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. Andy Resnick

    Insights Digital Camera Buyer's Guide: Compact Point and Shoot - Comments

    Andy Resnick submitted a new PF Insights post Digital Camera Buyer's Guide: Compact Point and Shoot Continue reading the Original PF Insights Post.
  2. thecourtholio

    Interaction Energy For Two Point Charges

    Homework Statement Find the interaction energy ( ##\epsilon_0 \int \vec{E_1}\cdot\vec{E_2}d\tau##) for two point charges, ##q_1## and ##q_2##, a distance ##a## apart. [Hint: put ##q_1## at the origin and ##q_2## on the z axis; use spherical coordinates, and do the ##r## integral first.]Homework...
  3. moenste

    Reaction between a track and a ball at a point

    Homework Statement The diagram shows a section of a curtain track in a vertical plane. The curved section, CDE, forms a circular arc of radius of curvature 0.75 m and the point D is 0.25 m higher than B. A ball-bearing of mass 0.060 kg is released from A, which is 0.50 m higher than B. Assume...
  4. Z

    Kepler's Third Law and Motion of Two Point Masses

    I posted this before but I think it was in the wrong, place, so sorry for the duplicate :O I'm trying to work through some equations in the paper 'Gravitational Radiation and the Motion of Two Point Masses' (Peters, 1964) but I can't get out the right values 1. Homework Statement For a binary...
  5. Z

    Kepler's Third Law and Motion of Two Point Masses

    I'm trying to work through some equations in the paper 'Gravitational Radiation and the Motion of Two Point Masses' (Peters, 1964) but I can't get out the right values 1. Homework Statement For a binary star system with each mass = 1 solar mass, the equations give the results: Separation ~ 10...
  6. K

    Does a limit exist on a graph at (-1,0) if the point

    Does a limit exist on a graph at (-1,0) if the point is solid, and has a right sided limit, but there is nothing left of the point? I understand that if the left sided limit and the right sided limit are different then it doesn't exist, but on my graph it shows a line coming from the right...
  7. M

    MHB How to find the Location of the point P

    A portion of a sketch drawn by a student to obtain the location of a point P which lies at a distance of 3cm from the given straight line AB and 5cm from the point A, is shown in the figure. Complete the sketch and mark the location of the point P. This is is already included. And the...
  8. R

    Calculate the lateral strength / buckle point of a cylinder

    Hello. Here is my problem: I want to mount 2 solar panels on a horizontal 10 foot bar. (panels are 5' and 20-40/lbs each - so 80lbs) is there a formula i can use on each material option to figure out what material to use for the 10 foot bar (what won't break under the weight of the panels or...
  9. H

    General pathline of a particle x at point epsilon at time ta

    Homework Statement Show that the path line of a particle at point x currently, and point ξ at time τ is given by ξ(τ) = x + (τ-t)Lx Homework Equations Pathline is solution to dx/dt = u x(t)|t=τ = X L is the velocity gradient and is a 2nd order tensor Lij = dui/dxj The Attempt at a...
  10. Frankenstein19

    Question about electric field of a point figure in book

    I'm studying for my physics exam and there is a figure on the book physics for scientists and engineers 4th edition by giancoli that i just don't understand. I know that the electric field of a negative charge points toward it and the electric field of a positive charge points away from it Also...
  11. B

    A point source isotropically emitting 10^8 fast nuetrons

    Homework Statement A point source isotropically emitting 10^8 fast nuetrons per second falls out of its shield onto a railroad platform 3 m horizontally from the track. A train goes by at 60 mph. Ignoring scattering and attenuation, what is the fluence of nuetrons that would strike a passenger...
  12. Mr Davis 97

    I Constructing a vector from a point

    So I have solved the following problem: Consider two points located at ##\vec{r}_1## and ##\vec{r}_2##, separated by distance ##r = |\vec{r}_1 - \vec{r}_2|##. Find a vector ##\vec{A}## from the origin to a point on the line between ##\vec{r}_1## and at ##\vec{r}_2## at a distance ##xr## from the...
  13. Mr Davis 97

