipping down the incline. Further assume that he is not moving horizontally when he hits the top of the ramp.
a) If Peter is making 2 revolutions per second when he reaches the bottom of the incline, what was his angular momentum when he hit the top of the incline?
What torque is required to stop Peter's rolling at the bottom of the ramp in 1 second?

10. Isabelle Hamori shrieks in the heat of combat at 13,000 Hz. The gym is set up with pairs of two meter wide strips three meters apart, with six meters between each pair.
a) If Isabelle is fencing in the middle of strip 11 at the far end of the gym from the Bout Committee table, which is 10 meters from strip 1, how much longer will it take the Chairman of the Bout Committee to wince than Isabelle's referee, who is standing halfway between strips 10 and 11? (This is at the 1988 Chicago Nationals, where the ambient temperature is approximately 40 degrees C. Take the speed of sound in air at 20 degrees C to be 340 m/s and remember that the speed of sound is related to the square root of the temperature in degrees Kelvin.)
Isabelle's opponent is MJ O'Neill, also known for her dulcet tones on the strip. MJ screeches while fleching at Isabelle, who attempts to retreat, at full voice. The referee, who is maintaining his original position relative to Isabelle, notices that the combined shrieking is producing 2 beats per second. If MJ screeches at 12,980 Hz, what is her minimum velocity relative to Isabelle?

Leave it to an MIT student to make a kick-butt exam. All credit goes to Ms Ezzell!

--AlphaGeek :fight)
After watching all of Walter Lewin's videos as well as Mr. Fullerton's, I've come to the conclusion that Mr. Fullerton's videos are more straightforward and earlier to understand that Lewin's. For those of you who swear Lewin isn't speaking English, here's a summary of the video content. I will be listing content in order of the A Plus Phys. video titles, so that if anyone needs elaboration they can refer to the corresponding video. :star: If even that doesn't work, the textbook & practice problems for each chapter might help, too.

Note: There are some concepts that I can't put in, like RHR and other exercises that require visuals. For these, please reference the vids!

Magnetism

Moving Charges in Magnetic Fields
Forces on Current-Carrying Wires
Fields due to Current-Carrying Wires

PSSC Magnet Laboratory

[*]Biot-Savart Law
[*]Ampere's Law

Moving Charges in Magnetic Fields

-Magnetism= force caused by moving charges
-Magnets= dipoles (always both N & S; no dipole discovered)
-like poles repel, opposites attract
-mag. domains = clusters of atoms
~Random domains = no net B (mag. field)
~Organized domains = had net B
-1 tesla (T) = N*s/C*m
-non-SI unit = 1 Gauss = 10-4 Tesla
*Bearth = 1/2 Gauss
-Mag. field lines point noth to south
-Density B= mag. flux
-FB= q(vXB)
-lFBl - qvBsinθ
For a particle affected by a FB, the radius of its circular path r = mv/qB

Lorentz Force:Ftot= Fe + FB = q(E + v x
For a particle traveling perpendicular to the E field, v = E/B

Current Carrying Wires in Mag. Field

FB= ∫I dl x B
**watch video for RHR, elec. motor and examples.

Mag. field for current carrying wire

B = μ0 I / 2πr
μ0 =4π x10-7

Max's 2nd Eqn AKA Gauss's Law for magnetism:

Φ (mag flux) ∫ B • dA = 0 ***note: integral over the CLOSED SURFACE

The Biot- Savart Law

dB = / 2πr (dl x r)
...This one is hard to understand without the vid, because it involves derivation with examples, and the solution changes with each situation.

Amphere's Law

You can skip this video if you've seen Walter's video lecture 15, as it's content is the same in both Fullerton & Lewin's versions.

∫ B • dl = μ0 Ipenetrating
Watch the video for elaboration with examples.
Also see the either video for information on a solenoid (slinky).

...I hope that was moderately helpful. If not, maybe I've at least convinced you to watch the videos. Good luck on the independent unit, everyone! Stay on top of things!

--AlphaGeek

I'm not sure if this is cliche, but I saw this on television once and thought it deserved a physics-rundown (It was a future weapons episode).

This bulletproof vest, called "Dragon Skin," is manufactured by Pinnacle Armor. It was designed for military use, though it failed Army inspection (the heat test: the vest was heated up to 170 degrees F and was shot at afterward. The clay material backing couldn't withstand the heat, and the design lost its overlapping shape. The integrity of the vest was lost, thus the vest was deemed unsafe). HOWEVER, despite this subtle detail, the vest's design is truely ingenious.

