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AlphaGeek

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  1. 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 [tex] before it and [/tex] 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 [tex] and [/tex] it looks like: [tex] /pi [/tex]


    I hope thats helpful! If you have any questions, Mr. Fullerton or I would be glad to help
  2. AlphaGeek

    Test Prep Splash

    By AlphaGeek,

    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: Steady State
    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

    RC Steady State:
    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!
  3. AlphaGeek
    ...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.

    [ATTACH=CONFIG]542[/ATTACH]
    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)
  4. AlphaGeek
    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 =
    Faraday's Law:
    =
    =

    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!
  5. AlphaGeek

    Military Physics

    By 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
  6. AlphaGeek
    *yawn* It's a beautiful Tuesday morning and you've awoken from camping in the jagged pass. You stow your tent into the key items pocket and continue on your trek to Lavaridge Town. You're on your merry way, thinking fondly of a dip in the hot springs, when the grass in front of you begins to rustle!


    Oh my, a Spoink appeared! Adrenaline pulses through your veins as you shout, "Go, McNugget!" (Mc.Nugget is none other than your lvl 98 torchic).

    You quickly break out your pokedex, which informs you that a spoink's tail can stretch up to .3 m from when it rests at equilibrium. You also remember hearing from a passing hiker that it's angular velocity between attacks is 6 rad/sec. Due to torchic's smaller size, it can only use scratch when spoink is closest to the ground. Asuming that torchic can strike as soon as you order him to attack, how soon after spoink starts oscillating from equilibrium position (moving up first, then down) should you tell torchic to use scratch?

    ...And for those of you who wonder why you didn't delete scratch for a cooler move, I think scratch is the bomb.
    Respond now, a cookie is on the line! (This one is larger than the one awarded from the first challenge )

    --The Geek
  7. AlphaGeek
    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
  8. AlphaGeek
    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
  9. AlphaGeek
    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

    Watch Ramesh Raskar's presentation below:

    http://www.ted.com/talks/ramesh_raskar_a_camera_that_takes_one_trillion_fra mes_per_second.html
  10. AlphaGeek

    Electric organ

    By 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!
  11. AlphaGeek
    Credit to Mr. Powlin (who read this last year about the same time) and Snopes.com, where I found this humorous commentary once again. For those of you who did not hear this last Christmas or those who want to get into the spirit of the physics-filled holiday season, I thought I'd post this up for a few giggles. Happy Holidays, all! :snowman:

    No known species of reindeer can fly. BUT there are 300,000species of living organisms yet to be classified, and while most of these areinsects and germs, this does not COMPLETELY rule out flying reindeer which onlySanta has ever seen.
    There are two billion children (persons under 18) in the world.BUT since Santa doesn't appear to handle the Muslim, Hindu, Jewish and Buddhistchildren, that reduces the workload to 15% of the total — 378 million according to Population ReferenceBureau. At an average (census) rate of 3.5children per household,that's 91.8 million homes. One presumes there's at leastone good child in each.
    Santa has 31 hours of Christmas to work with, thanks to thedifferent time zones and the rotation of the earth, assuming he travels east towest (which seems logical). This works out to 822.6visits per second.
    This is to say that for each Christian household with goodchildren, Santa has 1/1000th of a second to park, hop out of the sleigh, jumpdown the chimney, fill the stockings, distribute the remaining presents underthe tree, eat whatever snacks have been left, get back up the chimney, get backinto the sleigh and move on to the next house. Assuming that each of these 91.8 million stops are evenly distributed aroundthe earth (which, of course, we know to be false but for the purposes of ourcalculations we will accept), we are now talking about .78 miles per household, a total trip of 75½ million miles, not counting stops to do whatmost of us must do at least once every 31hours, plus feeding andetc.
    This means that Santa's sleigh is moving at 650 miles per second,3,000 times the speed of sound. For purposes of comparison, the fastestman-made vehicle on earth, the Ulysses space probe, moves at a poky27.4miles per second — a conventional reindeer can run, tops, 15 miles per hour.
    If every one of the 91.8 million homes with good children were toput out a single chocolate chip cookie and an 8ounce glass of 2% milk, the total calories (needless to sayother vitamins and minerals) would be approximately 225 calories (100 for the cookie, give or take, and125 for the milk, give or take). Multiplying the number of calories per houseby the number of homes (225 x 91.8 x 1000000), we get the total number ofcalories Santa consumes that night, which is 20,655,000,000 calories. To breakit down further, 1 pound is equal to 3500 calories. Dividing our total number of caloriesby the number of calories in a pound (20655000000/3500) and we get the numberof pounds Santa gains, 5901428.6, which is 2950.7tons.
    The payload on the sleigh adds another interesting element.Assuming that each child gets nothing more than a medium-sized lego set (twopounds), the sleigh is carrying 321,300 tons, not counting Santa, who isinvariably described as overweight. On land, conventional reindeer can pull nomore than 300 pounds.Even granting that "flying reindeer" (see above) could pull TEN TIMESthe normal amount, we cannot do the job with eight, or even nine. We need214,200 reindeer. This increases the payload (not even counting the weight ofthe sleigh) to 353,430 tons. Again, for comparison, this is four times theweight of the Queen Elizabeth.353,000 tons traveling at 650miles per second createsenormous air resistance — this willheat the reindeer up in the same fashion as spacecraft re-entering the earth's atmosphere. The lead pairof reindeer will absorb 14.3QUINTILLION joules ofenergy. Per second. Each.
    In short, they will burst into flame almost instantaneously,exposing the reindeer behind them, and create deafening sonic booms in theirwake. The entire reindeer team will be vaporized within 4.26 thousandths of a second. Santa, meanwhile, will besubjected to centrifugal forces 17,500.06 times greater than gravity. A250-pound Santa (which seems ludicrously slim) would be pinned to the back ofhis sleigh by 4,315,015 pounds of force.
    In conclusion: If Santa ever DID deliver presents on ChristmasEve, he's dead now.
  12. AlphaGeek
    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

