# AlphaGeek

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## Blog Entries posted by AlphaGeek

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!
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

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
For those of you who don't know, there is a video section of the Aplus site that features videos of physics-y origin. You can get there by clicking the word "videos" on the top blue bar of the site.

http://aplusphysics.com/community/index.php/videos/view-340-vector-despicable-me/

When I first saw this video, it was floating among intense brain-teasing physics vids and real life examples of the science. I thought it deserved some defense for its place on the site, so let me explain what this despicable me mini clip has to do with physics.

The most notable physics-feature of the video is that the geeky character's name is Vector, as he explains both verbally and through body language. A vector quantity is a magnitude with direction. For example, velocity is a vector quantity. A velocity of 3 m/s to the right has both units (meters per second) and direction (to the right). 3 m/s alone, a speed, is not a vector quantity because even though it has units, it does not have a direction. We call this a scalar quantity.

I hope that explains Vectors joke, "I'm committing crimes with both direction and magnitude!" If he were the evil Dr. Scalar, it would only have magnitude. Haha! Ha. Ha... Ha.

...And I didn't notice this before, but when Vector first comes into the scene he crosses his arms while doing the "vulcan salute," which is actually the nerdfighter salute (You know! Vlogbrothers on youtube). I thought that was really cool. I wonder if it wasn't even supposed to be there in the first place, but some nerdy producer put it in

Not familiar with vlogbrothers? Do acquaint yourself via nerd humor:

...Just for the record, my favorite part of the movie is as follows:

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.
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 ]

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
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
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
Having trouble on the 4 minute drill? Need to consolidate your thoughts for the Mechanics part of the AP-C exam? Have no fear! I've sifted through my notes to find a good portion of the mechanics equations. If you find anything missing/incorrect, PLEASE give feedback in the comments section! I'll edit the changes in ASAP. Thank you

MECHANICS

Vectors etc.

A B = lAl lBl cos Ө
A x B = - (B x A)
lA x Bl = lAl lBl sin Ө

Kinematics

V= Vo+ at
Δx = Vo t + (1/2) a t2
V2 = Vo2 +2aΔx

Δx = ∫ v(t) dt
Δv= ∫ a(t) dt
Vavg = Δx/Δt = (Xf - Xo)/(tf - to)
V= dx/dt
a= dv/dt

UCM/gravitation

Fc = mv2/ r
ac = v2/r = w2/r

Fg= GMm/r2
Ug= -GMm/r

Rotational Motion

S= Өr
v= wr
a= αr

w= ΔӨ/Δt
ac= w2r
Vlinear = 2πr/t

parallel axis theorem: I = Io +md2

KEroll= (1/2)Icmw2 + (1/2)mv2

Angular momentum (L) = r x p = mvrsinӨ = mwr2sinӨ
dL/dt = Torque
L= w I
K= (1/2) I w2

Moment of Inertia

I = Σmi(ri)2 = ∫r2dm
Isolid disk = (1/2) mR2 (also works for a cylinder about its axis)
Ihoop = mR2
Isolid sphere = (2/5)mR2
Ihollow sphere = (2/3)mR2

Torque (T)

x >> Ө
v >> w
a >> α
m >> I
F >> T

ΣF = ma >> ΣT = Iα
T = rxF = rFsinӨ

Center of Mass

rcm = Σmr/Σm = ∫r dm / Σm
Xcm = (m1x1 + m2x2)/ (m1+m2)

Drag force

Fd = bv = cv2
VT = mg/b
V = VT(1 - e(-b/m)t)= (mg/b)(1 - e(-b/m)t)

Friction
Ff= μ Fn

...As to not get long and confusing, I'll make another blog post with all of the Electricity & Magnetism equations that I have in it. Check that post out, too!

--Alpha Geek
Part 2 of the equation posts: E&M. Again, if you see any mistakes or have a few equations to add, make sure to utilize the comment section! I'll add it in right away.

