Flat Definition and 496 Threads

What does it mean in general relativity that space is locally flat. It means that in neighbourhood of each point we can chose such coordinates that the metric is flat or we can do it only in a point (not in neighbourhood - open set).

I recently saw a YouTube video of a 2009 lecture by Lawrence Krauss. He says that we now know unambiguously that the universe (and I just mean the universe that we can see, back to the big bang, not the multiverse or the foam or any of that) is flat. But it seems to me that a flat universe with...

Homework Statement Dear all, I am trying to find find out the shape of a drop on a flat surface. The requirement is the constant volume of the drop. Any help is greatly appreciated! Homework Equations The Attempt at a Solution I know that for the solution I must find out the...

Sorry 'bout posting so many topics but there are too many things that are unclear to me. CMBR measurements suggest the universe is pretty much FLAT, but I don't see it as flat, and forget our planet, all those vast spaces in every spatial dimension - all that is flat? It obviously has depth...

I've been watching http://www.youtube.com/watch?v=Pm5ROyoaMZA" on youtube where it was shown that a cone has curved space (from about 1:05:00). Using the same method of parallel transportation it seems to me that every 3d shape, other than a plane has some points of curvature. However the same...

Why all two dimensional manifolds are conformally flat? Why all manifolds with constant sectional curvature are conformally flat? Does anyone know proofs of above statements. Thanks in advance.

Homework Statement Flat capacitor with a plates distanced d = 2 cm and the area A= 1 dm² are connected to a voltage U = 100 V. We disconnect the source of voltage and we increase the distance between the plates to twice the distance. How much work is done? Homework Equations C= ε*A/ d...

Homework Statement - How much is the electrical voltage U between two evenly electrified parallel flat plates, distanced d = 1 cm, if the strength of electric field between them is E = 1 V / m? - What is the strength of electric field to the left and to the right of each plate? - What is...

I think I read on these forums that a small piece of relatively flat 4D spacetime of General Relativity can be embedded in ten dimensions. What happens if the small piece of spacetime is not very flat, does this change the number of embedding dimensions required? Thanks for any help!

I recently did an experiment to measure the activity of a radioactive source, with the idea that the activity can be found by treating the detector area as part of the surface area of a sphere. In the actual experiment, we plotted u = \frac{n d^2}{\Delta t} against d, where d is the...

Hi everyone. I have studied Lanthanum Hexaboride (LaB6) cathodes the last few days, but I have a few questions regarding them. There are two types of these cathodes; the flat one and one which is in the shape of a cone. I have attached two documents which handles about these two types of...

I was wondering what equiptment would be best suited to using multiple strian gauges on specific points on a flat steel plate with a central hole in tension. What is the simplest experimental set-up to obtain reading from the gauges? Any points/links would be appreciated.

This is a concept question from a previous final physics exam. The correct answer is (b) and my professor confirms this, but I can't understand why the answer is not (d) since the question seems to imply that the ball is already in rolling motion and that friction would not play a part. A...

Homework Statement I want to be able to calculate how far a ball has traveled when rolled along a flat surface at each second. Lets say the values are 2kg for ball, the force is 100N and co-efficient of friction is 0.2. Homework Equations F(n) = -mg a = f/m The Attempt at a...

Hello all, I have a simple question about copper windings on generators. From what I have read, magnetism only penetrates the skin of a copper wire by very little. Dosnt this mean that most of the copper in a winding is wasted ? And wouldn't it be more efficient to use flat wire "-"...

Homework Statement The problem is Hartle 18.3 and reads: "Consider a flat FRW model whose metric is given by (18.1). Show that, if a particle is shot from the origin at time t*, with a speed v* as measured by a comoving observer (constant x,y,z), then asymptotically it comes to rest with...

Homework Statement In a circular flat track of radius 50m, what is the maximum speed car can go without sliding? Assume the coefficient of static friction between the car tire and the road is 0.4. Homework Equations Unsure of what equations to use. The Attempt at a Solution Unsure...

I might have to move from my cottage to a flat in a town near by, so i can be near to services not available where i live now, gone will be my vista of the hills . the quiet country life and quirky cottage

Hi All, I have a question about the agitator tank. What is the different between dish-bottom, flat-bottom and cone-bottom tank? Regards.

1. A 10 kg box on a level surface is attached by a weightless, unstretchable rope to a 7.0 kg box which rest on a 30 degree incline. Find the rate of acceleration and the tension in the rope if... a) there is no friction ... b) coefficient of friction is 0.10 for both boxes 2. F_net...

Homework Statement We are looking at an isotropic flat universe, with the metric ds^2 = dt^2 - b(t)^2(dx^2 + dy^2 + dz^2) I need to write down the energy conservation equation \frac{dV}{V} = -\frac{d\epsilon}{\epsilon + p} We have been given the solution to be 3\ln(b) = -\int...

A curved space and be flattened out locally, giving a flat "subspace" while increasing the curvature around that "subspace", right? If so, wouldn't it be possible to make any curved space flat everywhere and concentrate all the curvature at a single point and move that point infinately far away...

If geometry of spacetime is flat in intergalactic medium (empty space), why then there is no point of asking where is the center of Universe?

Why does Saturn's dust form a flat ring and not a sphere around it?

1. The present universe is observed to be flat. Was it always flat, before it started its accelerated expansion? 2. Is the Riemann tensor zero for this flat universe? Is its geometry that of special relativity? I'd appreciate if Marcus or Ich or any other science advisor weighed in on...

Homework Statement We have a tank of air at 7 atm, 30 L, and 293 Kelvin(20 Celsius), and a flat tire with 1atm, 10 L (Constant), also at 293 K (20 C). The valve is opened quickly and closed quickly when the pressure in the tank reaches 6 atm. Using the Ideal gas assumption and taking the...

