Flat Definition and 469 Threads

In this paper they state that a flat universe with no dark energy (##\Lambda = 0##) was favoured before the measurement results -- as they convey in the paper -- implies that the universe can not be flat and have a zero cosmological constant simultaneously. What reasons were there, before this...

Homework Statement N men, each with mass m, stand on a railway flatcar of mass M. They jump off one end of the flatcar with velocity u relative to the car. The car rolls in the opposite direction without friction. (a) What is the final velocity of the flatcar if all the men jump off at the same...

hi guys, why our solar system is flat? why planets are not revolving arround the sun as atoms revolve arround the nucleus? why galaxys are flat? why black holes are flat?

So I learned recently that pressure gradient in the flow direction for flow over a flat plate is zero. However I don't understand this, because there has to be something that sets the flow in motion in the first place, and for fluids this has to be a pressure gradient. Could someone explain why...

Hi, I have became confused about a "certain situation" belonging to Bernoulli Principle :a "wind on flat roof situation" more precisely. The big misconceptions on the net didn't make it easier. There are very contradicting explanations from experts on the web. However, I hope to...

Is space-time flat and if the answer is yes then how can objects orbit vertically and diagonally(since gravity is the warping of space-time)? Or does it exist from all sides?

In the FRW model with Euclidean 3-space, k=0 in the first Friedmann equation makes the density(times a constant) directly related to the Hubble parameter. The Hubble parameter is the time derivative of the scale factor divided by the scale factor itself. My question is: does a Euclidean...

Hello All, Pertaining to General Relativity, what do we mean when we tell a space is flat or space is curved? Is it that sum of the angles of a triangle = 180? Or is it something else. I was listening to a lecture of Prof.Susskind, when he takes a piece of paper, makes certain points on the...

Homework Statement A box of mass 72 kg is at rest on a horizontal frictionless surface. A constant horizontal force of magnitude F then acts on the box, accelerating it to the right. You observe that it takes the box 3.4 seconds to travel 13 meters. What is the magnitude of the force F? m =...

I have an idea about an extension cord that is flat and sticky like tape. This will cause less mess around the TV and will prevent anyone from tripping over any cords

So my question is, am I missing any key thing in answering the question below? Question: An electric flux of 149 N.m2/C passes through a flat horizontal surface that has an area of 0.67 m2. The flux is due to a uniform electric field. What is the magnitude of the electric field if the field...

Hello all, would like to ask what seems to be a general rule when designing power systems. In the power system design program which I just started to use. It is advised that: For every simulation to be run, it is best to have a "flat start" before running any faults. I'm not too sure...

I have been deriving multiple Riemann tensors for the past few days (in 2D for simplicity sake) and for some reason, no matter what functions I put in the vector of transformation properties, I always get all 0's for my Riemann tensors. Note: By vector of transformation properties, I am...

Homework Statement Consider a particle moving through Minkowski space with worldline x^\mu(\lambda). Here \lambda is a continuous parameter which labels different points on the worldline and x^\mu = (t,x,y,z) denotes the usual Cartesian coordinates. We will denote \partial/\partial \lambda by a...

I am trying to calculate heat transfer from an inclined flat plate under free convection conditions. I am referring to Fundamentals of heat and mass transfer (Incropera / DeWitt). The equations and a spreadsheet which can be used to find a solution can be found at...

Hello everyone, I am wondering how to properly go about calculating the stresses caused by bending on an angled flat plate. Two approaches seem valid but they give much different results: -Calculate the new area moments of inertia for the angled flat plate (new Ixx, ymax)...

hello, I am trying to find the new moment of inertia total for a rectangular stainless steel 4"x2"x.25"thk with a 3"x.75" flat bar welded on top of the short side of the rectangular tube. The short side (.75") of the flat bar will be welded to the center of the rectangular tube to the 2"...