    Derive vector that moves uniformly from one point to another

    Homework Statement Consider two points located at ##\vec{r}_1## and ##\vec{r}_2##, and separated by distance ##r = |\vec{r}_1 - \vec{r}_2|##. Find a time-dependent vector ##\vec{A} (t)## from the origin that is at ##\vec{r}_1## at time ##t_1## and at ##\vec{r}_2## at time ##t_2 = t_1 + T##...
  14. G

    Distance from a point to a plane

    Homework Statement What is the distance from the point P to the plane S? Homework Equations ## S = \left \{ r_{0} + s(u_{1},u_{2},u_{3})+t(v_{1},v_{2},v_{3}) | s,t \in \mathbb{R} \right \} ## The Attempt at a Solution [/B] I found an expression for the general distance between point P and a...
  15. Artlav

    I Is there a point during star formation when gas is 1 atm?

    I have been reading about and contemplating the early stages of star formation lately. An interstellar cloud collapses under it's own gravity to form a star. There is much data about what could trigger it, and what happens when the gas heats up or starts fusing. However, i couldn't find...
  16. G

    A Strings vs. point interactions in QFT?

    Hi all! I have a question regarding the principal difference between QFT and string theory according to popular accounts. It is said that QFT deals with point particles leading to the well-known infinities in calculating the transition amplitudes whereas in string theory the interaction is...
  17. Hercuflea

    Moving point and it's distance relative to a fixed point

    Not sure whether this is an intro physics or intro calculus/related rates problem. 1. Homework Statement Suppose a point P lies at (x,y)=(0,1) meters. A car is traveling at 30 meters/second along the x-axis towards +∞. Define r to be the distance between P and the car at any time t. I...
  18. X

    I What is the predicted location of gravity in orbiting objects?

    Because gravity travels at the speed of light in GR, there seem to be a few possible places where gravity could pull you towards if dealing with some massive object in some orbit, which one of these is predicted?1. To the position where you see the object (the place where the object used to be)...
  19. TheCapacitor

    Finding focal point of concave lens using concave and convex

    Homework Statement [/B] I was doing http://stao.ca/VLresources/sci-tie-data/lessons/1400_1499/DivergingLensExperimentDeta.pdf experiment. Let's look at this image: Suppose we get the imaginary object at dv by convergence lens. And this object is like a real object for the concave lens. Then...
  20. J

    Problem about existence of partial derivatives at a point

    Homework Statement I have the function: f(x,y)=x-y+2x^3/(x^2+y^2) when (x,y) is not equal to (0,0). Otherwise, f(x,y)=0. I need to find the partial derivatives at (0,0). With the use of the definition of the partial derivative as a limit, I get df/dx(0,0)=3 and df/dy(0,0)=-1. However, my...
  21. P

    Jackson 3.7 Three point charges (q,−2q,q) are located in ....

    Homework Statement Hi, The question is " Three point charges (q,−2q,q) are located in a straight line with separation a and with the middle charge (−2q) at the origin of a grounded conducting spherical shell of radius b ..." I have found a solution for this problem but there is one step that...
  22. F

    I Understanding Point Masses: Key Concepts and Limitations for A Level Physics

    "Understand that, for a point outside a uniform sphere, the mass of the sphere can be considered a point mass." This is one of the "learning outcomes" in the A level Course. Here are my questions related to it. 1.) What is meant by point masses? (A definition will do) 2.) Why do we consider...
  23. M

    Correlating density and melting point in alkenes

    Quoted data for cis and trans 1,2 dichloroethenes shows cis has higher density but lower melting point than trans. How can this be explained? Packing arguments are clearly unable to rationalise these observations. Other 1,2 disubstituted ethenes show similar pattern. Thank you.
  24. M

    Electric Field of Plate to a Point

    Homework Statement [/B] A square plate of side-length L, charged with uniform surface charge density η. It is centred at x = y = z = 0 and orientated in the z = 0 plane. The task is to determine the z-component of the electric field at the point r(x, y, z) = r(x, 0, d), offset along the...
  25. shintashi

    For gear teeth to linear shaft, what's the center point?

