Say It With Science is an educational blog that serves to teach readers about science, in general, but more specifically about physics and mathematics.
It is run by members who each have their own unique specialty and interests in science and mathematics. Some of us are merely high school students and some of us are university students. This enables us to provide high quality information about lower and higher level subjects.
You may contact us via ask box or at sayitwithscience@gmail.com
We welcome questions, feedback, and submissions; and we will clarify any concepts for readers.Say It With ScienceTumblr (3.0; @sayitwithscience)http://sayitwithscience.tumblr.com/Dream Souls. Photo By David...<img src="http://31.media.tumblr.com/c55c32d45b28cdf025d0e2200325f6ae/tumblr_mt7cvd80Tm1qmyxvuo1_500.jpg"/><br/><br/><p><span>Dream Souls. Photo By David Hanjani</span></p>
<p><span><a href="http://www.photographyofdavidhanjani.tumblr.com" target="_blank">www.photographyofdavidhanjani.tumblr.com</a></span></p>http://sayitwithscience.tumblr.com/post/63352188057http://sayitwithscience.tumblr.com/post/63352188057Mon, 07 Oct 2013 01:26:05 -0400scienceastrologysubmissionphotographyofdavidhanjaniHi! Love your blog. I enjoy math, but the unfortunate thing is that Im really bad at it. Is this strange to be bad at a subject you enjoy? And what do you recommend someone does if they want to get better at math?<p><span class="GingerNoCheckStart"></span>No that’s not strange at all! Part of the fun of learning something new is its difficulty. If you want to learn more about math all you have to do is stay curious. Wikipedia is always a great resource. Talk to your math teachers or professors and see what fields would be most useful at your age and see if you can buy some text books relating to the subject. Youtube has useful channels too, like <a href="http://www.youtube.com/user/khanacademy" target="_blank">Khan Academy</a>. </p>
<p><span class="GingerNoCheckEnd"></span></p>http://sayitwithscience.tumblr.com/post/38589350326http://sayitwithscience.tumblr.com/post/38589350326Sat, 22 Dec 2012 20:47:35 -0500aangotaskmathematicscontemplatingstardustWhat are the 9 types of energy<p><span class="GingerNoCheckStart"></span>Hey there,</p>
<p>Why don’t you check out <a href="http://sayitwithscience.tumblr.com/post/8104049758/there-are-9-types-of-energy-chemical-energy#notes-container" target="_blank">this</a> earlier post. It should give you a brief intro to the types of energy, the law of conservation of energy, efficiency and Sankey diagrams!</p>
<p>Hopefully that will help you out and thank you for the question!</p>
<p>As always, feel free to submit any other inquiries to our <a href="http://sayitwithscience.tumblr.com/ask" target="_blank">ask</a>.</p>
<p><span class="GingerNoCheckEnd"></span></p>http://sayitwithscience.tumblr.com/post/32858876488http://sayitwithscience.tumblr.com/post/32858876488Thu, 04 Oct 2012 01:08:54 -0400sayitwithsciencescienceenergyaskask boxquestionsankey diagramsticktocktickertapeHi! I stumbled upon your tumblr, and I'd like to start off by saying how amazing it is, and thank you for making this tumblr! Moreon to my issue, i'm currently studying crude oil in Chemistry. Could you please help me understand"cracking" in terms of crude oil? From what I understand, 'cracking' is the CHEMICAL process of breaking down large molecules into smaller ones. And they 'crack' crude oil to refine it into petroleum; fractional distillation being a PHYSICAL process. More info please?<p>It sounds like you’re a bit confused between fractional distillation and cracking. It’s true that cracking is a chemical process and fractional distillation is a physical process, but by saying that I mean to show you that they’re two entirely different processes.</p>
<p>When crude oil is first extracted from the ground, is made up of a variety of different hydrocarbons (chemical compounds that only consist of carbon and hydrogen), some very short (ethene) and some long (decane), and is entirely useless in this state. Hydrocarbons can be separated into two groups: <a href="http://en.wikipedia.org/wiki/Alkane" target="_blank">alkanes</a> and <a href="http://en.wikipedia.org/wiki/Alkene" target="_blank">alkenes</a>. An alkane is saturated, meaning it holds as many hydrogen atoms as possible, whereas an alkene is unsaturated and contains a double carbon bond. </p>
<p>Fractional distillation serves to separate the longer hydrocarbons from the shorter hydrocarbons by their boiling points. This works because the longer the hydrocarbon, the higher the boiling point and viscosity and the lower the flammability.</p>
<p><a href="http://en.wikipedia.org/wiki/Fractional_distillation" target="_blank">Fractional distillation</a> takes place as follows:</p>
<ol><li>Crude oil is vapourised and fed into the bottom of the fractionating column.</li>
<li>As the vapour rises up the column, the temperature falls.</li>
<li>Fractions with different boiling points condense at different levels of the column and can be collected.</li>
<li>The fractions with high boiling points (long chain hydrocarbons) condense and are collected at the bottom of the column</li>
<li>Fractions with low boiling points (short chain hydrocarbons) rise to the top of the column where they condense and are collected.</li>
</ol><p><span>To see a diagram of the fractional distillation process, click </span><a href="http://www.bbc.co.uk/schools/gcsebitesize/science/images/5_fractional_distillation.gif" target="_blank">here</a><span>.</span></p>
<p>Cracking on the other hand, breaks long alkanes down into shorter, more useful alkane and alkene molecules. It requires a catalyst (a substance that causes or accelerates a chemical reaction without itself being affected) and a high temperature. This is done mainly to assuage the high industrial demand for the shorter molecules. The alkenes are typically converted into polymers (plastics) while the alkanes are sought after as a fuel source. Cracking is an example of a thermal decomposition reaction.</p>
<p>I hope that helps clear up some of your confusion.</p>http://sayitwithscience.tumblr.com/post/29018337124http://sayitwithscience.tumblr.com/post/29018337124Wed, 08 Aug 2012 20:55:00 -0400crude oilchemistrysciencealkanealkenefractional distillationcrackinghydrocarbonsHCsaturatedunsaturateddecaneethenedouble carbon bondboiling pointflammabilityviscosityfractionating columnvapourtemperaturecondensecondensationshort chainlong chaincatalystpolymerplasticfuelthermal decomposition reactionticktocktickertapeDemons in the History of Science
Part one of two: Laplace’s...<img src="http://38.media.tumblr.com/tumblr_m7zly9R2zL1qmyxvuo1_r1_400.jpg"/><br/><br/><p><strong>Demons in the History of Science</strong></p>
<p><em><strong>Part one of two: Laplace’s Demon</strong></em></p>