The overlapping-disk design distributes the impact of a bullet to multiple plates, whereas on a single plated vest the force is absorbed by only one plate. On the specific epistode of Future weapons where this armor is featured, it withstood a number of tests, including shots from an AK-47 and an M67 grenade. In the case of the grenade, even though the vest itself was ripped to shreds, the armor itself was still intact.

The vest was officially declared to provide "level 3 protection," which means that it can protect agains 9.6 g bullets traveling at 847 m/s, give or take a few m/s.

...For those of you with an interest in physics and no occupation to apply it to, the military is looking for creativity

--Alpha Geek
Ever wonder why studying hard or taking multiple AP tests in a row makes you exhausted, or even hungry? This is because when you think, your brain has to work hard to send "messages" through the neurons to different parts of the brain and body. Cellular respiration turns your food (glucose) into adenoside triphosphate, or ATP. This is the molecule that many body functions require to do work, such as facilitated diffusion, muscle movement, and yep, you guessed it-- thinking. The sodium ion pump that creates a gradient in the nerve cell interior allows for the inside of the cell to become positive. The combination of the ATP- using sodium ion gradient and the two diffusion- using potassion and sodium channels allows for the neuron to send an electric pulse down the axon. At the end of the cell, called the transmitter, the pulse allows ligands (or signal molecules) to continue the message to the next brain cell, which will send another signal until the message reaches its intended destination.

...Scientific evidence as to why proctors should let us return to our bags for food. Physics says we need it to think! Post below with your favorite brain food! Mine is a chocolate chip cliff bar, and I know a certain math geek's is a buffalo chicken wrap

Happy post-APs, everyone!

--Alpha Geek
Just thought we could benefit from some review on moment of inertia, because it was a pretty extensive topic and wasn't really mentioned in physics B. Not to mention that the variable is a different expression for each object.

The general form of the equation is I = ∑i miri² = ∫r² dm .

Below are the moment of inertia equations for a few different objects. If you have another object in mind to share, please do add it in the comments!

Isolid disc = 1/2 mr2
Icylinder about its axis = 1/2 mr2
Ihollow disk/hoop = mr2
Isolid sphere= 2/5 mr2
Ihollow sphere= 2/3 mr2
Irod about it's center= 1/12 ml2
Irod about it's end= 1/3 ml2

Though these shortcuts are great, make sure to know how do derive the moment of inertia of an object. For review, here's how to calculate the moment of inertia of a rod from it's end (Also in the textbook p 273 as well as in the notes packets).

The linear mass density (λ) = M/L, where M is the mass of the uniform rod with length L.

dm = M/L dx, or the mass density times the little wee bit of rod.

Using the general equation, we know I = ∫oLx2 dm, where x is the length of the rod from x=0 to x=L.
By substituting for dm, we then know I = ∫oLx2 (M/L) dx.
The constant comes out, leaving I = (M/L) ∫oLx2 dx.
And using calculus, we get I = (M/L) (1/3)x3 evaluated from L to 0, which leaves us with
I = (1/3) (M/L) (L3)
I= (1/3) ML​2

Note: If you need further assistance on this topic, the unit packet for Rotation (with the frog on a unicycle in it) and the packet titled "Chapter 6: Rotation" are useful. However, for visuals and more elaborate derivations, I recommend reading Tipler p. 272 and the pages following and/or watching this video again:

http://www.aplusphysics.com/courses/ap-c/videos/MomentOfInertia/MomentOfInertia.html

...Which I always find extremely helpful. I'll probably post another unit summary again, since our midterm is looming in the near future. Best of luck, all!
Soooo, because this is my last blog post for this year ( ), I thought it would be fitting to do a course reflection on the AP-C physics class this year. I thought I'd do it in a "bests-vs-worsts" top 5 format, kind of like you could find on collegeprowler.com when viewing different schools.