    I used to know this back when i worked with Flash animation, but I'm working on a 3D printer now and I've forgotten, when you have a set of teeth, on a linear shaft and a gear, do you base the unrolling of the gear, for the spacing, on the inside of the teeth, the outside of the teeth, or the...
  26. M

    MHB Determine the location and nature of turning point

    whats the turning point of (1-lnx)², x>2 is it a minimum or maximum. can someone help me
  27. S

    Point source of light, opaque screen with hole. photodiode problem

    Homework Statement A point source of light (wavelength \lambda = 600 \, \text{nm} ) is located a distance x = 10\,\text{m} away from an opaque screen with a small circular hole of radius b. A very small photodiode is moved on an axis from very far away toward the screen. The first...
  28. C

    Fixed Point Theorem: Necessary & Sufficient Conditions for Convergence

    Homework Statement Let be ##f \in C^{1}(\mathbb{R}^{n}, \mathbb{R}^{n})## and ##a \in \mathbb{R}^{n}## with ##f(a) = a##. I'm looking for a suffisent and necessar condition on f that for all ##(x_{n})## define by ##f(x_{n}) = x_{n+1}##, then ##(x_{n})## converge. Homework Equations ##f(a) =...
  29. M

    Magnetic Field at any Point Around a Finite Straight Wire

    Hello, First of all, this question does stem from graduate work, but it seems far too simple to tag "advanced". I am looking to write some code to simulate the force from a railgun. The first step in doing this (from a tutorial I found) is to find the magnetic field at any point within the...
  30. F

    Angle properties: Angles centered at a point?

    Homework Statement Find what x is I don't understand what to do with (3x-23). Homework Equations Angles centered at a point: a+b+c+d=360 https://drtayeb.files.wordpress.com/2011/03/igcse-math-extended.pdf (here is a pdf of the textbook, Exercise 4A, 3i.)[/B]The Attempt at a Solution...
  31. B

    MHB Coordinates of Point H in GH Line

    GH is a straight line. the coordinates of G are (-2,8) the midpoint of GH is (5,-3) work out the coordinates of H
  32. C

    Calculating Moment about a Point: Understanding FDCsin and FDCcos

    1. The problem sttatement, all variables and given/known data i don't understand the moment FDC(sin18.43)(9) , I think it's wrong.. http://imgur.com/a/qPdqf Homework EquationsThe Attempt at a Solution I think it should be FDC(cos18.43)(3), am i right? the r is the perpedicular distance from D...
  33. C

    Moment about a point in trusses

    Homework Statement for moment about C, the author gave [-F_(FE)](5) +3.2(4) +6(5) = 0 i don't understand the working...Why there are moment 3.2(4) and also moment 6(5)? Homework EquationsThe Attempt at a Solution for moment 3.2(4) , it's centered at F, so ,it's not moment about C ,am i...
  34. P

    B Irradiance at a point on a surface

    My book states that "irradiance at a point on the outer sphere is less than irradiance at a point on the inner sphere." The formula is simple, it's flux/area. So in case of a sphere its flux/4π*r^2. But my question is what exactly does this formula give? does it give the total flux that...
  35. T

    Why the temperature remains constant at the boiling point?

    Why all liquids vaporize on boiling point instead of the temperature gradually increasing along with vapor pressure and more liquid atoms evaporating? Does it have anything to do with formation of vapor bubbles?
  36. F

    MHB Proof Quest: Non-Equilateral Triangle Enclosing Point Z

    I am struggling with this question, it would be easy enough if the triangle was equilateral but that is not necessarily the case. Let (ha, hb, hc) be heights in the triangle ABC, and let Z be a point inside the triangle. Further to this, consider the points P, Q, R on the sides AB, BC and AC...
  37. Jeffack

    A Create function which meets slope and point requirements

    I am trying to create a function of A and x which has the following properties. A is a scaling parameter that determines the shape of the function. I write the function below in f(A,x) form 1) f(A,1)=1 always 2) For all x>1, 0<f ' (x)<1 3) As A approaches some upper bound (which could be...
  38. E