<p>Some might say that the modern day physicists have it easy; they can appeal to the public with their stories of eleven-dimensional universes, time travel, and stories of a quantum world that is stranger than fiction. But the basis of such appeal remains the same as the appeal for pursuing science always was and will be: a greater understanding of the environment, ourselves, and knowledge itself.</p>
<p>Just like <em><a href="http://en.wikipedia.org/wiki/Schr%C3%B6dinger's_cat" target="_blank">Schrödinger’s cat</a></em>, a popular thought experiment by famous physicist Erwin Schrödinger, Laplace’s Demon and Maxwell’s Demon are two other thought-experiments in scientific thinking which are important for what they reveal about our understanding of the universe. It may only interest you to learn of these thought-experiments for the sake of reinforcing the philosophical relevance and beauty that science has always sought to provide.</p>
<p><em><a href="http://en.wikipedia.org/wiki/Jim_Al-Khalili" target="_blank">Jim-Al Khalili</a></em>, author of <em><a href="http://www.amazon.com/Quantum-Perplexed-Dr-Jim-Al-Khalili/dp/1841882380" target="_blank">Quantum: A Guide for the Perplexed</a></em>, affirms that fate as a scientific idea was disproved three-quarters of a century ago, referring to the discoveries of quantum mechanics as proof, of course. But what does he mean when he says this? Prior to such discoveries, it may have been okay to argue for a <em><a href="http://en.wikipedia.org/wiki/Determinism" target="_blank">deterministic</a></em> universe, meaning that scientists could still consider the idea of a world in which one specific input must result in one specific output and thus the sum all these actions and their consequences could help “determine” the overall outcome, or fate, of such a world.</p>
<p><em><a href="http://en.wikipedia.org/wiki/Pierre-Simon_Laplace" target="_blank">Pierre-Simon Laplace</a></em>, born on March 23, 1794, was a French mathematician and astronomer whose work largely founded the statistical interpretation of probability known as <em><a href="http://en.wikipedia.org/wiki/Bayesian_probability" target="_blank">Bayesian Probability</a></em>. He lived in a world before Heisenberg’s Uncertainty Principle and Chaos Theory and thus he was allowed to imagine such a deterministic universe:</p>
<blockquote>
<p>We may regard the present state of the universe as the effect of its past and the cause of its future. An intellect which at a certain moment would know all forces that set nature in motion, and all positions of all items of which nature is composed, if this intellect were also vast enough to submit these data to analysis, it would embrace in a single formula the movements of the greatest bodies of the universe and those of the tiniest atom; for such an intellect nothing would be uncertain and the future just like the past would be present before its eyes.</p>
<p>Laplace, A<em> Philosophical Essay on Probabilities</em></p>
</blockquote>
<p>Laplace thought about what it would be like if it were possible to know the positions, masses, and velocities of all the atoms in existence and hypothesized a being, later known as <strong><em><a href="http://en.wikipedia.org/wiki/Laplace's_demon" target="_blank">Laplace’s Demon</a></em></strong>, which would be able to know such information and such calculate all future events. </p>
<p>With our knowledge of physics, <em><a href="http://en.wikipedia.org/wiki/Heisenberg_Uncertainty_Principle" target="_blank">The Heisenberg Uncertainty Principle</a></em> and <em><a href="http://en.wikipedia.org/wiki/Chaos_theory" target="_blank">Chaos Theory</a></em>, such a being could not exist because such information about atoms cannot be observed with enough precision to calculate and predict future events. (By the way, “enough” precision means infinite precision!) This might be good news for those who believe in free will as its concept would not be permitted in a deterministic universe governed by Laplace’s demon. </p>
<p>Interestingly enough, The Heisenberg Uncertainty Principle and Chaos Theory are not the only restrictive challenges that scientists have faced in trying to understand the properties and bounds of our universe. <em><a href="http://en.wikipedia.org/wiki/Second_law_of_thermodynamics" target="_blank">The Second Law of Thermodynamics</a></em> is also of concern to scientists and philosophers alike, as we will learn with the birth of another mind-boggling demon.</p>http://sayitwithscience.tumblr.com/post/28373411829http://sayitwithscience.tumblr.com/post/28373411829Mon, 30 Jul 2012 20:24:00 -0400Pierre-Simon LaplaceLaplaceLaplace's Demondemondemonslaplaces demonsciencephysicssayitwithscienceeducationinformationfactscertaintyfatedeterminismhistoryhistory of sciencescience historyphilosophyafossilizedblogTwo days ago, the CERN team announced that they had found a new...<br/><br/><p><span class="GingerNoCheckStart"></span>Two days ago, the CERN team announced that they had found a new particle whose properties are consistent with the long sought-after Higgs Boson’s. Whether or not it <em>is</em> the elusive boson however, is still to be determined by further research. To read more about the event, follow this <a href="http://www.bbc.co.uk/news/world-18702455" target="_blank">link</a> to the new BBC article.</p>
<div>If you have no clue what this is about, the above video is a quick and nice introduction to the Higgs Boson submitted by one of our followers, the lovely <a href="http://oh-yeah-and-what.tumblr.com/" target="_blank">oh-yeah-and-what</a>. Thanks for the awesome submission!</div>
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<div>SIWS loves feedback from followers and we’ll do our best to respond. If you have any questions, ideas, or concerns, feel free to <a href="http://sayitwithscience.tumblr.com/ask" target="_blank">drop us a message</a>, email us at sayitwithscience@gmail.com or like and post on <a href="http://www.facebook.com/SayItWithScience" target="_blank">our Facebook page</a>. You can even make <a href="http://sayitwithscience.tumblr.com/submit" target="_blank">a submission post</a> and we might publish it and credit you, like we did with this one!</div>
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<div>Take care and happy science-ing! </div>
<p><span class="GingerNoCheckEnd"></span></p>http://sayitwithscience.tumblr.com/post/26605029309http://sayitwithscience.tumblr.com/post/26605029309Thu, 05 Jul 2012 23:25:00 -0400current eventseducationfactsknowledgenewsparticle physicsphysicsquantum physicssciencescientistssubmissionvideosayitwithsciencemovealong-nothingtosee-deactivaHey everyone! The Say It With Science team invites you to...<img src="http://38.media.tumblr.com/tumblr_m6efy951Ju1qmyxvuo1_500.jpg"/><br/><br/><p>Hey everyone! The Say It With Science team invites you to “like” our new <a href="https://www.facebook.com/SayItWithScience?bookmark_t=page" target="_blank">Facebook page</a>.</p>
