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DavidStack

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  1. DavidStack
    After spending about two hours in a hot tub the other night and therefore having excessively pruney hands, the question that I've always been curious about came to mind: why does our skin get pruney when it's been under water for a long time? I looked up some things, and discovered that at first, scientists believed that it was simply due to the different layers of skin we have. The outermost layer of the outermost layer of our skin has cells that are filled with keratin, a protein that keeps your skin hydrated by absorbing a lot of water. When underwater, the keratin absorbs a lot of water, expanding the cells and thus expanding the size of the skin layer. Since the second outermost layer doesn't expand, the outermost layer has to fold in order to adjust to the new size difference. This made perfect sense until it was discovered that people with severed nerves don't get pruney, which leads to the claim that our nerves "tell" our skin to get pruney. When our nerves sense that our skin is underwater for a long time, information is transferred along the neural pathways to "fold" our outermost layer of skin in order to increase the mew of our skin and thus increase the force of friction between our hand and whatever we touch (since F(f) = (mew)F(n)), thus giving us better grip in order to fight against the now slipperiness that the water on our hands creates. It's crazy how intelligently our bodies were created!
  2. DavidStack

    Final Blog Post

    By DavidStack,

    Well this is kind of bittersweet, finally being done with blog posts but also realizing that high school is completely over, as is Physics C with a fantastic teacher. I've learned so much during the year, from angular analogs to retarding forces to induction to the sheer brilliance of Walter Lewin's ability to draw a dotted line; it's been quite a year. I've appreciated this blog posts as much as I've hated them, mostly because they forced me to truly learn the stuff that I write about. And now when I struggle with physics in college, I'll always be able to go back to APlusPhysics and ask for help. I encourage anyone and everyone to take Physics C - it's certainly challenging but I can't see how you could regret it. So, farewell APlusPhysics, I'll likely come crawling back in no time at all.
  3. DavidStack

    Hot Rod

    By DavidStack,

    Hot Rod, arguably the greatest movie ever created, actually has quite a bit of physics incorported into it. The part I will focus on is when Rod fails miserably to jump the local pool. Barely making it halfway, Rod slowly spins forward while in the air and lands face first, bike and all, into the pool. His demise results from two things - lack of kinetic energy and conservation of angular momentum. While he did have a ramp leading up to the jump, it was not nearly big enough to clear the pool. Thus, his moped could not gain enough speed, which meant that he didn't have enough kinetic energy to convert to potential energy to reach the height he needed to reach. Not only so, but while taking off the jump, Rod leaned forwards. Since he couldn't control his angular momentum while suspended in the air, that initial momentum continued while he was in the air, leading him to rotate forward ever so slowly and land on his face. All in all, Rod clearly didn't understand the physics needed to clear this jump. However, he made the movie hilarious, which is all that matters I guess.
  4. DavidStack
    As I go off to Tufts University in the fall, one of the things that I'm looking most forward to is joining the qudditch team, where I will be a chaser. One of the tuft-est (see what I did there?) things about being a chaser is that one hand has to hold the broom while you run, meaning that you have to catch the ball with solely the other hand. This is difficult for two reasons: 1) The force felt from the ball is directed onto one hand instead of two, so the force is spead across a smaller plane, making it more difficult to control the force. And 2) With two hands you can catch the ball on its sides, forcing the ball to naturally slow down over a longer period of time, thus decreasing the force felt. But with one hand, you have to find a balance between securing the ball and moving your hand with the ball in order to increase the period of time of the ball slowing down. This is a tricky maneuver and can easily lead to one dropping the ball. So even though quidditch can be difficult, I'm very excited to play.
  5. DavidStack
    So apparently there's more to dropping a ball than just gravity... who would have thought?! Well, for starters, when the ball is above the ground it has potential energy, due to the equation U = mgh. (See? Gravity is key!) As the ball comes closer and closer to the ground though, that potential energy is steadily converted to kinetic energy in the form of velocity (k = .5mv^2). Since m is in both equations, the mass of the object does not affect how fast the ball falls nor the time it takes the ball to fall. HOWEVER, an important thing we learned in Physics C this year is that not all of the potential energy is converted to kinetic energy, due to the fact that a drag force acts against the object falling. This drag force creates friction, which heats up the object, and that heat accounts for the "lost" energy. So that is the physics of dropping a ball, although, as I previously stated, it can really be summed up by this: gravity.
  6. DavidStack
    1) Don't worry about the time, it will just make you work slower.
    2) If Mr. Fullerton says it's going to be on the AP, it's probably going to be on the AP.
    3) Since the AP changes every year, test taking strategies can often come in handy more than trying to hammer in every single thing we ever learned in the entire year.
  8. DavidStack
    There are a few things that come into the physics of punching something. First off, impulse plays a huge role in punching somehting. Obviously as you punch something such as a wall or a person you will experience an impulse as you have a change in momentum. Therefore the thing that you are punching will feel the force of the punch as well as the impulse delivered from the punch. Due to Newton's second law you, the puncher, will also feel a force driving backwards in your direction as every reaction has an equal and opposite reaction. This describes the physics of punching something with your fist.
  9. DavidStack
    There's more to the knowledge I've gained from the Kerbal Space Program. First of all I couldn't get the screen shot to work; I guess the F1 key doesn't like me. And yes, I did just use a semi-colon outside of English class. But another very important thing I've learned through the Kerbal Space Program is how to correctly get a lot of power to your rocket. While I previously thought that either putting one engine at the bottom of stack of fuel tanks (the engine doesn't have enough power to lift up that much weight) or that having layer upon layer upon layer of fuel tanks and engines (the rocket becomes too wobbly and essentially combusts) were good ideas, I soon discovered that two separate layers of fuel tanks and engines, one on top of the other, is a great combination since it makes the rocket not too wide so that it flies straight but also gives the rocket enough power to get off the ground and eventually orbit in space.
  10. DavidStack