    Centripetal Force for non point particle

    Say there is a point particle attached to a rope of radius r. It spins with angular velocity w. As a result, the force needed to keep this particle in circular motion is: F = mv2/r However, now say I have some irregular shaped object, with say some constant surface density for simplicity. It...
  39. M

    MHB Geometry proof Mid point theorem

    Hi,I have been stuck on this problem The midpoints of the sides AB and AC of the triangle ABC are P and Q respectively. BQ produced and the straight line through A drawn parallel to PQ meet at R. Draw a figure with this information marked on it and prove that, area of ABCR = 8 x area of APQ. I...
  40. A

    A Prep for Hawking/Ellis: Point Set Topology Needed

    I'm trying to prepare to read The Large Scale Structure Of Space-time by Hawking and Ellis. I've been reading a General Topology textbook since the authors say "While we expect that most of our readers will have some acquaintance with General Relativity, we have endeavored to write this book so...
  41. E

    I At what point does classical EM become relativistic EM?

    By classical E&M I mean the general basics. I understand that even classical EM has clear relativistic undertones, but is anything taught classically actually wrong under the context of relativistic EM? For example, does Faraday's law continue to hold etc. I know this is kind of a broad...
  42. MMircky

    Studying Nonlinear Optics Starting Point

    I am currently an undergrad studying physics and am doing research on PPLN and nonlinear optics. I have a basic understanding of the math involved with my research, but would like to know more on nonlinear optics and why these materials behave the way they do. I am currently reading...
  43. Zack K

    Gravitational field strength at a point

    Homework Statement What is the gravitational field strength at a point 6.38x106 m above the Earth's surface? Homework Equations The Attempt at a Solution Ok so I plug in all the known variables. G= 6.67x10-11. M= 5.98x1024. r= 6.38x106 and so I multiply that by 2 because the given point is...
  44. A

    Force at a point by continuous charge distribution....

    Homework Statement This is more of a general question, but a simple example would be find the force on a test charge q at the center of a ring of charge with a total charge Q and a charge distribution given as λ(θ) =ksin(θ) where θ is measured clockwise with respect to the positive x-axis. The...
  45. K

    B Calculating Length Along X-Axis for a Rotating Line

    Hi (I hope this is the right place to post this - sorry if it's not) Say I have a line (in the graphic is shown a 10mm but it could be any length) and it rotates around a point at one end. What is the formula to get the length along the x-axis directly under the other end of the line. In the...
  46. JasMath33

    Using the Intermediate Value Theorem to Find Fixed Points

    Homework Statement Homework EquationsThe Attempt at a Solution I started looking at this problem and I think I am going to have to use the intermediate value theorem for this proof, but I am not quite sure. I started looking at possible examples of these functions, but I know this is not good...
  47. C

    Understanding the Direction of Electric Fields: A Question of Convention?

    Hello. I have a doubt regarding the direccion of the electric field in a point from a source charges. Let be a charge in the left of the plane. In a point to the right of the charge, the direction of the electric field vector is to the right. If the charge is negative, the direction of the...
  48. T

    Highest boiling point between organic compounds

    Homework Statement The following substances have approximately the same relative molecular mass. Which is likely to have the highest boiling point? Homework EquationsThe Attempt at a Solution I chose E as it has OH so therefore hydrogen bonding. Is this correct? Cheers!
  49. K

    B Constructing Space Without Points: Agree/Disagree?

    For sake of argument, let me define a point as a mathematical entity that occupies no space. Then, how can one hypothetically construct space, through addition of points, when each point contributes no space? Because adding points together consists of merely adding nothing to nothing, the most...
  50. F

    Yield Point: Why Can Material Elongate without Force?

    Homework Statement I was told that yield point is a point which the material which will have an appreciable elongation of yielding without any increase in load . my question is If there's no increase load , how can the material elongate ? Homework EquationsThe Attempt at a Solution IMO ...
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