<p>Thanks for following!</p>http://sayitwithscience.tumblr.com/post/26167727412http://sayitwithscience.tumblr.com/post/26167727412Fri, 29 Jun 2012 18:03:00 -0400Facebooksayitwithsciencehadron94-deactivated20130407Maximum Entropy Distributions
Entropy is an important topic...<img src="http://31.media.tumblr.com/tumblr_lyxhg8WUKQ1qmyxvuo1_500.png"/><br/><br/><p><strong><span class="GingerNoCheckStart"></span>Maximum Entropy Distributions</strong></p>
<p><a href="http://en.wikipedia.org/wiki/Entropy_in_thermodynamics_and_information_theory" target="_blank">Entropy</a> is an important topic in many fields; it has very well known uses in <a href="http://en.wikipedia.org/wiki/Statistical_mechanics" target="_blank">statistical mechanics</a>, <a href="http://en.wikipedia.org/wiki/Thermodynamics" target="_blank">thermodynamics</a>, and <a href="http://en.wikipedia.org/wiki/Information_theory" target="_blank">information theory</a>. The classical formula for entropy is Σ<sub>i</sub>(p<sub>i</sub> log p<sub>i</sub>), where p=p(x) is a <a href="http://en.wikipedia.org/wiki/Probability_density_function" target="_blank">probability density function</a> describing the likelihood of a possible microstate of the system, <em>i</em>, being assumed. But what is this probability density function? How must the likelihood of states be configured so that we observe the appropriate macrostates?</p>
<p>In accordance with the second law of thermodynamics, we wish for the entropy to be maximized. If we take the entropy in the limit of large N, we can treat it with calculus as S[φ]=∫dx φ ln φ. Here, S is called a <a href="http://en.wikipedia.org/wiki/Functional_(mathematics)" target="_blank">functional</a> (which is, essentially, a function that takes another function as its argument). How can we maximize S? We will proceed using the methods of <a href="http://en.wikipedia.org/wiki/Calculus_of_variations" target="_blank">calculus of variations</a> and <a href="http://en.wikipedia.org/wiki/Lagrange_multiplier" target="_blank">Lagrange multipliers</a>.</p>
<p>First we introduce three constraints. We require <strong>normalization</strong>, so that ∫dx φ = 1. This is a condition that any probability distribution must satisfy, so that the total probability over the domain of possible values is unity (since we’re asking for the probability of <em>any possible event</em> occurring). We require <strong>symmetry</strong>, so that the expected value of <em>x</em> is zero (it is equally likely to be in microstates to the left of the mean as it is to be in microstates to the right — note that this derivation is treating the one-dimensional case for simplicity). Then our constraint is ∫dx x·φ = 0. Finally, we will explicitly declare our <strong>variance</strong> to be σ², so that ∫dx x²·φ = σ².</p>
<p><a href="https://netfiles.uiuc.edu/liberzon/www/teaching/cvoc/node38.html" target="_blank">Using Lagrange multipliers</a>, we will instead maximize the augmented functional S[φ]=∫(φ ln φ + λ<sub>0</sub>φ + λ<sub>1</sub>xφ + λ<sub>2</sub>x²φ dx). Here, the integrand is just the sum of the integrands above, adjusted by Lagrange multipliers λ<sub>k</sub> for which we’ll be solving.</p>
<p>Applying the <a href="http://en.wikipedia.org/wiki/Euler-Lagrange_equation" target="_blank">Euler-Lagrange equations</a> and solving for φ gives φ = 1/exp(1+λ<sub>0</sub>+xλ<sub>1</sub>+x²λ<sub>2</sub>). From here, our symmetry condition forces λ<sub>1</sub>=0, and evaluating the other integral conditions gives our other λ’s such that q = (1/2πσ²)<sup>½</sup>·exp(-x² / 2σ²), which is just the <a href="http://en.wikipedia.org/wiki/Normal_distribution" target="_blank">Normal (or Gaussian) distribution</a> with mean 0 and variance σ². This remarkable distribution appears in many descriptions of nature, in no small part due to the <a href="http://en.wikipedia.org/wiki/Central_limit_theorem" target="_blank">Central Limit Theorem.</a></p>
<p><span class="GingerNoCheckEnd"></span></p>http://sayitwithscience.tumblr.com/post/26114861112http://sayitwithscience.tumblr.com/post/26114861112Thu, 28 Jun 2012 22:34:00 -0400educationentropyeuler-lagrangeinformation thoerymathmathematicsmathsmaximum entropy distributionsprobabilitysayitwithsciencesciencestatisticssymmetrythermodynamicsoptimizationmaximizationstatistical mechanicsmechanicsalbanhouseHappy π Day math lovers!<img src="http://upload.wikimedia.org/wikipedia/commons/2/27/Pi_digits_distribution_update.png"/><p>We here at Say It With Science would like to celebrate it with you by sharing some interesting trivia about one of our favorite physicists, Richard Feynman, and one of our favorite constants, π (pi). The Feynman Point is a sequence of six 9’s beginning at the 762nd decimal place of π, named after Nobel Prize winning physicist Richard Feynman. Feynman had memorized π to this point so that he could end his recitation of the mathematical constant by saying “nine nine nine nine nine nine and so on…”. At this point someone less knowledgeable about mathematics might assume the number continues this way forever, however we know better. It is believed that π is a <a href="http://en.wikipedia.org/wiki/Normal_number" target="_blank">normal number</a>, meaning that its digits are as uniformly distributed among the digits 1 through 9 (or the digits of any other base you choose to use). If π is a normal number then the chances of coming across six 9’s in a row is 0.08%. Strange occurrences like this are what makes math beautiful. π Day is a perfect reason to start memorizing as many digits of π as you can! Happy π Day!</p>http://sayitwithscience.tumblr.com/post/19291538926http://sayitwithscience.tumblr.com/post/19291538926Wed, 14 Mar 2012 11:23:29 -0400pipi daymathcontemplatingstardustCharge, Parity and Time Reversal (CPT) Symmetry
From our...<img src="http://31.media.tumblr.com/tumblr_lxkc84nQom1qmyxvuo1_r1_400.jpg"/><br/><br/><p align="center" class="MsoNormal"><strong><span>Charge, Parity and Time Reversal (CPT) Symmetry</span></strong></p>
<p class="MsoNormal"><span><span> </span>From our everyday experience, it is easy to conclude that nature obeys the laws of physics with absolute consistency. However, several experiments have revealed certain cases where these laws are not the same for all particles and their <a href="http://en.wikipedia.org/wiki/Antiparticle" target="_blank">antiparticles</a>. The concept of a symmetry, in physics, means that the laws will be the same for certain types of matter. Essentially, there are three different kinds of known symmetries that exist in the universe: charge (C), parity (P), and time reversal (T). The violations of these symmetries can cause nature to behave differently. If C symmetry is violated, then the laws of physics are not the same for particles and their antiparticles. P symmetry violation implies that the laws of physics are different for particles and their mirror images (meaning the ones that spin in the opposite direction). The violation of symmetry T indicates that if you go back in time, the laws governing the particles change.</span></p>