    Bottle Rocket

    By DavidStack,

    To add to the benefits of the Kerbal space program listed in the previous post, it helped us with the bottle rocket mostly in terms of simplicity. Instead of trying to make a giant rocket with parachutes or other complicated things, we decided to go with a simple rocket because we learned the importance of aerodynamics and weight balance through the Kerbal space program. My group realized that all we really needed was the two liter bottle, fins, a nose cone, and some weight on the top of the rocket in order to help it fly straight and keep the center of mass towards the top of the rocket, while the drag force was towards the bottom of the rocket. Even though our rocket wasn't flashy or put together incredibly precisely, it had the basic components to fly for over 5 seconds, which I consider to be a success.
  11. DavidStack

    Kerbal Project

    By DavidStack,

    After spending quite some time with the Kerbal project, I am starting to understand how the program works. Building the biggest rocket possible doesn't really work; you need a balance between fuel level and weight, aerodynamics, the necessary seperators and parachutes, and a knowledge of when to hold off on burning fuel, the SAS, and other things like that. Thus, our first couple launches were not as successful as we would have hoped, but as we learned to decrease the drag force by improving the rocket's aerodynamics and not just stack fuel tank on fuel tank, which decreased the engine power by making the rocket too heavy, we started having successful launches. While the game is definitely interesting and I have learned a decent amount about space exploration, I wouldn't mind if there were other options for an end of the year project, given that I don't necessarily love video games and there are topics that I would love to explore, especially things involving engineering. So to recap, the program is a great idea for an end of the year project, but other options would be much appreciated.
  12. DavidStack
    As my last blog post of the quarter, I think the only reasonable thing to do is get sentimental (at least as sentimental as you can in a physics blog post) and think of the ups and downs of this quarter. This quarter was mostly independent units, which was good in the fact that it helped prepare us for college but also made me realize that I need to change a lot of my study habits - or lack there of. I also have discovered that E&M comes a lot less naturally than mechanics did, so studying for the E&M AP could take a good deal of time. And even though I thought I could stay up to date on my blog posts after the first quarter, I failed that once again, so hopefully the fourth quarter can finally be the quarter of success. Thus, I have learned that more work on physics outside of class would be benefitial, as well as staying more focused during work time in class. Fourth quarter here I come!
  13. DavidStack

    Einstein's Hair

    By DavidStack,

    Although Albert Einstein's mess of hair was most likely due to the fact that he rarely slept well because of the equations constantly rattling through his brain and that he really didn't care what he looked like since he was too busy making history, electricity also plays into effect.