There are numerous references to the basic equation for the torque capacity of a keyed shaft, but I can find absolutely nothing for a simple D-shaped shaft (ie a plain shaft with a single, plain, parallel flat milled on it) with a D-shaped hub. No set screw. It seems so simple but there's...

Hello, I've been using mathematica the past few days and quite like it. I am trying to add some figures to my word document. Because of the size of the exported emf file from mathematica, I am unwilling to have a document that is 100MB in size. So I was searching for some ways to export...

Is there a fundamental incompatibilty between a flat universe and the possibility of a Big Rip-type scenario of the evolution of the universe? Does a Big Rip necessarily imply a hyperbolic universe? What relationship is there -if any- between the shape of the universe and the equation of...

Mathematically, ##T_{\mu_\nu} = 0## ##\Rightarrow~R_{\mu\nu} - \frac{1}{2} R g_{\mu\nu} = 0## Now multiply both sides by ##g_{\mu\nu}## with definition of ##R = g^{\mu\nu} R_{\mu\nu}## ##R - \frac{1}{2} R = 0## ##R = 0## Is that my imagination wrong? I thought 'empty space' might not...

I'm just wonder what would happen for a physics project. I have looked everywhere and can't find anything helpful on the internet. Theoretically, I just need to find what would happen to a large humanoid object reaching a terminal velocity of 100-125 mph and falling down around 4km into said...

I'm trying to calculate the aerodynamic forces on a flat plate as a function of angle of attack. The drag force is calculated from F = ρ*V^2*A*Cd/2 Where ρ is the air density, V is the wind velocity, A is the area, and Cd is the drag constant. Simple question: For A, should I use...

Homework Statement A case is being pulled along horizontal ground by means of a rope. Show that the force required to keep the case moving at constant velocity along the ground is a minimum when the rope is at an angle θ to the horizontal, where: tanθ=\muk Homework Equations...

Hello, I am having a problem about the nature of the measurements of the intervals ds's forming out of infinitesimal displacements dx's of the coordinates and the actual meaning of the measurements of the same dx's, in flat metric spaces. I am certain that this must be a trivial problem...

In the Wiki article on the FLRW metric, http://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metric it says "the universe is nearly an isotropic and homogeneous FLRW spacetime". OK, so spacetime is globally flat, which implies that space is too. This...

Homework Statement (a) I hold a flat mirror 22.4 cm in front of my face. There is a freckle on my face 1 mm in diameter. Find the angular size of the freckle on the image of my face as viewed by my eye. (b) Repeat for a concave mirror which has a focal length of 39 cm. (c) What is the...

Hello friends , I have some conceptual problems in understanding the difference between Minkowski spacetime and the spacetime of general relativity. The general spacetime of GR is defined as a smooth manifold which is locally like Minkowski spacetime . What does this statement mean ? Does...

http://www.space.com...map-aas223.html Recent measurements suggest the universe it is flat and infinite The new results, presented by Schlegel and his colleagues here today (Jan. 8) at the 223rd meeting of the American Astronomical Society, also provide one of the best-ever determinations...

When rolling on a flat floor when the velocity at the circumference equals the velocity at the centre of mass the friction stops acting right? SO why does't that happen when it is rolling down an inclined plane? If it is rolling shouldn't the point in contact with the inclined plane be...

Here's a random question from the depths of my brain. :) Say that there's a sailboat on a flat, empty lake. Now imagine that on the boat, behind the sail there's a big electric fan. If the fan were to be switched on(blowing towards the sail), would the boat move forwards, backwards, or...

I am a little stumped with this question:( I have attached my solution so far which has just been to deduce the acceleration on the flat surface

Homework Statement There are 4 railway flat cars each of mass M traveling with speed v. They're not attached but move together initially without friction. A man of mass m starts at the end and jumps onto the second car, then to the third, and then on to the fourth.When he lands on the 4th it...