<p class="MsoNormal"><span><span> </span>There were two American physicists by the names of Tsunng-Dao Lee and Chen Ning Yang suggested that the <a href="http://en.wikipedia.org/wiki/Weak_force" target="_blank">weak interaction</a> violates P symmetry. <span> </span>This was proven by an experiment which was conducted with radioactive atoms of colbalt-60 that were lined up and introduced a magnetic field to insure that they are spinning in the same direction. In addition, it was also found that the weak force also does not obey symmetry C. Oddly enough, the weak force did appear to obey the combined CP symmetry. Therefore the laws of physics would be the same for a particle and it’s antiparticle<span> </span>with opposite spin.</span></p>
<p class="MsoNormal"><span>Surprise, surprise! There was a slight error in the previous experiment that was just mentioned. A few years later, it was discovered that the weak force actually violates CP symmetry. Another experiment was conducted by two physicists named Cronin and Fitch. They studied the <a href="http://en.wikipedia.org/wiki/Decay_(physics)" target="_blank">decay</a> of neutral <a href="http://en.wikipedia.org/wiki/Kaons" target="_blank">kaons</a>, which are <a href="http://en.wikipedia.org/wiki/Mesons" target="_blank">mesons</a> that are composed of either one down <a href="http://en.wikipedia.org/wiki/Quark" target="_blank">quark</a> (or antiquark) and a strange antiquark (or quark). These particles have two <a href="http://en.wikipedia.org/wiki/Decay_mode#Decay_modes_in_table_form" target="_blank">decay modes</a> where one will decay much faster than the other, even though they all have identical masses. The particles with the longer lifetimes will decay into three <a href="http://en.wikipedia.org/wiki/Pions" target="_blank">pions</a> (denoted with the symbol </span><span>π</span><span>0</span><span>), however the kaon ‘species’ with the shorter lifetimes will only decay into two </span><span>pions. They had a 57 foot beamline, where they only expected to see the particles with slower decay rate at the end of the beam tube. In astonishment, one out of every 500 decays where from the kaons species that had a shorter lifetime. The main conflict with seeing the short-lived mesons at the end of the beam tube is because they are traveling <a href="http://en.wikipedia.org/wiki/Special_relativity" target="_blank">relavistic</a> speeds and therefore ignoring the <a href="http://en.wikipedia.org/wiki/Time_dilation" target="_blank">time dilatation</a>that they are supposed to undergo. Thus, the experiment has shown that the weak force causes a small CP violation that can be seen in kaon decay.</span></p>
<p class="MsoNormal"><span><img src="http://hyperphysics.phy-astr.gsu.edu/hbase/particles/imgpar/cronin2.gif"/></span></p>
<p class="MsoNormal"><span><img height="259" src="http://hyperphysics.phy-astr.gsu.edu/hbase/particles/imgpar/cronin3.gif" width="512"/></span></p>http://sayitwithscience.tumblr.com/post/15780283840http://sayitwithscience.tumblr.com/post/15780283840Fri, 13 Jan 2012 12:20:00 -0500physicssayitwithscienceantimatterantiparticlesCPTsymmetryweak interactionweak forceparticlesquantumkaonsmesonsrelativitytime dilatationsciencehadron94hadron94-deactivated20130407Refraction
Light waves are part of the EM wave spectrum. When...<img src="http://33.media.tumblr.com/tumblr_lsxdt4ZS7b1qmyxvuo1_r1_500.gif"/><br/><br/><p><strong>Refraction</strong></p>
<p>Light waves are part of the EM wave spectrum. When moving through an optical medium (i.e. air, glass, etc. …), the E field of the wave excites the electrons within the medium, causing them to oscillate, as a result, the light wave slows down slightly due to the loss of some of its kinetic energy. Its new speed is always less than that of the speed of light in a vacuum (v<c). Materials are characterized by their ability to bend as well as slow down light, which is known as optical refractive index (n).</p>
<pre> c
n = -
v
speed of light in a vacuum
= ----------------------------
speed of light in the medium</pre>
<pre>n = 1 in a vacuum
n = more than 1 in all other media</pre>
<p>Refraction itself occurs when light passes across an interface between two media with different indices of refraction. As a general rule (which can be derived by Snell’s law below), light refracts towards the normal when passing to a medium with a higher refractive index, and away from the normal when moving to a medium of lower refractive index.</p>
<p>Snell’s Law:</p>
<p>n₁sinα = n₂sinβ</p>
<p>where n₁ is the refractive index of the first medium</p>
<p><strong>Reflection</strong></p>
<p>One of the properties of a boundary between optical media is that some of the light that’s approaching the interface at the angle of incidence (α) is reflected back into the first medium, while the rest continues on into the second medium at the angle of refraction (β).</p>
<p>Angle of incidence = Angle of Reflection</p>http://sayitwithscience.tumblr.com/post/14627563939http://sayitwithscience.tumblr.com/post/14627563939Thu, 22 Dec 2011 14:31:05 -0500refractionsay it with sciencephysicsreflectionopticsincidencemediumsnells lawrefractive indexspeed of lightlightsciencegcseaqaa levelsatsvcesticktocktickertapeThe Hamilton-Jacobi Equation
This blog has posted more than a...<img src="http://38.media.tumblr.com/tumblr_lwdhjuiXtO1qmyxvuo1_400.png"/><br/><br/><p><b>The Hamilton-Jacobi Equation</b></p>
<p>This blog has posted <a href="http://sayitwithscience.tumblr.com/post/9850296271/when-describing-the-trajectory-of-a-point-particle" target="_blank">more</a> <a href="http://sayitwithscience.tumblr.com/post/8055994226/originally-in-the-newtonian-formulation-of" target="_blank">than</a> <a href="http://sayitwithscience.tumblr.com/post/7579883360/fbds-an-intro-to-newtons-laws" target="_blank">a</a> <a href="http://sayitwithscience.tumblr.com/post/11612109741/f-ma" target="_blank">few</a> <a href="http://sayitwithscience.tumblr.com/post/12039067599/the-virial-theorem-in-the-transition-from" target="_blank">times</a> in the past about classical mechanics. Luckily, classical mechanics can be approached in several ways. This approach, which uses the <a href="https://en.wikipedia.org/wiki/Hamilton%E2%80%93Jacobi_equation" target="_blank">Hamilton-Jacobi equation (HJE)</a>, is one of the most elegant and powerful methods.</p>
