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DavidStack

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Blog Entries posted by DavidStack

  1. DavidStack

    Ping Pong

    By DavidStack,

    Ping Pong has always been one of my favorite leisure time activities, and after embarrassing a good friend of mine yesterday with my ping pong skills, the physics of the sport came to mind, especially the physics of my favorite shot: the top spin shot. Arguably the most effective shot, the top spin increases your accuracy with more powerful shots, and is very difficult to return. But why?

    The picture above shows how the spin of the ball forces the air below the ball to take a longer path, while the air going over the top of the ball has a shorter path, thus moving faster relative to the ball. Because of this difference in air speed, there is a downward force on the ball, causing the dip of the ball soon after it is hit. With this dip, the ball falls faster than a non spinning ball, allowing the player to hit the ball with more force and still have it drop on the other side of the table, giving the player more accuracy. Then, the forward spin of the ball couples with the forward direction of the ball upon impact of the table, increasing the speed of the ball and making it difficult to return. This very interesting physics of air flow is used in designing air planes, rockets, and many other inventions.
  2. 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.

  3. DavidStack

    Stunts!

    By DavidStack,

    If you have ever watched Hot Rod, one of my favorite movies, then you understand the joy/hilarity of poorly thought out stunts. My brother and I have always enjoyed puting ourselves in harms way of the purpose of an awesome video, but I've discovered that we are much more willing to do painful stunts if we are landing into water instead of on solid ground. Most recently, we went to some pier that was 10 or so feet off of the ground and attempted backflips on our bikes off of a ramp into the water. I don't think either of us would even imagine attempting such a thing if we were to land on the ground. But why? It has to do with molecular structure, a topic concentrated in Chemistry but still very important in Physics, as molecular structure impacts things like air drag and electrical forces. So, solids have a stiffer molecular structure than liquids because they move less, and likewise liquids are stiffer than gasses because they move less. Due to the cusion-like complection of water, it is safer to land in than landing on dirt. Professional stuntmen in movies often land in air-water mixtures instead of actual water because the air-water mixture is much less stiff, breaking the fall of the stuntman more than water would. This all relates back to impulse, as the less stiff structures provide a longer time of impact, reducing the force felt on the person.
  4. 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!
  5. DavidStack

    Tap Dancing

    By DavidStack,

    Because I have a habit of bouncing from activity to activity, I chose to participate in the musical this winter after my body failed to push through the pain of indoor track. With my luck, it turns out that the musical this year has multiple tapping numbers, so i get to learn how to tap dance! So I wondered, how do tap shoes make the noise that they do?
    The physics is really quite simple. When the tapper pushes their foot to the ground, a lot of the kinetic energy is converted into sound energy, dispersing a crisp noise. Tap shoes produce a louder and crisper sound that the average shoe because tap shoes have metal plates on the ball of the foot and the heel, and metal (because of its free flowing electrons) conducts sound better than rubber - the typical bottom of a shoe - does. Tap shoes also produce a crisper sound when just the ball or heel of the foot makes contact with the floor because when the whole foot is on the floor some of the path way of the sound is trapped under the foot, creating a somewhat muffled noise that sounds more like a stomp than a precise tap. Thanks to physics, I understand how to make a crisper noise when I dance which will hopefully improve my dancing ability!
  6. 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).
  7. DavidStack

    Tetherball!

    By DavidStack,

    Continuing with the physics of recreational sports, I'd like to talk about the physics of tether ball, a sport I'm not quite as good at. But, tether ball clearly demonstrates centripetal motion, and is very interesting to delve into. A player will hit the ball with a horizontal force F. Neglecting air resistance, this will temporarily be the only force on the ball, and will equal the mass of the ball times the acceleration of the ball. Centripetal acceleration equals (v^2)/r, so, given that the mass of a tetherball is about .27 kg, and the rope (which is the radius in this situation) is about 2.25 m, F = (.27 * v^2) / 2.25. Given this equation for velocity, a player can figure out how to beat his or her opponent. If a player know that his or her opponent cannot return a ball traveling greater than 10 m/s, he or she knows to hit the ball with at least an F = (.27 * 10^2) / 2.25, or F = 12 N, assuming the rope can sustain this force. So, even though air drag has an affect on the ball, and it is very hard to figure out how to hit something with a certain force in newtons, the physics of centripetal motion helps one to attain a better knowledge of how tetherball works.
  8. 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.