    Due to the likely rolling around that Einstein did at night, his hair felt a lot of friction from his pillow case and the sheets on the bed. This frictional force led to the passing of electrons from the sheets to his hair, and since like charges repel each other, the electron build up in his hair led to his hairs repelling each other and standing up on end. And with a known charge of electrons, the repelling force between his strands of hair could actually be calculated with the equation F = kqq/r^2. Someone needed to get that man a comb!
  14. DavidStack

    My Cookie Problem

    By DavidStack,

    Alright Charlie, I'm writing a cookie problem just for you. You might need to get the cookie from Liz, but I figured I'd write one since you said there haven't been enough. And continuing with the Disney theme, here goes: Mulan is curious about how current and magnetic field interact, so she inserts her charge filled sword (which temporarily acts like a wire) of length 1 m into a magnetic field of 2 T in the postitive i direction, creating a force a 2.5 N in the positive k direction.

    1) What is the direction and magnitude of the current?
    2) What is the funniest movie David has ever seen? (You need to answer this correctly in order to get the cookie.)
  15. DavidStack
    Reiterating Charlie's most recent blog post, this independent unit has certainly seemed more difficult conceptually than the previous independent unit. As soon as I thought I understood something in the unit, another curve ball was thrown at me. Luckily, I discovered that Ampere's law and the Bio-Savart law are extremely helpful and applicable in this unit, and the right hand rules and simple force equations (like F=q(v x and F = I(B x L)) are easier this year because of the practice we had with them last year. Thus, knowing those equations and laws should help greatly on the test, as well as being able to apply things from previous units. It is pretty interesting when mechanics, electricity, and magnetism all come together in a problem as centripital force is equal to the force of two charges attracting/repelling each other which is equal to the force due to the magnetic field. Making the connections between the two halves of the year is becoming easier and it is cool to think that famous theories are created by doing what we are doing - making comparisons to two different fields and seeing how they are related. So all in all, while this is a more difficult independent unit, it is certainly fascinating as well.
  16. DavidStack
    Looking at the stationary bike that my dad bought for my mom for too much money, I realized that all these bikes do is take a normal bike and add friction to it in order to give the feeling like you are actually riding a bike. The friction of some material on the bike tire requires work to overcome it, and since W=Fd, the work required is the frictional force times the distance traveled, so you burn however many calories the work you did is equivalent to. Therefore, you can create one of the machines yourself very cheaply. You can create a simple frame using 2x4's, making a base and two A frames that the bike frame can rest into. Then take whatever bike you use during the summer time and attach a cork or something of the sort to the back of the frame that can put pressure on the back wheel and shabam! You have a beautiful exercise bike for your living room like the one below.

  17. DavidStack
    Although I personally believe that the Nintendo 64 is the greatest game system ever, playing Mario Tennis and my understanding of physics has led me to realize that a big reason why "better" game systems have been created is the lack of realism in the physics world in games such as Mario Tennis. The game creators didn't exactly take momentum into account given the fact that the ball is only hit with 4 speeds with 4 shots - a top spin, a slice, a lob, and a smash. In the real game of tennis, players derive much of the speed on their shot from the speed of their opponents shot. By shortening their swing, they rely less on building power through their wind up and instead get power by redirecting the power of the shot they are returning. Thus, the ball speed often increases during the point as more and more momentum is redirected. In Mario Tennis, on the other hand, the button you press is the only thing that determines the ball speed - top spin shots do not increase in speed during points. All in all, even though Mario Tennis is extremely entertaining, physicists can see the clear lack of depth of understanding by the creators of momentum and the transfer of kinetic energy.

