What is Point: Definition and 1000 Discussions

The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at a standard pressure such as 1 atmosphere or 100 kPa.
When considered as the temperature of the reverse change from liquid to solid, it is referred to as the freezing point or crystallization point. Because of the ability of substances to supercool, the freezing point can easily appear to be below its actual value. When the "characteristic freezing point" of a substance is determined, in fact the actual methodology is almost always "the principle of observing the disappearance rather than the formation of ice, that is, the melting point."

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  1. Pipsqueakalchemist

    Engineering Solving for Angular Velocity at Point A - Confused!

    So for this question I got the right angular velocity. But I don’t get the same velocity for point A. I don’t understand why it’s cos30, problem asked for V_a when theta = 45 so I used cos45. I have my work below.
  2. rbh

    Point charges in a regular hexagon

    The answer should be (2×sqrt3×k×q^2)/a^2. What did I do wrong?
  3. G

    Could you point me to interesting videos about plasma balls?

    Could you point me to interesting videos about plasma ball?
  4. A

    Point Kinetics for Linear Insertion of Reactivity

    I want to use the point reactor kinetics equations to solve for the power as a function time when a reactivity of gamma*t is added to a reactor that is at equilibrium at time 0. I am also asked to consider the case where the transients are long-lived compared to the lifetime of prompt neutrons...
  5. Hosein Javanmardi

    A Implementing FDM Boundary Conditions at a Red Point

    in Finite Difference Method (FDM), the boundary conditions can be implemented by applying the continuity of parallel component of magnetic field intensity. when it comes to the interface of two areas, it is done at ease, but consider this case at the red point: in FDM we exactly require on...
  6. rcgldr

    I Rail gun - point of application of force?

    A rail gun exerts an electromagnetic force on a projectile. The only moving object is the projectile, so is the point of application of force moving? Update - For others reading this thread, after a re-think on this, for linear motion (as opposed to angular motion), the point of application of...
  7. J

    Electric Potential of point outside cylinder

    Edit: Below is my work but i believe i have chosen the wrong values of the separation vector in the s direction. Any ideas as to what it should be?
  8. H

    How is the energy tranported at a certain time and point?

    I get the first part of B, but why doesn't the transported direction not change as the electric field changes? Does it follow the right-hand rule?
  9. Llukis

    A Experimental point of view of this Hamiltonian

    Dear everybody, I am involved with a system of two spins and I ended up with the following Hamiltonian: $$H_c(t) = W\sin(2J_+ t) \big( \mathbb{1} \otimes \sigma_z - \sigma_z \otimes \mathbb{1}\big) + W \cos(2J_+ t) \big( \sigma_y \otimes \sigma_x - \sigma_x \otimes \sigma_y \big) \: ,$$ where...
  10. cianfa72

    I Principle of relativity: active vs passive point of view

    Hi, starting from this thread I'm a bit confused about the content of the principle of relativity from a mathematical point of view. Basically the "Galilean principle of Relativity" puts requirements on the transformation laws between Inertial Frame of Reference (IFR); thus they have to...
  11. L

    MHB Fixed point iteration convergence

    Question: For the following functions, does the fixed point iteration for finding the fixed point in $\left [ 0,1 \right ]$ converge for all first points $ p_{0} \in \left [ 0,1 \right ]$? Justify your answer. a.$ g(x) = e^{\frac{-x}{2}}$ b.$ g(x) = 3x - 1$ Let me attempt for part a first...
  12. A

    Calculating the force on an electron from two positive point charges

    So this is more of an intuitive question rather than a mathematical one. I present the problem. Assume I have 2 charges of charge +q at a distance r from each other on the z axis. Position of two charges is (0,0,r/2) and (0,0,-r/2). Assume now that I want to calculate the force these two...
  13. C

    I Expansion of a complex function around branch point

    I’m coming at this question with a physics application in mind so apologies if my language is a bit sloppy in places but I think the answer to my question is grounded in math so I’ll post it here. Say I have a function F(z) defined in the complex z plane which has branch points at z=0 and z =...
  14. A

    Point charges placed inside a charged sphere

    I traced a spherical X-ray Gaussian (green) where the negative charges were diametrically opposite. My question is this: I can transform the entire charge of the Gaussian sphere into a point charge placed in the center. So, can I analyze only the electrical forces of the two negative charges...
  15. C

    Fulcrum, balance point and scissor lift force

    My first question, so be kind (!) Long time since I was at school, and can only remember the basics of beams and fulcrums. In essence, that a 1kg weight 5mtrs from a fulcrum / pivot needs 5kg at 1mtr to counterbalance. But what about the beam itself? Do the two sides cancel each other out? For...
  16. M

    MHB Fixed point,, Jacobi- & Newton Method, Linear Systems

    Hey! :giggle: Question 1 : Let $g(x)-=x-x^3$. The point $x=0$ is a fixed point for $g$. Show that if $x^{\star}$ is a fixed point of $g$, $g(x^{\star})=x^{\star}$, then $x^{\star}=0$. If $(x_k)$ the sequence $x_{k+1}=g(x_k)$, $k=0,1,2,\ldots$ show that if $0>x_0>-1$ then $(x_k)$ is...
  17. Mr Davis 97

    Help with picking out which point has the most negative derivative

  18. H

    Potential difference between the center of a sphere and a point 4.0 cm away?

