Flat Definition and 496 Threads

Answer is probably not, but is there some connection between the inhomogeneous wave equation with a constant term and the spacetime interval in Minkowski space? $$ 1) ~~ \nabla^2 u - \frac{1}{c^2} \frac{\partial^2 u}{\partial t^2} = \sum_{i=0}^2 \frac{\partial^2 u}{\partial x_i^2} -...

Hi everyone! Brand new here. I'm sorry if this isn't the right place to post this but I am terrible with math/calculating anything so I thought I would reach out and ask! No harm done, hopefully. I'm building an 8' x 3' x 2" live edge walnut slab table and am trying to create some steel legs...

What is the coefficient of drag on a flat surface? Note: The body is in free fall The object has 2 wings planform (it's a paper helicopter), which is flat The object's wing spin in a circular motion (anti-clockwise) during free fall

From Ars Technica: https://arstechnica.com/science/2018/11/why-does-flat-earth-belief-still-exist/

Homework Statement A projectile is fired with an initial speed of 46.6 m/s at an angle of 42.2 ∘ above the horizontal on a long flat firing range. 1-Determine the maximum height reached by the projectile. 2-Determine the total time in the air. 3-Determine the total horizontal distance...

1. I am using FEA modelling for a vertical heated flat plate and I need to compare theoretical results with the FEA s/w modelling results. I have completed the model in FEA s/w and I'm getting different values for temperatures and because I can't work out theoretical values then I do not know...

Because I must be a masochist I would like to know how I could convince a flat earther that the Earth is a sphere. The way I've chosen to do it is the following. I live in Honolulu and the person I am trying to convince lives in south america. At an agreed upon time we both take a 50 cm stick...

Besides the Feynman lectures on gravitation, I'm looking for modern and complete treatments of the topic of classical gravitation as a field theory in flat space time. Any suggestion?

What sort of spaces are there that locally aren't necessarily flat? Do they have applications in physics? I agree this question seems naive, cause it seems we cannot say anything meaningful thing in such spaces, physically speaking. But I sense that for QG you would need to generalize the...

Homework Statement An impenetrable flat horizontal plate is radiated with 3000 W/m^2, where 500 W/m^2 is reflected. The surface temperature of the plate is 200 °C and it emits 500 W/m^2. Air with temperature 25 °C flows over the plate, and the heat transfer coefficient due to this is 20...

Homework Statement By hand, find the 4 square roots of 340 mod 437. (437 = 23 * 19). Homework Equations Chinese remainder theorem (CRT) The Attempt at a Solution So this is the wrong way I did it was first I solved ##x^2 \equiv 340 (\operatorname{mod} 19)## and ##x^2 \equiv 340...

1. Homework Statement Question attached 2. The attempt at a solution Time-like killing vector is associated with energy. ## \frac{d}{ds} (\frac{\mu^2\dot{t}}{R^2})=0## Let me denote this conserved quantity by the constant ##E=\frac{\mu^\dot{t}}{R^2}## where ##\mu=\mu(z)## . similarly we...

Homework Statement Homework Equations Energy band diagram equations. The Attempt at a Solution This is more of a conceptual question, but this is the problem I am working on for reference. What is the difference between a flat band diagram and an equilibrium diagram? I know what they will...

Homework Statement Homework Equations look up, look down, your pants fell down The Attempt at a Solution [/B] Time-like killing vector is associated with energy. ## \frac{d}{ds} (\frac{\mu^2\dot{t}}{R^2})=0## Let me denote this conserved quantity by the constant...

I am trying to explain to someone why there is still gravitational time dilation at the centre of the Earth. (Not to over-tax the rubber sheet analogy, but...) When we sketch a gravity well, we show a curve that starts out nearly flat, then curves away from the flat plane, until it reaches the...

Hi everyone, I work at a small high school that does one-on-one teaching with students who often have some kind of attention related learning difficulty or have trouble in a large classroom. The physics and chemistry courses I have been teaching have very inadequate lab sections and we have very...

Homework Statement Homework Equations F=ma F=Gm1m2/r2 Gauss' Law? The Attempt at a Solution I'm not sure if I should be using Gauss' Law for this question, because I've never heard of it or learned about it. I'm currently taking multi-variable calculus (gradients, vectors, etc.). From what I...