  18. DavidStack

    Tennis Serve

    By DavidStack,

    I recently played a very poor serving tennis match and sit here thinking about why my serve was and often is so inconsistent, realizing that it comes mostly from my toss. The racket should contact the ball when the hitting arm is fully extended, but I often toss the ball short and contact the ball while my arm is still bent. By hitting the ball at the highest possible point, I maximize power and accuracy - the ball is at its maximum potential energy so more kinetic energy results when it is converted, and a higher height above the net means that the ball can be hit at a greater range of angles and still land in the service box. With a higher toss I can improve the consistency and pop of my serve so that I'll be like this guy :glee: and not this guy :banghead).
  19. DavidStack

    Mind Teaser

    By DavidStack,

    In my computers class we looked up mind teasers on mindcipher.com in order to improve our problem solving abilities, but problem solving is certainly applicable to physics and this question was relatively easy but got your brain working a little bit:
    You need to tell time for 30 seconds but all you have is a non homogenous rope (some parts burn faster than others) that you know burns for 60 seconds and a match. How do you tell time for 30 seconds?

    And if that one is too easy:
    It's said that a number N with 4 digits is a double-square number when it equals the sum of the squares of two numbers: one formed by the first two digits of N, in the order they appear in N and the other formed by the two last digits of N in the order they appear in N.For example, 1233 is a double-square number since 1233 = 12^2 + 33^2. Find another double-square number.

    Enjoy!
  20. DavidStack

    Muscle Memory

    By DavidStack,

    The brain is an incredible thing and is refered to as the world's smartest computer for a reason. I've recently grown to love muscle memory as it helped me greatly in performing in a musical this past weekend. Even though I would be in the middle of a dance number and completely forget the next part of the dance, my body would do the moves for me without me even thinking about it. This is because of muscle memory, as our bodies build neural pathways after doing a certain activity over and over again so that when presented in a situation, we naturally do what we've done so many times. Even though I would feel like I didn't know what to do, the procedual memory of doing certain moves on the stage on cue with certain music and lights led to me somehow almost always doing the right thing. I also just realized that this doesn't necessarily have to do with physics, but I still think it's a really cool topic, and now I can dance like this guy.

  21. DavidStack
    Many people do not enjoy plane rides because of the uncomfortable feeling of their ears popping as the plane rises. This has to do with air pressure, a concept that is not really covered in AP-C Physics but we did deal with last year and is certainly important in understanding the general physics around us. As the airplane increases in altitude, the air becomes less dense (since less air is being pushed down by gravity), leading to a decrease in air pressure. Because of this, the air trapped in your inner ear will cause your eardrums to push outward, causing a discomfort. To compensate for this, your body naturally allows some air from your inner ear to escape through the Eustachian tubes, two small channels that connect the inner ears to the throat, creating the "pop" of your ears. You likely had no interest in this, or you were already aware of why your ears pop, but thanks for reading anyways!
  22. DavidStack
    So, we've lasted half the year in Physics, and what better time than now to discuss how I'm feeling about this class?
    Ups:
    1) Physics C and BC Calc go well together. As I learn more in one class, it helps me understand something better in the other class.
    2) It's interesting to learn more real world physics, such as air drag and taking friction into account, instead of learning the mechanics of a perfect world that doesn't exist.
    3) Doing the same topics as last year, like momentum and energy and kinematics, has helped me to gain a better understanding of those concepts.

    Downs:
    1) Rotational mechanics makes energy and pulleys much more complicated. Before, it was very easy to find the kinetic energy of a ball rolling down a hill or the acceleration of an pulley with a massed string (when we were allowed to ignore things we no longer are allowed to ignore), but there are more components now.
    2) I have to study. In Physics B, I could do little to no work prior to taking a test and still do very well, but that is not the case now.
    3) Webassigns are longer and more difficult.