If there is topic "Is Our Universe Flat" poin me. I bet:) no

I'm trying to make a computer simulation of ship. Currently I'm working on a force of wind on the ship sail. This is the page which describes the basics: http://www.animations.physics.unsw.edu.au/jw/sailing.html Basicaly, wind (W - vector) has some staring direction and then that...

Rencently, I found myself confused by some fundamental concepts in GR. I hope someone can help me with that. We all know the vacuum Einstein equation (VEE) without the cosmological constant \Lambda is Rab=0. Since I learn GR the words " matters bend the spacetime " been told again and...

http://theflatEarth'society.org/forum/index.php?PHPSESSID=61505d6b5883a9d597b57940b118119b&topic=13876.0

The size of bouncing lumps of charged particles, DM and photons in the primordial universe is supposed to be around 220,000 lyr (= sound horizon since the acoustic speed is approx 0.6c). If we then calculate the angle A subtended at Earth by a lump of this size W, some 13.8 Glyr away D (back at...

There is a scientific word/explanation for the phenomenon that a out and back course in a cycling time trial race can actually be faster than a flat course. For example, in Cherokee Park in Louisville, there is a 10 kilometer course that starts on the flats, then has a pretty good downgrade...

1.A hurricane wind blows across a 7.8 m x 12 m flat roof at a speed of 180 km/hr. What is the pressure difference? How much force is exerted on the roof? 2. Homework Equations P= p_atmos + pgd P=F/AThe Attempt at a Solution p = 101300Pa - (1.3)(9.8m/s^2)(93.6m^2) p = 100107.5Pa 100107.5Pa =...

So, I now understand based on things that I have read, and things that people on this forum have told me, that the universe if flat, must be infinite. My question is, when we look into the farthest reaches of space, we see a small, finite baby universe. How is this possible? The universe can...

Homework Statement A car of mass M = 1200 kg traveling at 40.0 km/hour enters a banked turn covered with ice. The road is banked at an angle theta, and there is no friction between the road and the car's tires. What is the radius r of the turn if \theta = 20.0 degrees (assuming the car...

I know this is a commonly adressed topic, but it is one in which things are hazy to me. If the universe is finite (which we don't know), and flat (which we are becoming ever more increasingly certain about), then how could a light beam fired from a point in the universe not ever reach its...

For understand general relativity, they tell me, there have flat space, add a sun, the flat space changed to curved space, so the Earth move around. There has a question, if we admit mass, space , time is created, how could exist flat space? there have space-time and there have nothing?

[b]1. The problem statement: a 100kg homogeneous cylinder of radius .3m. Starts at rest. Is pulled by a string wrapped around the cylinder coming off the top with a force of 500N. Find the angular velocity after the cylinder has rolled one revolution? The attempt at a solution: I=...

If someone were to remove all of the gravitational influences on a portion of space would that essentially make that portion of spacetime flat? (Just curious):-p

Homework Statement Two optically flat plates of glass are separated at one end by a wire of diameter 0.250 mm; at the other end they touch. Thus, the air gap between the plates has a thickness ranging from 0 to 0.250 mm. The plates are 15.0 cm long and are illuminated from above with light...

How do you take the moments of inertia of a flat body? I know howto take it if it's a 3d body. And the 2d case should be really simpe,but I'm too stupid to figure it out. Can you help me? For example.. Say we have a body that's a rectangle of mass m on |x| < a, |y| < b..? Thanks so much.

Homework Statement A car, mass m=2500kg, rounds a curve on a flat road at a speed v= 15 m/s. The radius of curvature of the curve is r= 60m. There is obviously (static) friction between the road and the car tires, or the car would not stay on the curve. Homework Equations a.)...

Hi gang, I've been reading for a while and now need some help of my own. Thanks in advance. I'm currently planning a project where I'm going to demonstrate a launch of an RC car off the flat roof of a building. The height of the roof is 25'. I figure the RC car will go 25mph, and the ramp...

Is it possible to define gravitons corresponding to flat spacetime? If so, how? Flat spacetme has zero curvature. How does one mathematically describe gravitons for flat spacetime?

hi to everyone! i want to ask that, why it is easier to swing a linear flat stick rather than any other heavy stick or a baseball stick ? what is the reasion of this question in physics point of view?

http://www.news.cornell.edu/stories/March06/CMB_Timeline300.jpg If space-time is flat (on large scale), why then does this [above] picture show curves? Would I be right to say that space time at anyone moment is flat but over time it is curved?

Hi there, I am a physics idiot, but I still want to know where my object will end up ;) Here is what I am trying to do: I have a block that is sliding on a flat surface. I know the friction coefficient, the mass of the block, its initial position, gravity and initial velocity. How...

Hello everyone; here is the problem that I’m currently working on: =============================================== a- Find the principal moments of inertia of a flat rectangular plate (mass = 30g, a = 80 mm, b = 60 mm) that rotates about a diagonal with velocity ω = 15 rad/s. b- What are...

1.a) Concider the 2-space consisting of a spherical shell at constan radius, r. In polar coordinates the line element on the surface can be written (a, b,∈ 1,2) Homework Equations ds2=gabdxadxb=r2dθ2+sin2θdφ2 calculate gab, Γabc, R1212, R2121, R11, R22, R The Attempt at a Solution...

I have been reading John Baez's introduction to General Relativity (http://math.ucr.edu/home/baez/einstein/einstein.pdf). This part got me thinking: My question is, in order to completely model our universe's (3+1)-dimensional spacetime as a manifold in a higher-dimension, flat spacetime, how...