  13. AlphaGeek
    How would I determine the drag coefficient of an organic shape, such as a blob of pudding or a chicken or a Looney Tunes character?

    I wanted to do a blog post on the terminal velocity of Wile E. Coyote falling off of a cliff. I went back into my notes and found the following equations:

    Air resistance = Fdrag = bv = cv2
    VT= (mg)/b
    V = VT ( 1 - e(-b/m) )

    Notice the constants, b and c. I turned to google, thinking that the constants would be relatively easy to find.

    It turns out, the equation for Vterminal is a little more complex than I thought.

    [ATTACH=CONFIG]532[/ATTACH]

    Finding V terminal involves the mass of the object, acceleration due to gravity, the density of the medium that the object is traveling through, the area effected, and, of course, a drag coefficient. In my quest to find the drag coefficient, I found that the coefficient is related to the shape of the affected surface area. The lower the drag coefficient, the more easily the object can move through the air. The following table helps illustrate this:

    [ATTACH=CONFIG]533[/ATTACH]

    That's fine and well if you're trying to find the terminal velocity of a UPS box falling from a cargo plane in air of know density, although there are a few complications in the Wile E. Coyote situation. My number one probelm is as follows: unless the furry critter assumes a fetal position and magically transforms his body into a perfect sphere, his coefficient is difficult to determine.

    Any suggestions? :dontknow)
  14. AlphaGeek

    Happy New Year!

    By AlphaGeek,

    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!
  15. AlphaGeek
    Have you ever wondered how trampolines work? Anything fun or worthwhile has physics behind it, so let’s take a peek at the gymnast’s best friend:

    [ATTACH=CONFIG]497[/ATTACH]

    I hope you all enjoy my art skills. Read it and weep. :victorious:

    The magic behind a trampoline can be explained in terms of energy. Let’s say that a child is bouncing up and down on the trampoline. When the child is at a maximum height, his/her potential energy due to gravity is at a maximum. Because PE= mgh, with acceleration due to gravity and mass constant, his/her PE is the greatest because height is at a maximum. However, their kinetic energy is at a minimum of 0 because the child has a velocity of zero and KE= (1/2) m v^2. When the child is in contact with the trampoline and is as low as he/she will travel, his/her PE due to gravity is now at a minimum of zero because the height is zero. However, at this point the child’s kinetic energy is greatest because the velocity at this point is at a maximum. In addition, the potential energy due to the trampoline’s springs is at a maximum. Uspring (potential energy of the spring) is greatest at this point because the displacement x of the spring is greatest at this point and Uspring = (1/2) k x^2. In other words, the spring is at its maximum stretch possible for the child and wants to return to its state of rest, so it sends the child back into the air.

    If that AP B review didn't click, try watching the specimen V. vulpes exploring this bouncy apparatus. (CAUTION: Video has sound. If you're in the school library, please adjust volume level accordingly before proceeding).





    Ah, discovery.

    --'Geek out!

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