<p>Why is the HJE so powerful? Consider a dynamical system with a <a href="https://en.wikipedia.org/wiki/Hamiltonian_mechanics" target="_blank">Hamiltonian</a> <i>H</i>=<i>H</i>(q,p,t). Suppose we knew of a <a href="https://en.wikipedia.org/wiki/Canonical_transformation" target="_blank">canonical transformation (CT)</a> that generated a new Hamiltonian <i>K</i>=<i>K</i>(Q,P,t) which (for a local chart on phase space) vanishes identically. Then the canonical equations would give that the transformed coordinates (Q,P) are constant in this region. How easy it would be to solve a system where you know that most of the important quantities are constant!</p>
<p>The rub is in finding such a canonical transformation. Sometimes it can’t even be done analytically, but nevertheless this is the goal of the Hamilton-Jacobi method of solving mechanical systems. In the equation given above, <i>S</i> is the <a href="https://en.wikipedia.org/wiki/Generating_function_(physics)" target="_blank">generating function</a> of the CT. Coincidentally, it often comes out to just equal the classical action up to an additive constant! This is due to the connection between canonical transformations and mechanical gauge transformations; it turns out that the additive function used to define the latter <i>is</i> the generating function of the former. In general the HJE is a partial differential equation that might be solvable by additive separation of variables… but don’t get too hopeful! Oftentimes the value of the HJE comes not in finding the actual equations of motion but in revealing symmetry and conservation properties of the system.</p>http://sayitwithscience.tumblr.com/post/14377156575http://sayitwithscience.tumblr.com/post/14377156575Sat, 17 Dec 2011 19:06:01 -0500physicssciencemechanicsclassical mechanicshamiltonianlagrangianhamilton-jacobijacobihamilton-jacobi equationcanonical transformationmechanical gauge transformationgauge transformationgenerating functionalbanhouseVariable Star Astronomy
Variable stars are stars whose...<img src="http://33.media.tumblr.com/tumblr_luo93515tH1qmyxvuo1_500.gif"/><br/><br/><p><strong>Variable Star Astronomy</strong></p>
<p>Variable stars are stars whose brightness changes because of physical changes within the star. There exist more than 30,000 variable stars in just the Milky Way. Variable star astronomy is a popular part of astronomy because amateur astronomers play a key role. They have submitted thousands of observed data and these data are logged onto a database. American readers can find information on it on the <a href="http://www.aavso.org/" target="_blank">American Association of Variable Star Observers page.</a> </p>
<p>One of such variable stars are called <strong><a href="http://en.wikipedia.org/wiki/Cepheid" target="_blank">Cepheids</a></strong>. Cepheids are pulsating variable stars because they undergo a “repetitive expansion and contraction of their outer layers” [<a href="http://www.amazon.com/Universe-Definitive-Visual-Martin-Rees/dp/0756613647" target="_blank">1</a>]. In Cepheids, the star’s period of variation (about 1-70 days) is related to its luminosity; the longer the period, the higher the luminosity. In fact, when graphed, the relationship is shown by a straight line (as can be seen on the title image).<em><a href="http://en.wikipedia.org/wiki/Henrietta_Swan_Leavitt" target="_blank"> Henrietta Swan Leavitt</a></em>, an American astronomer, first discovered this and understood the significance of this knowledge. Combined with understanding of the star’s apparent magnitude <a href="http://sayitwithscience.tumblr.com/post/7634997621/absolute-magnitude-of-a-star-out-of-all-the-stars" target="_blank">(a previously written post on this subject can be found here)</a>, astronomers can use this information to find a star’s distance from Earth. Cepheids are famously known for their usefulness in finding distances to far-away galaxies and other deep sky objects. Leavitt died early from cancer but was to be nominated for the Nobel Prize in Physics by Professor Mittag-Leffler (Swedish Academy of Sciences). </p>
<p>Edwin Hubble used Leavitt’s discovery to prove that the Andromeda Galaxy (M31) is not part of the Milky Way, but was able to find the distance to the Andromeda Galaxy (between 2-9 million light years away). At first his calculation was incorrect (900,000 light years) because he observed <a href="http://en.wikipedia.org/wiki/Cepheid#Classical_Cepheids" target="_blank">Type I (classical) Cepheid Stars</a>. Type I Cepheid stars are brighter, newer Population I stars. Hubble later used <a href="http://en.wikipedia.org/wiki/Cepheid#Type_II_Cepheids" target="_blank">type II Cepheids</a> (also called <em>W Virginis stars)</em>, which are smaller, dimmer, Population II stars, and he was able to make more accurate calculations.</p>
<blockquote>
<p>To determine the star’s distance, <a href="http://www.astronomynotes.com/starprop/s3.htm#A1.2" target="_blank">use the inverse square law of light brightness.</a> </p>
<p><img src="http://www.astronomynotes.com/starprop/invsq1.gif" width="508" height="275"/></p>
</blockquote>
<p>A similar type of star are <strong><a href="http://en.wikipedia.org/wiki/RR_Lyrae_variable" target="_blank">RR Lyrae</a></strong> Variable Stars. They are smaller than Cepheids and have a much shorter period (from a few hours to a day). On the other hand, they are far more common. Likewise, they can be used to solve for distances as well. Low mass stars live longer, and thus Cepheid stars are generally younger because they are more massive. </p>
<p>Both Cepheids and RR Lyrae Variable stars are referred to as standard candles: objects with known luminosity. If you’ve ever wondered how astronomers came to those enormous figures when describing how far away galaxies and stars are from us, you can now better understand why and how. </p>http://sayitwithscience.tumblr.com/post/12807425257http://sayitwithscience.tumblr.com/post/12807425257Mon, 14 Nov 2011 18:09:00 -0500astronomyastrophysiciscepheidseducationinformationphysicistsphysicsrr lyraesayitwithsciencestarstar luminositystarstumblrsciencezeroinfinite-deactivated2012012How can I be a writer on sayitwithscience?<p>Hey, there!</p>
<p>At this moment, we’re not sure if we can immediately recruit someone new to the team. There are a few things, however, that you can do to show us your interest:</p>
<ul><li>Email us! Message us (non anonymously)! Let us know who you are! Tell us about your science interests!</li>
<li>Write posts you would like to submit. You may then submit them to us via <a href="http://sayitwithscience.tumblr.com/submit" target="_blank">the submission box</a>! If you let us know beforehand, we can guide you through the process. Remember: they must be original work and must include links to any sources you use.</li>
<li>Provide general feedback and comments.</li>