    I can use integral as attached to obtain the answer. However, I wonder why I can not use " V = KQ / r " the get the correct answer. Thanks.
  19. Z

    Why Are There Floating Point Errors When Parsing Data in Pandas?

    I am parsing data from an excel sheet using pandas ExcelFile class and read_excel function. I noticed that when reading the file, there are floating point errors in the results. For example, a cell that contains 25.15 will be 25.149999999999977 in the pandas data frame. The value 25.15 in Excel...
  20. F

    I Can we prove the correspondence between a real number with a point ?

    Can we prove that there is a corespondence 1-1 between a real number with a point of straight line?It seems to me that we can use Dedekin's cut to prove each point of line corresponds a real number. But how about the reverse?
  21. F

    Software simulator for point charges & conductors

    Is there any software/simulator of point charges/conductors? Preferably with features enabling dynamics of charges.
  22. T

    Solving the operating point of a transformer with a nonlinear B-H curve

    What are the possible ways of solving the operating point of air gapped transformer with nonlinear B-H curve? For example let's consider 3C90 E34 sized core with 0.5 mm airgap. I know that the magnetomotive force over the ferrite part can be formulated as function of the reluctances...
  23. duchuy

    Point mechanics -- Tension in a rope from a hanging mass

    Hi, I'm being asked to determine the tension of a rope only knowing that g = 9.8 m.s-2. I understand that in order to calculate tension, I would need to multiply mass with acceleration. But i don't understand how i would in this case. This is the question for reference. Thanks for your help...
  24. murshid_islam

    B Finding the Point: Unveiling Euclid's 'Elements' Redux

    The following proof (in the image below) is from the book Euclid's "Elements" Redux (by Daniel Callahan and John Casey). I did not understand why the point D or the ##\triangle \rm ABD## was necessary. (I mean, what was the "point" of D? :-p) Joking apart, wasn't this sufficient: suppose we have...
  25. M

    Value that gives more than one stationary point

    Here is my attempt at a solution I don’t know where to go from here, as the equation is a quadratic it will have two solutions which means there will be two stationary points, but I don’t know how to solve for z.
  26. person123

    I Computing the Density of a Point Cloud

    Hi. For some background, I am running molecular dynamics simulations of silica fracture in LAMMPS. Each point represents the location of a broken bond. I would like to find regions where many bonds are breaking, which I speculate would be locations of crack formation. These computations are...
  27. greg_rack

    Horizontal inflection point of a parametric polynomial function

    For ##x=-1## to be an *horizontal* inflection point, the first derivative ##y'## in ##-1## must be zero; and this gives the first condition: ##a=\frac{2}{3}b##. Now, I believe I should "use" the second derivative to obtain the second condition to solve the two-variables-system, but how? Since...
  28. Y

    Why is Point A in Compression in a Simplified Crankshaft with a Load P?

    Summary:: I'm stuck on a year 2 mechanics question. I have this simplified crankshaft with a load P at the end. The solutions to the exercise have said that point A is in compression due to a bending moment but I don't understand why that is. The solutions and my calculations have both come out...
  29. danielhaish

    I Is Teleportation & Time Travel Possible?

    the idea of time dilatation is that the light should cross 299 792 458 meters each second relative to all point of views , so when there is two observer that one of them is in rest and the other observer is moving , the clock ticking of the moving observer would be slower(by clock ticking i...
  30. greg_rack

    B Find Local Max/Min: 2nd Derivative=0

    How do I distinguish between a point of local maxima or minima, when the second derivative in that point is equal to zero?
  31. J

    Fundamental Forces: Placing charges along a line so that the net force is zero at a point

    So 0=q1q3/r+q2q3/(13.6-r) 0=-8.5/r-3.63/13.6+3.63/r 0.2669=3.63/r-8.5/r r=-18.2465, but the answer is supposed to be 8.24
  32. S

    Calculating moments about a support point

    Moment about X2 to calculate force at X1: x1 * 9 = (250 * 2) Therefore, x1 = 500/9 = 55.5N The book however gives force at x1 as 110N. So I figured I have not understood a concept somewhere
  33. pipeguy

    A Stress point calculation some practical help

    https://www.physicsforums.com/attachments/273785 What are the possible materials (beams, H-profiles) etc that can be used to withstand the pressure of the whole system if there are 12 tonnes of wheat in the pipe at all times? How you would make this system work? There always has to be an angle...
  34. greg_rack