Homework Statement Two beetles run across flat sand, starting at the same point. Beetle 1 runs 0.50 m due east, then 0.80 m at 30◦ north of due east. Beetle 2 also makes two runs; the first is 1.6 m at 40◦ east of due north. What must be (a) the magnitude and (b) the direction of its second run...

Hello. I've been doing some reading about Friedmann's three types of GR solution that yield closed, open and flat universes. I think I can grasp the closed solution best. As far as I understand it a closed universe is finite in both time and space. It begins, grows and then collapses in upon...

Homework Statement Two large, flat metal plates are held parallel to each other and separated by a distance d.· They are connected together at their edge by a metal strip. A thirt plastic sheet carrying a surface charge \sigma per unit area is placed between the plates at a distance 1/3*d from...

I am trying to understand the derivation of the Casimir energy from https://en.wikipedia.org/wiki/Casimir_effect#Derivation_of_Casimir_effect_assuming_zeta-regularization. At one point, the derivation writes the following: The vacuum energy is then the sum over all possible excitation modes...

Homework Statement Killing Equation is: ##\nabla_u K_v + \nabla_v K_u =0 ## In flat s-t this reduces to: ##\partial_u K_v + \partial_v K_u =0 ## With a general solution of the form: ##K_u= a_u + b_{uc} K^c ## where ##a_u## and ##b_{uv}## are a constant vector and a constant tensor...

I thought about the geometry of Earth by definition and thought of the implications of 'curved space-time'. I understand a trajectory of a satellite and other objects to be straight geodesics through warped space-time. Would the Earth then therefore be able to described as a straight surface in...

Hi , please i want to know if i have flat plate (rectangle) but its fixed like pendulum (fixed from one end and free at the other end and can rotate around this fixed end) i want to know the drag force due to air on this plate?

Let's say we have a rolling without slipping (e.g. mentioned ball) on flat surface. Does it mean that the ball will accelerate to infinity ?

I am currently new in this forum and just a senior high graduate, so pardon me if I have some errors in writing this questions. I have a cylindrical pressure chamber with approximately 80 cm in diameter and the length of 1.5 m and metal wall 1 cm in thickness. I know most pressure chamber have...

I've been trying to understand how we know that the observable universe is flat, and I'm having difficulty finding any sources that explain exactly how the calculations were done. On this WMAP website (https://map.gsfc.nasa.gov/mission/sgoals_parameters_geom.html), it says: "A central feature of...

It's been said that the universe is flat. How can it be flat if flatness has no volume?

This is probably a stupid question but so as ##r \to \infty ## it is clear that ##-(1-GM/r)dt^2+(1-GM/r)^{-1}dr^2 \to -dt^2 +dr^2 ## However how do you consider ## \lim r \to \infty (r^2d\Omega^2 )##..? Schwarschild metric: ##-(1-GM/r)dt^2+(1-GM/r)^{-1}dr^2+r^2 d\Omega^2## flat metric ...

Let some point P on the y-axis and let the disk of radius "a" to lie on the z-x plane perpendicular to y axis. All charge elements in a thin ring shaped segement of the disk lie at same distance from P. If s denotes the radius of such a annular segment and ds is its width, then the area is...

Kansas I have been told has the flattest and biggest flat spot on earth. Any actual data on the size/area and flatness. Is it an outlier in Earth curvature. Most google info was junk. Looking for science data if this is even true. Posted here cos I am not sure if science or junk science.

If I push an object such as a cylinder of wood along a flat table (flat face of cylinder in contact with the table) through it's center of mass, the friction or energy required is not dependent of the surface area the block makes with the table, Friction = μ N, correct? And the energy required =...

I'd appreciate some help finding the monostatic RCS of a flat strip of length a. I have figured out the necessary expression for the scattered field but am having difficulties figuring out the requisite approximations for arriving at the desired sinc result. If I provided all the expressions...

Homework Statement Find the electric field at a distance z above the center of a flat circular disk of radius R Homework EquationsThe Attempt at a Solution My attempt to solve this was take the line integral from the center of the circle to the edge. Then, knowing the circle is symmetrical...