</ul><p><br/>If things end up going great, you may land yourself a position on the team! Thank you for your interest! This message put a smile on our faces. (:</p>
<p>-Say It With Science</p>http://sayitwithscience.tumblr.com/post/12078395615http://sayitwithscience.tumblr.com/post/12078395615Sat, 29 Oct 2011 13:04:06 -0400sayitwithscienceanonymousask boxaskszeroinfinite-deactivated2012012The Virial Theorem
In the transition from classical to...<img src="http://38.media.tumblr.com/tumblr_ltj8vtQSUO1qmyxvuo1_500.png"/><br/><br/><p><strong>The Virial Theorem</strong></p>
<p>In the transition from classical to statistical mechanics, are there familiar quantities that remain constant? The <a target="_blank" href="https://en.wikipedia.org/wiki/Virial_theorem">Virial theorem</a> defines a law for how the total kinetic energy of a system behaves under the right conditions, and is equally valid for a one particle system or a <a target="_blank" href="https://en.wikipedia.org/wiki/Mole_(unit)">mole</a> of particles.</p>
<p><a target="_blank" href="https://en.wikipedia.org/wiki/Clausius">Rudolf Clausius</a>, the man responsible for the first mathematical treatment of <a target="_blank" href="https://en.wikipedia.org/wiki/Entropy">entropy</a> and for one of the classic statements of <a target="_blank" href="https://en.wikipedia.org/wiki/Second_law_of_thermodynamics">the second law of thermodynamics</a>, defined a quantity <em>G</em> (now called the Virial of Clausius):</p>
<p>G ≡ Σ<sub>i</sub>(<strong>p</strong><sub>i</sub> · <strong>r</strong><sub>i</sub>)</p>
<p>Where the sum is taken over all the particles in a system. You may want to satisfy yourself (it’s a short derivation) that taking the time derivative gives:</p>
<p>dG/dt = 2T + Σ<sub>i</sub>(<strong>F</strong><sub>i</sub> · <strong>r</strong><sub>i</sub>)</p>
<p>Where <em>T</em> is the total kinetic energy of the system (Σ ½mv<sup>2</sup>) and dp/dt = F. Now for the theorem: the Virial Theorem states that if the time average of dG/dt is zero, then the following holds (we use angle brackets ⟨·⟩ to denote time averages):</p>
<p>2⟨T⟩ = - Σ<sub>i</sub>(<strong>F</strong><sub>i</sub> · <strong>r</strong><sub>i</sub>)</p>
<p>Which may not be surprising. If, however, all the forces can be written as power laws so that the potential is <em>V</em>=<em>ar</em><sup>n</sup> (with <em>r</em> the inter-particle separation), then</p>
<p>2⟨T⟩ = n⟨V⟩</p>
<p>Which is pretty good to know! (Here, <i>V</i> is the total kinetic energy of the particles in the system, not the potential function <i>V=ar</i><sup>n</sup>.) For an inverse square law (like the <a target="_blank" href="https://en.wikipedia.org/wiki/Gravitational_force">gravitational</a> or <a target="_blank" href="https://en.wikipedia.org/wiki/Coloumb_law">Coulomb</a> forces), F∝1/r<sup>2</sup> ⇒ V∝1/r, so 2⟨T⟩ = -⟨V⟩.</p>
<p>Try it out on a <a target="_blank" href="https://en.wikipedia.org/wiki/Simple_Harmonic_Oscillator">simple harmonic oscillator</a> (like a mass on a spring with no gravity) to see for yourself. The potential <em>V</em> ∝ <em>kx²</em>, so it should be the case that the time average of the potential energy is equal to the time average of the kinetic energy (n=2 matches the coefficient in 2⟨T⟩). Indeed, if <em>x</em> = <em>A</em> sin( √[k/m] · <em>t</em> ), then <em>v</em> = <em>A</em>√[k/m] cos( √[k/m] · <em>t</em> ); then <em>x</em><sup>2</sup> ∝ sin² and <em>v</em>² ∝ cos², and the time averages (over an integral number of periods) of sine squared and cosine squared are both ½. Thus the Virial theorem reduces to</p>
<p>2 · ½<em>m</em>·(<em>A²k/2m</em>) = 2 · ½<em>k</em>(<em>A²/2</em>)</p>
<p>Which is easily verified. This doesn’t tell us much about the simple harmonic oscillator; in fact, we had to find the equations of motion before we could even use the theorem! (Try plugging in the force term <em>F=-kx</em> in the first form of the Virial theorem, without assuming that the potential is polynomial, and verify that the result is the same). But the theorem scales to much larger systems where finding the equations of motion is impossible (unless you want to solve an Avogadro’s number of differential equations!), and just knowing the potential energy of particle interactions in such systems can tell us a lot about the total energy or temperature of the ensemble.</p>http://sayitwithscience.tumblr.com/post/12039067599http://sayitwithscience.tumblr.com/post/12039067599Fri, 28 Oct 2011 14:30:05 -0400albanhousecoulombeducationeducationforcesgravityinformationmathmathematicssayitwithsciencesciencethe viral theoremtheoremthermodynamicsmechanicsanalytical mechanicskinetic energypotential energyenergyphysicsclausiusrudolf clausiusvirialvirial theoremstatistical mechanicsn-bodymultiparticlestatistical lawtime averageharmonic oscillatoralbanhouse∑ F = ma<p>… is a differential equation:</p>
<p><img src="http://media.tumblr.com/tumblr_lt8ku9Y0Jj1qf37hs.gif"/></p>
<p>where acceleration <strong>a</strong>(t), velocity <strong>v</strong>(t), and displacement <strong>s</strong>(t) are all vectors and functions of time. This equation is second-order in position because the highest derivative is the second time derivative of position. Combined with the right boundary conditions, <strong>s</strong>(t) (also called the trajectory: path through space and time) can be determined.</p>
<p>This differential equation can be solved one component, or dimension, at a time. Let us focus on one of these, and call it the x component. The equations for y and z can be found exactly the same way.</p>
<p><strong>Constant acceleration</strong></p>
<p>If the graph of a(t) signifying acceleration in the x direction is constant</p>
<p><img src="http://media.tumblr.com/tumblr_lsu1jw2t0W1qf37hs.jpg" width="350"/></p>
<p><img src="http://media.tumblr.com/tumblr_lt8meaSBhV1qf37hs.gif"/></p>
<p>then the graph of v(t), the velocity in the x direction, is a straight line with slope a<sub>0</sub></p>
<p><img src="http://media.tumblr.com/tumblr_lsu1louzB81qf37hs.jpg" width="350"/></p>
<p><img src="http://media.tumblr.com/tumblr_lt8m9rmxXF1qf37hs.gif"/></p>
<p>and the graph of x(t), the position along the x axis, is a parabola</p>
<p><img src="http://media.tumblr.com/tumblr_lsu1mwHn4R1qf37hs.jpg" width="350"/></p>
<p><img src="http://media.tumblr.com/tumblr_lt8m7z44NO1qf37hs.gif"/></p>
<p>It is also possible for the acceleration, or either of the initial velocity or initial position, to be negative. Thus the displacement/projectile motion formula is derived.</p>http://sayitwithscience.tumblr.com/post/11612109741http://sayitwithscience.tumblr.com/post/11612109741Tue, 18 Oct 2011 10:05:06 -0400accelerationcalculusconstant accelerationderivativederivativesdifferential equationdisplacementgraphsintegralintegralskinematicsphysicspositionprojectile motionprojectile motion formulavelocitytrajectorynebulae12Going superfluid!