Electrostatics

E= Fe/q = kq/r

λ = Q/L
ρ = Q/V
σ = Q/A

Electric potential

Ue = kq1q2/r
F = -dU/dl
V = k ∑ qi/ri = W/q
∆V= Vb - Va = ∫ab E dl = ∆U/q

Gauss's Law:

Conductors

Esurface=

Vinside =

Einside =

Capacitor

C=Q/V =

Uc =

Ue = field energy density =

Energy = V/d

C= w/ = Dielectric constant

Circuits

I =

I = V/R

I = NqVdA = NeVdA

Current density (J) = NqVd = I/A

charging up: w = I2V
charging down: U = CV2

= RC
5 = 99% charged/discharged

Resistance

R=
E=
P=IV
E= J
W= qv

Series Circuit

Ceq =

I= constant
V=IR
Req= R1+R2+...
Q=CV

Parallel Circuit

Ceq = C1 + C2 + ...
I = V/R
V= constant

Q=CV

Batteries:

Videal = V = mf = VT
V with resistance = Iri = VT
Pbattery = W/t = = I
Pexternal resistor = I2R
Pinternal resistor = I2ri

Magnetism

Gauss's Law: dA = O
Amphere's Law: Ipen
...for a wire of radius R, B =
Biot-Savart law: dB = (dl X r)
...for a loop of wire, B =
=
=

solenoid: B =
toroid: B =

Mag. moment () =NIA =NIR2
Mag. torque=

...sorry that took so long to post up, jeez that code takes a while to type up ^-^ Feel free to add/correct in the comments section!
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
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
Hi everybody!

I haven't done a cookie problem in a while, so here it goes! The problem is related to the current unit. First correct answer gets a cookie. I think Charlie is the only one who answers these things, but I enjoy writing them and he likes cookies, so... It all works out :glee:

Slinky the dog is bored (since Andy is off at college and all), so he decides to watch Walter Lewin's video Lecture 15. Slink thinks the solenoid example is really cool and decides to try it out himself. If he hooks his middle into a circuit of 1A and stands with his front feet .5 m away from his back feet, what is the magnetic field inside of Slinky? Note: Slinky's center is composed of 100 turns, and each turn is uniformly spaced.

I love Disney! Bonus question: What is the name of the arcade in Toy Story?

Have fun!

--Alpha Geek
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
Recently, a friend has confessed to me that he has been diagnosed with stage one senioritis. We've all heard of this virus: common symptoms include drowsiness, in-class headaches, increased social tendencies, and worst of all, characteristic decreases in effort and GPA. Though some have better immune systems than others, this sickness is in fact contagious and most seniors contract a mild case. Because knowledge is the number one prevention factor, I intend to explain--using science and graphical representation-- what is known about this common yet dangerous disease. For those of you who seek protection (at least until the end of AP week), please read on.

Diagnosis: How do I know if I have Senioritis?

Illnesses ending in -itis indicate irritation and inflammation. Senioritis specifically refers to inflammation of the "give-a-care" gland, and inflammation seems to increase as the temperature rises outdoors. Senioritis is most common in ages 16-18, however some people are simply born with it. In this case, the illness is refered to as "chronic procrastination," an entirely different animal.

Though little is known about the causes of Senioritis, there are key variables that contribute to the intensity of the illness. Inflammation level of the GAC gland (I) is directly proportional to t, the amount of time (in hours) left until the end of the year. It can be represented by the equation

I = (2 π t2 P h) /f3

where
P= the constant of procrastination. This constant varies, dependent upon personality type.
t = time (in hours) left until graduation
f= the number of friends infected
h= the amount of homework (in kg) the student is assigned each night.

For a student with a moderate course load and average amount of friends, the constant of procrastination tends to triple after AP week due to a weaker mentality.

Observe the below graph exhibiting the relationship between academic wellness over time. The decreasing trends are due to GAC flareups, a common side-effect of senioritis. Students with a high P constant are especially susceptible to the virus. Note how in students with relatively high tendencies of procrastination, the AP period provides a brief spike of academic rigorousness, followed by a devastating relapse. We call this spike of functionality "cramming."