So I know what these are 4 translation : ##\frac{\partial}{\partial_ x^{u}} = \partial_{x^u}## 3 boost: ##z\partial_y - y \partial_z## and similar for ##x,z## and ## y,x## 3 rotation: ##t\partial_x + x\partial_t ## and similar for ##y , z## however I want to do it by solving Killing equation...

Homework Statement A fish watcher at point P watches a fish through a glass wall of a fish tank. The watcher is level with the fish; the index of refraction of the glass is 8/5, and that of the water is 4/3. The distances are d1=8.0 cm, d2=3.0 cm, and d3=6.8 cm (a) To the fish, how far away...

Would there be any way in a universe expanded into flat space, with no celestial objects visible, for a spacecraft to determine its velocity? It could know its acceleration was say 10 M/sec in a certain direction (though it could also be decelerating), and from this determine its minimum...

As the universe expands and is per definition gravitationally decoupled on long distances and the overall metric therefore is "flat" and apparently no gravitational background exists, the question in some discussion arose: Can GWs propagate in in a gravitational empty space at all? If not, and...

< Mentor Note -- thread moved to HH from the technical engineering forums, so no HH Template is shown > Hi! We are looking for alternative ways to heat up flat iron without using electricty or fuel or solar panel. So we came up with friction (as it is also a source of heat). But we don't know...

Hello everyone 1. Homework Statement I have a homework question where I need to find out if the geometry is flat or not. The metric is shown below. Homework Equations The Attempt at a Solution So far I have written the metric in the form guv but and I am trying to find coordinates in which...

Homework Statement A child pulls a trolley by the handle along a flat sidewalk. She exerts a force of 80.0 N at an angle of 30.0° above the horizontal while she moves the wagon 12 m forward. The force of friction on the trolley is 34 N. Homework Equations (a) Calculate the mechanical work done...

I wanted to calculate the best angle for a sail on a reach, but realized I had to make the calculation a bit unrealistic by using a flat sail and doing other simplifications to make it possible for me to solve. However it seems that wasn't enough ([emoji4]) since I ended up with a quite useless...

While Minkowski space and Euclidean space both have identically zero curvature tensors it seems that a flat Lorentz manifold in general, may not admit a flat Riemannian metric. Such a manifold is the quotient of Minkowski space by the action of a properly discontinuous group of Lorentz...

This is sort of SF in the sense that it's not scientifically accepted or reviewed. The "idea" comes from other people and it is that a usually round light beam of a certain diameter, like 30-50-100-similar cm diameter, extends slowly and even stops midair. It's flat at the end. Like a straight...

Why do we know space-time is locally flat,then we can use differentiable 4-manifold to describe the space-time?Do the Equivalence Principle say this?Classically space-time in GR is smooth,but what about if we consider the quantum effect?

Greetings All, I have a rather odd question which has been bothering me. If you have a perfectly round sphere sitting on a perfectly flat plane, what is the area of surface contact between the two? Is there an actual value, or is it something which can't be calculated. I'm assuming the diameter...

Hi, I'm trying to figure out the pressure that seawater would exert against a vertical surface (on average) over a large area but I am messing up somewhere. The height of the rectangle is 560 ft and the length is 20 miles, or 105,600 ft, giving an area of 2.12 mi². The weight of the water in...

I am having a hard time understanding flat band voltage in MOS capacitor. Please see the picture below about MOS capacitor. Q0 is a fixed effective charge at the surface between oxide and semiconductor and it is positive here. The voltage across the oxide caused by the fixed charge ψox =...

I'm doing research on why some galaxies and other phenomenon like Saturn's rings are flat. There seems to be two answers. Here they are in shortened form: 1) When the galaxy is forming, there is some net angular momentum in some direction.Over time, all particles with an angular momentum...

hi, I am just curious about, and really wonder if there is a proof which demonstrates that curvature tensor is 0 in all flat space coordinates. Nevertheless, I have seen the proofs related to curvature tensor in Cartesian coordinates and polar coordinates, but have not been able to see that zero...