A liquid goes superfluid when it suddenly...<img src="http://33.media.tumblr.com/tumblr_lsmm5oj7Yr1qmyxvuo1_r1_500.jpg"/><br/><br/><p><strong>Going superfluid!</strong></p>
<p>A liquid <a title="1:44 min video [scienceblogger]" href="http://scienceblogger.tumblr.com/post/8116205777" target="_blank">goes superfluid</a> when it suddenly loses all internal friction and gains near infinite thermal conductivity. The combination of zero viscosity but nonzero surface tension allows a superfluid to <a title="[ions-anions-and-cations]" href="http://ions-anions-and-cations.tumblr.com/post/2738487907" target="_blank">creep up</a> walls and back down the outside to drip from the bottom of open containers, or to <a title="Rollin film" href="http://en.wikipedia.org/wiki/Rollin_film" target="_blank">completely cover the inner surface</a> of sealed containers. Lack of viscosity also allows a superfluid to leak through a surface that is porous to any degree, because the molecules can slip through even microscopic holes. Superfluids furthermore exhibit a thermo-mechanical effect where they flow from colder to warmer temperatures, exactly the opposite of heat flow as stated by the laws of thermodynamics! That implies the remarkable property of superfluids of carrying zero entropy. Because of this, a <a title="Alfred Leitner explains the fountain effect" href="http://www.youtube.com/watch?v=YVMuI_shltE" target="_blank">perpetual fountain</a> can be set up by shining light on a superfluid bath just below a vertical open capillary tube, causing the fluid to shoot up through and beyond the tube until its contact with the air causes it to cease being a superfluid and fall back down into the bath, whereby it will cool back into the superfluid state and repeat the process.</p>
<p>So how does superfluidity work, exactly?</p>
<p><strong>Makings of a superfluid</strong></p>
<p>Physicists first got the inkling of something stranger than the norm when, around 1940, they cooled liquid helium (specifically, the <sup>4</sup>He isotope) down to 2.17 K and it started exhibiting the above-mentioned properties. Since the chemical makeup of the helium didn’t change (it was still helium), the transformation to a superfluid state is a physical change, a phase transition, just like ice melting into liquid water. Perhaps for cold matter researchers, this transition to a new phase of matter makes up for the fact that helium doesn’t solidify even at 0 K except under large pressure - whereas ALL other substances solidify above 10 K.</p>
<p><img src="http://ltl.tkk.fi/research/theory/He4PD.gif" height="256" width="310"/>[Phase diagram of <sup>4</sup>He, <a title="Research on helium at LTL/Helsinki University of Technology" href="http://ltl.tkk.fi/research/theory/helium.html" target="_blank">source</a>]</p>
<p>Helium is truly the only substance that never solidifies under its own vapor pressure.</p>
<p>Instead, when the temperature reaches the transition or <a title="2.17 K for 4-He" href="http://www.google.com/url?sa=t&source=web&cd=1&ved=0CBwQFjAA&url=http%3A%2F%2Fen.wikipedia.org%2Fwiki%2FLambda_point&ei=uMKQToNVh7nRAbbW7DQ&usg=AFQjCNHe6njlpNZPa07b4Z7tvyvIkPOIVQ&sig2=q8nReog4KJmQA28HeqtbEA" target="_blank">lambda point</a>, quantum physics takes hold and a fraction of the liquid particles drop into the same ground-energy quantum state. They move in <a title='"a single quantum object" [fuckyeahengineers]' href="http://fuckyeahengineers.tumblr.com/post/2738242883" target="_blank">lock-step</a>, behaving identically and never getting in each others’ way. Thus we come to see that superfluidity is a kind of <a title="though BECs usually refer to transitions from dilute gases rather than liquids" href="http://en.wikipedia.org/wiki/Bose%E2%80%93Einstein_condensate" target="_blank">Bose-Einstein condensation</a>, the general phenomenon of a substance’s particles simultaneously occupying the lowest-energy quantum state.</p>
<p>Read more:<br/>”<a href="http://www.aps.org/publications/apsnews/200601/history.cfm" target="_blank">This Month in Physics History: Discovery of Superfluidity, January 1938</a>”. <em>APS News</em>: January 2006</p>
<p><small>Based on a project by Barbara Bai, Frankie Chan, and Michele Silverstein at Cornell University.</small></p>http://sayitwithscience.tumblr.com/post/11435371561http://sayitwithscience.tumblr.com/post/11435371561Fri, 14 Oct 2011 10:00:05 -0400heliumlambda pointrollin filmsuperfluidquantum physicsphysicsentropyviscositysuperfluidityfountain effectthermomechanical effect4 He3 Heisotopesphase diagramphase transitionBose-Einstein condensationBECnebulae12Hypercubes
What is a hypercube (also referred to as a tesseract)...<img src="http://33.media.tumblr.com/tumblr_lsuyi4rH0h1qmyxvuo1_500.jpg"/><br/><br/><p><strong>Hypercubes</strong></p>
<p>What is a <a href="http://en.wikipedia.org/wiki/Hypercube" target="_blank">hypercube</a> (also referred to as a <a href="http://en.wikipedia.org/wiki/Tesseract" target="_blank">tesseract</a>) you say! Well, let’s start with what you know already. We know what a <a href="http://en.wikipedia.org/wiki/Cube" target="_blank">cube</a> is, it’s a box! But how else could you describe a cube? A cube is 3 dimensional. Its 2 dimensional cousin is a square. </p>
<p>A hypercube is just to a cube what a cube is to a square. A hypercube is 4 dimensional! (Actually— to clarify, hypercubes can refer to cubes of all dimensions. “Normal” cubes are 3 dimensional, squares are 2 dimensional “cubes, etc. This is because a hypercube is an n-dimensional figure whose edges are aligned in each of the space’s dimensions, perpendicular to each other and of the same length. A tesseract is specifically a 4-d cube). </p>
<p><img width="398" height="339" src="http://robertinventor.com/software/virtualflower/virtualflower/a_hyper_cube.gif"/></p>
<p>[<a href="http://robertinventor.com/software/virtualflower/virtualflower/tesseract.htm" target="_blank">source</a>]</p>
<p>Another way to think about this can be found <a href="http://www.geom.uiuc.edu/docs/outreach/4-cube/" target="_blank">here</a>:</p>
<blockquote>
<p><span>Start with a point. Make a copy of the point, and move it some distance away. Connect these points. We now have a segment. Make a copy of the segment, and move it away from the first segment in a new (orthogonal) direction. Connect corresponding points. We now have an ordinary square. Make a copy of the square, and move it in a new (orthogonal) direction. Connect corresponding points. We now have a cube. Make a copy and move it in a new (orthogonal, fourth) direction. Connect corresponding points. This is the tesseract.</span></p>
</blockquote>
<p><span>If a tesseract were to enter our world, we would only see it in our three dimensions, meaning we would see forms of a cube doing funny things and spinning on its axes. This would be referred to as a cross-section of the tesseract. Similarly, if we as 3-dimensional bodies were to enter a 2-dimensional world, its 2-dimension citizens would “observe” us as 2-dimensional cross objects as well! It would only be possible for them to see cross-sections of us.</span></p>