Top 5 Bests:

5.) Blog Posting [i thought this was really fun! I've never done anything like this before for a class. It brought up interesting physics applications and I thought it was fun to converse with classmates on the site ]
4.) Independent Units [As uncomfortable as I was at first, independent units forced me to manage my time, work harder than usual to learn the topic, and was great preparation for college. I feel like everyone sould experience this kind of a unit before graduating]
3.) Assigned practice problems from the readings [Assigned problems were REALLY helpful. I would've struggled a lot more than I did had I skipped doing the sample problems]
2.) Units with Lecture & book follow-up [This is my favorite way to learn things! The read-then-lecture method]
1.) VIDEOS <3 [Hands down the most helpful resource in Physics]

Top 5 Worsts:
...I think this is my biggest beef. I really don't have 5 things to complain about.
1.) Readings weren't assigned [When life gets busy in the middle of the year, especially with a number of APs, sports, etc., readings are the first thing to get cut out for me if they're not assigned. Confession: when the going got tough, I would often skim or not read. I reccomend assigning readings in the future. Kids will complain, but they'll thank you when they see better grades and their AP score.]

Overall, this was a successful year. A note to future students: This is by far the hardest AP course I've taken throughout high school. If you want to succeed, you must:

A.) Read the textbook and do some practice problems
B.) WATCH THE VIDEOS. Whether you're confused or simply want review, these are soooo outrageously helpful. It's like being in class a second time, except in 15 minutes or less instead of 42. Plus, you can skip over any sections that you feel you know solid.
C.) REVIEW THE EQUATIONS AND FREE RESPONSE BEFORE THE AP. I went through most of the E&M free response questions as well as both E&M and mechanics equations before the exam. KNOW THE EQUATIONS! I swear equations and key concepts are the majority of the test when it comes to the multiple choice Qs.

Any favorite parts of the year? Things you wanted to change? Post below with your opinion!

...I can't believe we only have 1 more day of physics
It snowed a little again today, which put me in the mood for some winter-related physics. :snowman: Some of you may be familiar with the movie "National Lampoons Christmas Vacation," a very silly yet amusing film about the holiday antics of the Griswold family. During one scene, Clark Griswold takes his brother and the children to go sledding. He decided to spray the bottom of his sled with a kitchen lubricant, significantly decreasing the friction between his sled and the snow.

For those of you that have never seen this clip before, skip to 1:20 for the sled action (before that is all the brother talking, he's kind of loopy).

So how much does greasing up an object truly effect friction?

Between two metals (lets use two hunks of aluminum for example), the coefficient of friction is roughly 1.05 to 1.35. When greased however, mu drops down to .3, which is anywhere from a third to a fourth of the original coefficient. The same goes for the coefficient of friction between snow and Clark's steel sled. The coefficient of friction between snow and steel is roughly .1. The Griswolds were sledding at night, so if the snow turned to ice the coefficient would be remarkably lower: 0.015. Add some canola or olive oil spray to the mix, and friction would be extremely small.

In other words, next time you break out the toboggan for some serious sled races, make sure to pack the pam!

P.S. I didn't pull those numbers out of a hat, my main source is http://www.engineeringtoolbox.com/friction-coefficients-d_778.html. Thanks, google!
...Or in coloquial terms, "My stars, is that ctenophore exhibiting bioluminescence?"

You might think that's all glow, but there's more to this jelly's luster. Bioluminescence occurs when a living organism's cells emit light. Common examples include fireflies and angler fish, who use light to find mates and attract prey respectively. These organisms convert chemical energy into light energy, just as a human body would convert chemical energy (like glucose) into mechanical or heat energy.

The above jelly fish Mertensia ovum, also known as the Arctic Comb Jelly or Sea Nut, does emit a small amount of blue and purple light. However, those fancy-dancey rainbow colored adornments on its side are actually caused by-- yep, you guessed it-- thin film interference. Sound familiar (think AP-?

Like light being "bent" into it's different rays of color on an oil spill or in a rainbow, the jelly fish has eight columns of cilia that have a similar effect. Besides their fashionable apearence, the cilia columns or "comb rows" also allow the jellyfish to move as well as sense changes in it's surroundings as would a bug's antenna.

...On a less-sciencey note, I found this bubble very enjoyable.

--AlphaGeek :tyrannosaurus:
...With all of this electricity and magnetism boggling our minds, it's nice to be reminded of the importance of mechanics once in a while. And by that I mean the force of friction: Ff = (normal force)(mu). Believe it or not, this commonly viewed as weak force can add up. Take the above myth busters clip for example, when the friction in between the sheets of a phone book in between the pages of a second phone book make them extremely difficult to separate. Try 8,000lb of force and two tank's worth of difficult! One of the tricks that creates so much of the friction is that each individual page is interlocked, increasing the surfaces that oppose each other as well as the weight of each page upon the next. Enjoy the clip :3
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