    Even though physics is definitely more challenging this year, I've made it this far with very few bumps and bruises, so the rest of the year can't be too bad (knock on wood). E and M here I come!
    I'd also like to give a shout out to Slim Shady/Mr. Jericho/Mr. Ditty - you've done a fabulous job looking important while Mr. Fullerton teaches, and we're really gonna miss you. Visit soon!
  23. DavidStack
    I have always failed at writing down 50 equations in 4 minutes, both last year and this year, and I was never really sure why because I do know a good deal of equations. But as I think about it, I usually try to think of every little equation - getting me flustered and slowing me down - instead of focusing on the general equations that can help me figure out other equations. So, here's a simplified equation dump of equations that can lead you to most any equation we've learned in mechanics.
    F = ma
    K = .5mv^2
    U = mgh
    p = mv
    J = F(delta)t = (delta)p
    W = Fx = (delta)K
    P = W/t
    U(s) = .5kx^2
    F = -kx
    F(g) = (GMm)/r^2
    F(f) = (mew)F(n)
    T = 2(pi)(m/k)^(1/2) -> for springs
    T = 2(pi)(l/g)^(1/2) -> for pendulum
    T = 1/f
    F© = (mv^2)/r
    I = mr^2
    I = I(cm) + Mx^2
    v = v(0) + at
    x = x(0) + v(0)t + .5at^2
    v^2 = v(0)^2 + 2a(x-x(0))

    And for rotational mechanics, the analogs are:
    torque = F
    theta = x
    omega = v
    alpha = a
    L = p
    I = m

    This does not cover every single equation, but it hits the most important ones and can centralize your equation focus while preparing for the midterm.
  24. DavidStack

    Feeding the 5000

    By DavidStack,

    In the famous Bible story of feeding the 5000, Jesus and his 12 disciples feed 5000 men with 5 loaves of bread and 2 fish. But, the 5000 only counts the men, and since these people have been following Jesus for over 2 days, it can be assumed that they are with their families. On average, we'll say that each man has a wife and two kids, speaking that some of the men weren't married and some of the men had much larger families. That means that this story is actually the feeding of the 20000.
    First, let's look at how much bread and fish would be needed to feed these people. According to Mark 6:42, the people "ate and were satisfied." These people hadn't eaten for 2 days as they've been following Jesus, so to eat and be satisfied, each person would need at least 2 fish and a loaf of bread. There needed to be 40000 fish and 20000 loaves of bread, meaning that Jesus multiplied the amount of fish by 20000 and the amount of bread by 4000, and that doesn't even include the extra 12 basket-fulls of bread that the disciples picked up afterwards. That's crazy!
    We can also see how long it would take to pass the bread around. The passage states that the people were divided into groups of 50 and 100, so if the groups are close to equal, there are 133 groups of 100 and 134 groups of 50 (267 groups in total). With large groups like that, the distance between the center of one group to the center of another was probably around 30 meters (close to 100 feet). The disciples had to pass around a lot of food to each group, so they likely stayed all together. Thus, they had to walk a 26700 meters to go to each group, and since the people ate and were satisfied, they probably wanted seconds, so that's actually 53400 meters with an average velocity of 1.4 m/s (the average walking speed). Using kinematics, we know that v = x/t, so t = x/v. Therefore, it would take 38143 seconds for the disciples to pass out that food, which is about 10 hours and 36 minutes. That's a lot of time!
    By understanding physics, you can appreciate the incredible awe of this miracle.
  25. DavidStack

    CYO Swag

    By DavidStack,

    As I perpare myself mentally, physically, and spiritually for the upcoming CYO basketball season, I can't help but think of the physics that partner with a fluid and successful basketball shot. Players that have perfected their basketball shot, like Ray Allen, have found a combination of enough leg bend, a straight-armed follow through, and an effective wrist flick. With these three key components, the ball travels with arc and backspin in a projectile motion, sailing through the hoop. The potential energy developed through the bend of the legs is stored in the leg muscles (which act much like springs), which is then transfered through the arm to the ball and converted into kinetic energy, giving the ball the energy to travel to the hoop. Basketball shots that have arc are much more effective than flat shots because the ball moves less in the horizontal plane in the time it takes the ball to travel through the hoop, so the ball has less of a chance of hitting the rim and bouncing out. Also, the backspin creates a softer bounce of the rim, giving the ball a greater chance of bouncing into the hoop off of the rim. So, through my knowledge of physics, I can focus on getting the power in my basketball shot from my legs instead of my upper body, and making sure I have an effective amount of arc and backspin.

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