    Derivative of a Point of Max: Finding a & b Conditions

    The derivative of a point of maximum must be zero, and since $$y'=3ax^2+2bx+2 \rightarrow y'(-1)=3a-2b+2 \rightarrow 3a-2b+2=0$$ we get the first condition for ##a## and ##b##. Now, since we want ##x=-1## to be a local maximum, the derivative of the function must be positive when tending to...
  35. Andrea Vironda

    I Position of a point as a function of angular position

    Hi, I have this scheme, in which there are 3 segments: - I is coaxial to c axis and free to rotate in the origin. Length d1 - II is coaxial with a axis and free to rotate around c axis. There a fixed angle θ between a and c axis. Length d2 - III is welded to II, it's the PM segment. α is fixed...
  36. M

    I Separating particles in a zero point energy field

    From my understanding, quantum fluctuations create particle pairs that are usually annihilated. Is it possible to use some kind of force (eg: electromagnetism) to direct and separate antiparticles from normal particles? I believe experiments have proven that it is possible to store positrons...
  37. K

    I Closest point on a plane to a point near the plane

    Suppose there is a 3d plane z=a*x+b*y+c. Suppose there is a point in space near, but not on the plane. (xo, yo, zo). What is the coordinate (x1,y1,z1) on the plane that is nearest the original point? My attempt uses minimization but the result is blowing up into large answer. I wonder if...
  38. V

    A Propagator from a space-time point to itself

    I am following a lecture note on the QFT. But am a little confused about some parts related to the vacuum bubbles. We define the Feynman propagator, ##D_{F}(x-y)##, as giving the amplitude for a particle emitted at ##x## to propagate to ##y## (where it can be measured). After following the...
  39. D.S.Beyer

    B Lagrangian Point in General Relativity

    Is there a relationship between the Lagrangian ‘hill diagram’ and the spacetime curvature embedment graphs? The Lagrangian map shows effective potential, which deals with centrifugal force. As centrifugal force is a fictitious force (and gravity is as well), I would assume the underlying...
  40. W

    [Statics] Determining the normal force at an internal point of a member

    For this problem (see image), I get the correct answer for the normal force at point E if I: 1) divide the frame into members AB and CB, 2) solve for the x and y components of the reaction force at point B, 3) make a free body diagram with the cut at point E forming member EB and setting the sum...
  41. il postino

    Chemistry Calculate the potential at the equivalence point

    Data ##E^{0}_{I_2/I-}= 536mV## ##E^{0}_{Cr_2O_7^{2-}/Cr^{3+}}= 1330mV## ##I_{2} + 2e- \Leftrightarrow 2I^{-}## ##Cr_2O_7^{2-} + 14H^+ + 6e^- \Leftrightarrow 2Cr^{3+} + 7H_2O## Total Reaction ##6I^{-} + Cr_2O_7^{2-} + 14H^+ \Leftrightarrow 3I_{2} + 2Cr^{3+} + 7H_2O##...
  42. P

    Electrostatic - electric potential due to a point charge

    Could somebody check my solution? I want to know is it correct.
  43. Baums Mizushala

    Point on a graph nearest to the origin

    The Attempt at a Solution I know the answer is supposed to be ##(-1,0)##. However when I differentiate the above expression I get. $$ 2x+{\frac 5 2} $$ Then the shortest distance would be when the expression equates to 0. $$ 2x+{\frac 5 2}=0 $$ I should be getting ##x=-1## but solving for ##x##...
  44. yecko

    Vapor-compression refrigeration system - boiling & critical point

    Is it the lower the boiling point the better? As room pressure and temperature already can change the phase without working at vacuum? And the higher critical point means it is hard to reach super critical phase? But there is transcritical CO2 cycle, which the critical point is so low, and...
  45. ElieQuebec10

    I Calculus: What is the derivative of ##\phi## at a point?

  46. B

    Calculating Moments About a Point

    Not sure if I answered this question right I took my clockwise moments as +ve and counter-clockwise moments as -ve Any help would be much appreciated! Thanks
  47. D

    Isoelectric Point: Definition & Calculation

    I am looking to understand how isoelectric point is defined and calculated. I read the paper "Isoelectric Point Calculator" (Kozlowski 2016) which gives the definition found everywhere 'isoelectric point pI is the pH value at which the net charge of a macromolecule is zero, and therefore its...
  48. M

    Three positive point charges are located on a line (Coulomb's Law)

    I need some help resolving the follow problem. I really don't know where to put the "twice as large as the resultant force on Q3" in order to build an equation. Thank you !
  49. willDavidson

    Electric field of a point charge

    I am trying to understand what a point charge is. Is it just an electron? Or is it just an idea?
  50. A

    MHB Determine its image point after the transformation

    The graph of y=x^2 was transformed to the graph of y=-3(x+5)-2. The point (-3, 9) lies on the graph of y=x^2. Determine its image point after the transformations.
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