1. A wheel rolls down a flat inclined surface that makes an angle $\alpha$ with the horizontal. The wheel is constrained so that its plane is always perpendicular to the inclined plane, but it may rotate about the axis normal to the surface. Obtain the solution for two-dimensional motion of the...

is there a straight formula for finding one of the natural (preferably fundamental) frequency of a steel flat with dimension say, L=1000 mm, B=100 mm and Thickness T=10 mm ? The flat is fixed at both ends by bolts and two weights of equal magnitude are suspended at 1/4 and 3/4 (i.e. 250 mm and...

Hi, I can't visualize how the universe could be infinitely flat according to the big bang theory... The only way I can visualize this is like a cone surface, one dimension supressed and left with a circle line (the universe), the other dimension thus forming the cone, is time. Now, in a...

So the observable universe is flat overall but warped (compressed) in locations of gravitational fields. Is it the expansion of space which counteracts the gravitational warping thereby making it flat on average?

Homework Statement Find the electric field a distance above the center of a flat circular disk of radius R, which carries a uniform surface charge σ. Homework Equations The Attempt at a Solution Basically, I want to solve this usually trivial problem without using symmetry arguments and the...

I'm thinking about a flat sheet-like structure approaching a massive object (maybe a black hole?). If it were to approach with the plane of the object tangent to the mass, it seems to me that eventually, with enough gravity, the sheet would end up looking much like those hokey "bowling ball...

why call 'flat' field spectrograph ? what does the meaning of 'flat'. another question: if I what to focus the ray from a point source, how to do it? anything help would be highly aprreciated. best wishes and thank you!

I am confused about this. I learned that the frictional force is pointing towards the center of the circle in circular motion (on a flat road) due to the tires. The thing is, I don't understand what the result of this friction is. The road is flat, and the motion of the car is straight...

Suppose I have a parameterized line ##\phi:\mathbb{R}\to\mathbb{R}^n## given by ##\phi(t) = (x^\mu(t))|_{\mu=1}^n##. How can I tell that the line is straight. My best answer so far is that at every time ##t## the acceleration (2nd derivative) is parallel to the velocity (1st derivative), i.e...

Hi, I'm new and uneducated. Would like to know how to reconcile space-time being considered asymptotically flat near the event horizon and beyond, with the observable fact that stars do indeed orbit the black hole. I understand those orbits are not near the horizon, and I believe all the...

Do you use more energy cycling on, say, a 10 km flat stretch, or on a stretch where you spend the first 5km cycling uphill and the second 5km coasting downhill? What if you spend the first 1km cycling up a pretty steep incline, and the remaining 9km coasting down a more gentle slope? What if you...

Hello PF: I noticed a thread on PF in which TOM STOER and others were discussing how to calculate the redshift for an arbitrary metric. I need to talk to Tom if he is still on this list. The question has arisen in an applied physics field whether the following conformally flat metric...

I know clouds are just condensed water vapor forming tiny droplets. Why do they have flat bottoms and all at the same plane? Is their a air density threshold that clouds form at? Thanks simply curious.

So as of 2013 physicist are 99.6% sure that the universe is flat, all evidence points to a flat universe, but we live on a spherical 3D planet... I'm having extreme issues comprehending the definition of flat in regards to the universe. Does it mean flat as in flat like the surface of the...

In his book on GR, Wald writes "the notion of the mass and spin of a field requires the presence of a flat background metric...which one has in the linear approximation but not in the full theory..." Why do we need a flat background metric to define a field's mass and spin?

Homework Statement Hello, I posted a similar question in the physics section but no one was able to help, I am first going to include a link to the older problem where I was attempting to find the ,(Finding the local flat space of the Poincare half disk metric), and explain what is different...

Greetings everyone. Does anyone know of any flat pressure switches? I am trying to build a keyboard sewn into a shirt and have not found any suitable sensors. There are force sensors (like this one https://www.sparkfun.com/products/9375) but it would become too expensive for a full keyboard and...