<p><span>Why would this be significant? </span>Generally, in math, we work with multiple dimensions very often. While it may seem as though a mathematican must then work with 3 dimensions often, it is not necessarily true. The mathematician deals with these dimensions only <em>mathematically</em>. These dimensions do not have a value because they do not correspond to anything in reality; 3 dimensions are nothing ordinary nor special. </p>
<p>Yet, through modern mathematics and physics, researchers consider the existence of other (spatial) dimensions. What might be an example of such a theory? String theory is a model of the universe which supposes there may be many more than the usual 4 spacetime dimensions (3 for space, 1 for time). Perhaps understanding these dimensions, though seemingly impossible to visualize, will come in hand. </p>
<p><span><a href="http://www.youtube.com/watch?v=lwL_zi9JNkE" target="_blank">Carl Sagan also explains what a tesseract is</a>. </span></p>
<p><span>Image: <span>Peter Forakis, <em>Hyper-Cube</em>, 1967, Walker Art Center, Minneapolis</span></span></p>http://sayitwithscience.tumblr.com/post/11361440387http://sayitwithscience.tumblr.com/post/11361440387Wed, 12 Oct 2011 14:45:05 -0400hybercubedimensionssciencespacephysicscubegeometrycarl sagansayitwithscienceeducationknowledgeinformationfactsmathematicszeroinfinite-deactivated2012012Hello followers!<p>The contributors have been on a bit of a break, but we’ll be back soon with a new batch of posts. In the meantime, we’d like to get to know our readers!</p>
<p>What subject areas do you study/where are you in your education/what classes are you taking?</p>
<p>Any requests for future Say it with Science posts?</p>http://sayitwithscience.tumblr.com/post/11316327134http://sayitwithscience.tumblr.com/post/11316327134Tue, 11 Oct 2011 10:15:06 -0400say it with sciencenebulae12Uniform circular motion describes an object that is travelling...<img src="http://33.media.tumblr.com/tumblr_lrt9uzGW9Q1qmyxvuo1_r5_500.gif"/><br/><br/><p><a title="UCM!" href="http://en.wikipedia.org/wiki/Uniform_circular_motion" target="_blank"><strong>Uniform circular motion</strong></a> describes an object that is travelling at constant speed in a circular pathway. Though the speed which is the magnitude of the velocity remains the same, the direction of velocity is constantly changing as the object curves along the edge of the circle. A changing velocity implies a nonzero acceleration, and thus a net force acting on the object. What might such an acceleration or force look like?</p>
<p>The equation which characterizes UCM is</p>
<p>||<strong>F</strong><sub>net</sub>|| = ma<sub>r</sub> = mv<sup>2</sup>/r</p>
<p>where m is the mass of the object, v the speed, and r the radius of the pathway. The acceleration a<sub>r</sub> = v<sup>2</sup>/r and net force point at all times toward the center of the circle. You might realize this is exactly Newton’s law with the added information that the acceleration is entirely radial.</p>
<p>Why would that be? If this were true, it means the external forces on the object <em>in sum</em> continually change the direction of an object’s motion but preserve its speed. This means the object never gains nor loses kinetic energy! It also means that acceleration, pointing radially toward the center of the circle, is always at odds to the velocity, which points along a tangent to the circle. By geometry the acceleration and velocity vectors for a particle in UCM are always perpendicular, because in a circle a radial line and a tangent must form a right angle at the circumference.</p>
<p>We gain more insight by looking to <a title="inertia, f=ma, equal and opposite force pairs" href="http://sayitwithscience.tumblr.com/post/7579883360/fbds-an-intro-to-newtons-laws" target="_blank">Newton’s laws</a>. The first defines inertia, which is the ability of an object with mass to remain at the same velocity unless acted upon by a force. So we know that, absent this curious radial net force, the object would speed through space not only at a constant speed but in a straight line. <em>With</em> the forces in place, a net acceleration arises that snaps the trajectory from line to circle.</p>
<p>The second law states ∑ <strong>F</strong> = m<strong>a</strong>, that the acceleration of the object is result of all the forces acting on it. UCM is usually <em>defined</em> using this equation, so there’s not much to glean here except to remember that the characteristic acceleration, v<sup>2</sup>/r, is constant in time because v and r are constant, and that we must sum all the forces before equating to mv<sup>2</sup>/r. But this is a sticking point: the net force in UCM is often called <a title='"center-seeking"' href="http://en.wikipedia.org/wiki/Centripetal_force" target="_blank">centripetal force</a>, but it is <em>not</em> actually present in the setup. When the initial conditions of the system are just right that they effect a constant radial acceleration, only then do we call the net force centripetal in nature. The UCM version of Newton’s second law conveniently allows us to solve for forces or other unknowns precisely because we know what the resultant net force must be.</p>
<p>Finally, Newton’s third law states for any force from a first object on a second, the force from the second on the first is equal in magnitude and opposite in direction. This is useful for looking at specific situations: celebrated (or not) problems include cars travelling on a circular track, cars travelling on a circular banked track, the carnival ride where a person is pressed against the inside of a <a title="Rotor (ride)" href="http://en.wikipedia.org/wiki/Rotor_%28ride%29" target="_blank">spinning cylinder</a>, a dish being carried on a <a title="Dim sum, anyone?" href="http://en.wikipedia.org/wiki/Lazy_Susan" target="_blank">lazy susan</a>, a puck or other object being spun around on a string, or an electron shot through some kinds of magnetic or electric fields. Forces between objects can be clues to weigh in the equation that equates to the centripetal (net) force.</p>
<p>If you have some <a title="#calculus, and remember your force diagrams as well" href="http://sayitwithscience.tumblr.com/tagged/calculus" target="_blank">calculus</a> under your belt, here is a fun exercise: imagine a <a title="Bicycle chain" href="http://en.wikipedia.org/wiki/Bicycle_chain" target="_blank">bicycle chain</a>, which is a closed loop made of metal links. You set the chain spinning so that it continues spinning in a perfect circle. If the chain has <em>n</em> links, what is the tension in each link? Then what is the tension in each link as <em>n</em> approaches infinity?</p>
<p>If you’ve been following the blog, try formulating UCM using the <a title="a natural choice for radial-transverse systems?" href="http://sayitwithscience.tumblr.com/post/9850296271/when-describing-the-trajectory-of-a-point-particle" target="_blank">Frenet-Serret equations</a>!</p>http://sayitwithscience.tumblr.com/post/10555193604http://sayitwithscience.tumblr.com/post/10555193604Fri, 23 Sep 2011 09:06:05 -0400physicsuniform circular motionUCMcentripetal forcecentripetal accelerationcentripetal motionnebulae12