  9. DavidStack
    A dear friend of mine and I have recently started playing NCAA Football 12 on his 360 and most of the time I win, thanks to my knowledge of physics. Because I understand the concepts of conservation of momentum, work, and air drag, I very often run the ideal football play - the running back slip screen. For this play, the offensive linemen break their blocks on some of the defensive players who are rushing the quarterback. Lured into the illusion that they can have an easy sack, these linemen and/or linebackers charge towards the quarterback. At this time, the running back breaks his block and turns towards the quarterback for the easy dump pass, like this:

    After that, the linemen that "missed" their blocks before now block for the running back, who has many blockers and open field in front of him. He does not need to worry about the defensive players behind him because of the law conservation of momentum: the defensive players are sprinting in one direction and they need a strong force (which comes from their muscles power) to reverse their momentum to the opposite direction. On the other hand, the running back's blockers are always moving in the right direction, so the do not need this strong force and their momentum aids them in delivering solid blocks. Not only so, but this play works much more consistently than a pass down-field because of the principles of work and drag force. The simple equation for work is force times displacement, so to throw the ball far down the field, the quarterback must do a lot of work. The correct amount of force needed for a far throw is difficult to consistently deliver, making this pass much harder. The drag force on the ball also applies because the ball travels in the air longer for farther passes, so the drag force acts on the ball for a longer period of time, affecting the consistency of the throw. Thus, because of my understanding of mechanics, I can lead a well poised offense to give my Michigan Wolverines the win every time.
  10. 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.
  11. DavidStack
    After not performing as well on the practice physics test as I would have hoped, I began to think about the physics of test taking, mainly using energy. We've learned that kinetic energy = .5mv^2 and that potential energy = mgh. In this instance, m = the question number, v = the speed that I answer questions, g = how easy the test is (the greater g is, the easier the test is), and h = my confidence. Therefore, my potential energy at the beginning before I take the test is converted to kinetic energy throughout the test taking process. Since mgh will equal .5mv^2 by the end of the test, the m's cancel out, showing that the question number does not matter in this scenario. By this equation, my velocity by the end of the test will equal (2gh)^(1/2). With this equation, it is clear that when I am more confident and have an easier test, I take the test faster and more efficiently. Also, my velocity can vary throughout the test since g is a constant but h is a variable as my confidence can rise or fall depending on if I get questions right or wrong. So, my goal for this midterm is to either study a lot and gain confidence, or just hope that we get a really easy test.
  12. 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.
  13. DavidStack

    Triple Jumping

    By DavidStack,

    Although I had to quit indoor track because of an ankle injury, I did learn a lot about the precision of triple jumping form from my coach. Previously, I had run my approach with my chest not perpendicular to the ground but slightly more forward and my knees did not move very high. With this form, my momentum (which points perpendicular from my chest) was pointed into the ground, preventing me from jumping as far as possible. My knees also needed tweaking, since a lot of the power from jumping comes from the knee drive. Because my knees moved little as I approached my jump, it was hard to suddenly rip my knee up when I took off, due to the conservation of momentum (it takes an external force to change the direction of my momentum, but if my momentum was already directed up and forward, that force would not be necessary). With a continued knee rise throughout my approach, I could develop a more fluid and powerful jump. While these adjustments to the approach slightly decreased my speed, they vastly improved my control and jumping power, greatly lengthening the distance I could jump.
    This world class jumper displays this approach form, leading him to a record breaking jump:




  14. 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!
  15. 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.
  16. DavidStack
    As the playoffs are underway, Bills fans (the sad category that I put myself under) have the same dilemma as they have since the 21st century began - which team are they going to root for in the playoffs this year? Year after year, the Bills struggle to qualify for the post season, a big reason being that they never have a strong quarterback. Their most recent excuse - Ryan Fitzpatrick. So lets look at why he's so awful:

    When you look at an elite quarterback like Tom Brady (as much as I hate to admit that he's good), you witness extremely precise and accurate throwing mechanics. He uses the potential energy from his lower body (by bending his knees, creating a buildup of muscle power, and then stepping with his opposite foot) to provide the power for his upper body to move fluidly and transfer this potential energy to the kinetic energy of the ball. With these mechanics, he can make 60 yard passes accurately and effortlessly (at least so it seems).

    Ryan Fitzpatrick, on the other hand, does not exhibit these mechanics. He uses mostly his upper body to deliver speed and distance to his throws, making him look like he throws in body in order to throw the ball. Because of this, he fails to use the huge supply of potential energy that his lower body has to offer, reducing the power of his throws. Not only that, but his upper body now has to focus of both the power and accuracy of the throw, which makes more of Ryan's throws off target. Even though Fitzpatrick graduated from Harvard, he seems to struggle with the concept of conservation of energy as he does not know how to efficiently convert the potential energy of his muscles to the kinetic energy of the ball, leading to many inaccurate, under-thrown passes and unhappy fans.

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