[ATTACH=CONFIG]618[/ATTACH]

Cures and Coping Techniques:

​1.) Senioritis is highly contagious, like influenza or ring worm. Try surrounding yourself by people with relatively low P constants to avoid infection.

2.) Create mini-deadlines for assignments as well as allotted time to study. Handling work in small bits reduces the chance of GAC flareups

3.) Wash your hands before eating finger food.

4.) Try self-medication: remind yourself that senior year is almost over, and in order to do well on APs you'll only have to fight the -itis a little longer. Stay strong, it's the final stretch!

Bueno suerte!
--Alpha Geek
...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:
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

...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
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!
Hi everyone, just figured that I'd post an accumulation of what I've been studying for the test tomorrow morning. It goes in video order because that's the order that I learned the material in. If something is too vague, I reccomed looking at the video for elaboration

Circuits

Current and Current Density
Resistors and Resistance
Circuits
Voltmeters and Ammeters
Ideal and Real Batteries
RC Circuits: Transient Analysis (Charging)
Current and Current Density:

Current measured in Amps, or charge per sec.
An electric field is applied to a conductor and a small field is created that opposes it
Avg. Velocity of electrons in that field= drift velocity (Vd)
Vol= Bh = Vd * change in time * Area
# electrons = Volume * volume density(AKA "N")= N *change in time* Vd * Area
I= N*q*Vd *A
Current density = J* N*q*Vd
I= integral (J * dA)
J= I/A

Resistance ("Howdy, Y'all!" made my day in that vid, btw)
R=V/I
p = row =resistivity
R= pL/A
V= IpL/A
E=energy=V/L = p (I/A) = pJ
W=qV
I= dQ/dt
P=IV=I2R=V2/R

Circuits:

Series: Req= R1 +R2...
Parallel: 1/Req= 1/R1 +1/R2....
Kirchhoff's Current Law= sum all current entering = sum all current exiting (conserv. charge)
Kirchhoff's Voltage Law= sum of all potential drops in a closed loop of a circuit = 0 (conserv. energy)

Voltmeters and Ammeters:

Voltmeters: measures v between two points, high resistance, connected in parallel
Ammeters: measures current, low resistance, connected in series

Ideal and Real Batteries:

Ideal: no internal resistance
V battery = Emf = change in V

Real: has internal resistance
V battery = change in V = IR = Emf- (I)®, where r = internal resistance

For battery:
W= change in Q * Emf
P= W/change in time = (change in Q)(Emf)/(change in time)
P resistor (works for both external and internal) = I2R

series: 1/Ceq= 1/C1 + 1/C2 ...
Parallel: Ceq = C1+C2...

RC Charging:
W= I2R
U=1/2 C V2

Time constant Tao=RC, occurs when quantity is 63% of its final value. 5 Tao= 99% final value (practically final value)

Note: an uncharged capacitor acts like a wire, a charged capacitor acts like a gap in the circuit (AKA no current)

...Okay, bed time. Good luck tomorrow, everyone!
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!
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
Often times, values in physics are abbreviated using metric prefixes, or SI prefixes. I found this table the other night and thought it would be helpful to post, in that I'm sure I'm not the only one who gets these mixed up sometimes.

Thanks to wiki for this table:

[TABLE="class: wikitable, width: 0"]
[TR]
[TH="bgcolor: #CCCCFF, colspan: 2"]Metric prefixes[/TH]
[/TR]
[TR]
[TD][TABLE]
[TR]
[TH="bgcolor: #EEDDFF"]Prefix[/TH]
[TH="bgcolor: #EEDDFF"]Symbol[/TH]
[TH="bgcolor: #EEDDFF"]1000m[/TH]
[TH="bgcolor: #EEDDFF"]10n[/TH]
[TH="bgcolor: #EEDDFF"]Decimal[/TH]
[TH="bgcolor: #EEDDFF"]Short scale[/TH]
[TH="bgcolor: #EEDDFF"]Long scale[/TH]
[TH="bgcolor: #EEDDFF"]Since[n 1][/TH]
[/TR]
[TR]
[TD]yotta[/TD]
[TD="align: center"]Y[/TD]
[TD]10008[/TD]
[TD]1024[/TD]
[TD="align: right"]1000000000000000000000000[/TD]
[TD]septillion[/TD]
[TD]1991[/TD]
[/TR]
[TR]
[TD]zetta[/TD]
[TD="align: center"]Z[/TD]
[TD]10007[/TD]
[TD]1021[/TD]
[TD="align: right"]1000000000000000000000[/TD]
[TD]sextillion[/TD]
[TD]trilliard[/TD]
[TD]1991[/TD]
[/TR]
[TR]
[TD]exa[/TD]
[TD="align: center"]E[/TD]
[TD]10006[/TD]
[TD]1018[/TD]
[TD="align: right"]1000000000000000000[/TD]
[TD]quintillion[/TD]
[TD]trillion[/TD]
[TD]1975[/TD]
[/TR]
[TR]
[TD]peta[/TD]
[TD="align: center"]P[/TD]
[TD]10005[/TD]
[TD]1015[/TD]
[TD="align: right"]1000000000000000[/TD]
[TD]billiard[/TD]
[TD]1975[/TD]
[/TR]
[TR]
[TD]tera[/TD]
[TD="align: center"]T[/TD]
[TD]10004[/TD]
[TD]1012[/TD]
[TD="align: right"]1000000000000[/TD]
[TD]trillion[/TD]
[TD]billion[/TD]
[TD]1960[/TD]
[/TR]
[TR]
[TD]giga[/TD]
[TD="align: center"]G[/TD]
[TD]10003[/TD]
[TD]109[/TD]
[TD="align: right"]1000000000[/TD]
[TD]billion[/TD]
[TD]milliard[/TD]
[TD]1960[/TD]
[/TR]
[TR]
[TD]mega[/TD]
[TD="align: center"]M[/TD]
[TD]10002[/TD]
[TD]106[/TD]
[TD="align: right"]1000000[/TD]
[TD="colspan: 2, align: center"]million[/TD]
[TD]1960[/TD]
[/TR]
[TR]
[TD]kilo[/TD]
[TD="align: center"]k[/TD]
[TD]10001[/TD]
[TD]103[/TD]
[TD="align: right"]1000[/TD]
[TD="colspan: 2, align: center"]thousand[/TD]
[TD]1795[/TD]
[/TR]
[TR]
[TD]hecto[/TD]
[TD="align: center"]h[/TD]
[TD]10002/3[/TD]
[TD]102[/TD]
[TD="align: right"]100[/TD]
[TD="colspan: 2, align: center"]hundred[/TD]
[TD]1795[/TD]
[/TR]
[TR]
[TD]deca[/TD]
[TD="align: center"]da[/TD]
[TD]10001/3[/TD]
[TD]101[/TD]
[TD="align: right"]10[/TD]
[TD="colspan: 2, align: center"]ten[/TD]
[TD]1795[/TD]
[/TR]
[TR="bgcolor: #EEEEEE"]
[TD="colspan: 2"][/TD]
[TD]10000[/TD]
[TD]100[/TD]
[TD="align: center"]1[/TD]
[TD="colspan: 2, align: center"]one[/TD]
[TD]–[/TD]
[/TR]
[TR]
[TD]deci[/TD]
[TD="align: center"]d[/TD]
[TD]1000−1/3[/TD]
[TD]10−1[/TD]
[TD="align: left"]0.1[/TD]
[TD="colspan: 2, align: center"]tenth[/TD]
[TD]1795[/TD]
[/TR]
[TR]
[TD]centi[/TD]
[TD="align: center"]c[/TD]
[TD]1000−2/3[/TD]
[TD]10−2[/TD]
[TD="align: left"]0.01[/TD]
[TD="colspan: 2, align: center"]hundredth[/TD]
[TD]1795[/TD]
[/TR]
[TR]
[TD]milli[/TD]
[TD="align: center"]m[/TD]
[TD]1000−1[/TD]
[TD]10−3[/TD]
[TD="align: left"]0.001[/TD]
[TD="colspan: 2, align: center"]thousandth[/TD]
[TD]1795[/TD]
[/TR]
[TR]
[TD]micro[/TD]
[TD="align: center"]μ[/TD]
[TD]1000−2[/TD]
[TD]10−6[/TD]
[TD="align: left"]0.000001[/TD]
[TD="colspan: 2, align: center"]millionth[/TD]
[TD]1960[/TD]
[/TR]
[TR]
[TD]nano[/TD]
[TD="align: center"]n[/TD]
[TD]1000−3[/TD]
[TD]10−9[/TD]
[TD="align: left"]0.000000001[/TD]
[TD]billionth[/TD]
[TD]milliardth[/TD]
[TD]1960[/TD]
[/TR]
[TR]
[TD]pico[/TD]
[TD="align: center"]p[/TD]
[TD]1000−4[/TD]
[TD]10−12[/TD]
[TD="align: left"]0.000000000001[/TD]
[TD]trillionth[/TD]
[TD]billionth[/TD]
[TD]1960[/TD]
[/TR]
[TR]
[TD]femto[/TD]
[TD="align: center"]f[/TD]
[TD]1000−5[/TD]
[TD]10−15[/TD]
[TD="align: left"]0.000000000000001[/TD]
[TD]billiardth[/TD]
[TD]1964[/TD]
[/TR]
[TR]
[TD]atto[/TD]
[TD="align: center"]a[/TD]
[TD]1000−6[/TD]
[TD]10−18[/TD]
[TD="align: left"]0.000000000000000001[/TD]
[TD]quintillionth[/TD]
[TD]trillionth[/TD]
[TD]1964[/TD]
[/TR]
[TR]
[TD]zepto[/TD]
[TD="align: center"]z[/TD]
[TD]1000−7[/TD]
[TD]10−21[/TD]
[TD="align: left"]0.000000000000000000001[/TD]
[TD]sextillionth[/TD]
[TD]trilliardth[/TD]
[TD]1991[/TD]
[/TR]
[TR]
[TD]yocto[/TD]
[TD="align: center"]y[/TD]
[TD]1000−8[/TD]
[TD]10−24[/TD]
[TD="align: left"]0.000000000000000000000001[/TD]
[TD]septillionth[/TD]
[TD]1991[/TD]
[/TR]
[TR]
[TD="bgcolor: #EEEEEE, colspan: 8"]

^ The metric system was introduced in 1795 with six prefixes. The other dates relate to recognition by a resolution of the CGPM.

[/TD]
[/TR]
[/TABLE]
[/TD]
[/TR]
[/TABLE]

These prefixes were developed to shorten extremely large or small values, such as the teeny tiny mass of an electron (9.11 E -31 kg, or .000911 yg) in contrast to the very large Avogadro's number (6.022E23 atoms, or .6022 yotta atoms in one mole). When tacked onto constants, sometimes we don't realize just how intense these prefixes really are. Here are a few examples to help grasp the largeness and smallness of these constants:

1. The Earth weighs 5,972 yotta grams, or 5.972E24 kg. It would take over 850 quintillion elephants to match this weight, or just over 81 moons.

2. The average mass of a human cell is 950 femto grams, or 9.5E-13 g. If you cut a penny into a trillion pieces of equal mass, the human cell would still have a lower mass than the penny bit. AND you would get arrested for defacing US currency. It's simply a lose-lose situation.

3. On earth, there are an estimated 7.059 G people (or 7.059 billion people). This is roughly 1000 times the number of pigeons in NYC. However, this is roughly 1/3 of the amount of hotdogs Americans consume in a year. (Yes really-- Americans chow down on an estimated 20 billion a year. That's about 70 hot dogs per person).

Hope that was enlightening if not helpful!

--Alpha Geek
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!
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