# Tag Info

63

The Foucault pendulum is a great experiment which does demonstrate that the Earth is rotating, but it was only introduced in 1851. The Earth had been known to rotate for several centuries before that, probably stimulated by Copernicus and Galileo pushing the heliocentric model of the solar system during the 16th century. A couple of decades before Faucalt's ...

45

Foucault pendulum. I don't know how the ancients did it, but it is surely pure classical mechanics. The animation describes the motion of a Foucault Pendulum at a latitude of 30°N.

36

Barring whatever fantastic energies would be required to stop the mass of the Earth from rotating and then changing the direction of the rotation, one of the major things I can see changing would be the expectations of weather patterns. Part of what affects our weather is known as the Coriolis Effect. While there would certainly be effects from the ...

33

The sun does rotate. We can see the rotation of the sun by the doppler shift of the light we get from the sun. . (Image from this page.) Since we know the characteristic spectrum of light from a hot body of a given temperature, we can use the same effect to determine if other stars rotate as well. Note that this only gives the spread in velocities along ...

25

Yes, the sun and nearly all other stars do rotate. One can see the rotation of the sun by looking at the motion of sunspots on its surface. Over time, the sunspots will move across the sun's surface - proof of its rotation. Furthermore, the rate of the sun's rotation is not constant throughout the sun; it is higher near the equator and slower near the ...

18

The day/night cycle is the first obvious effect: Days and nights would both be longer (EDIT Or shorter, apparently I got it backwards) if the speed of rotation is the same. All life on the surface of the planet has evolved for day/night cycles of roughly the length we experience now, with leeway for the difference between summer and winter. A sudden ...

15

I think the Foucault pendulum is the best answer, but for the sake of variety I'll add another very interesting one: the equatorial bulge affecting the figure of the Earth. This is the "pancaking" of the planet due to its rotation. You can measure the geometry of the Earth without leaving its surface, and find that it is bulging in accord with your ...

14

Well, for the basics of the Foucault pendulum, this wikipedia page does an adequate job describing how it works (Specifically read up on the Precession as a form of parallel transport section). This page also has a nice explanation of how they work: A Foucault pendulum is just like any other pendulum, nothing more than a weight attached to a wire; but ...

12

An indirect indication that the Earth rotates is the fact that the rotation varies over time. First of all, the orientation of the Earth's axis changes: long-term effects like precession and slow variations in the axial tilt, as well as small short-term variations like nutation. Precession was already known in the Ancient world (Hipparchus, Ptolemy,...) and ...

10

Given the rather large volume of the universe, I suppose it's possible. Not as an initial condition as far as I can tell though because of the conservation of angular momentum. However, given the right circumstances of impact events on a rogue planet (with no other bodies to perturb its non-rotation), I suppose it's possible. Highly unlikely, but ...

9

This is a note on why angular velocities are vectors, to complement Matt and David's excellent explanations of why rotations are not. When we say something has a certain angular velocity $\vec{\omega_1}$, we mean that each part of the thing has a position-dependent velocity $\vec{v_1}(\vec{r}) = \vec{\omega_1} \times \vec{r}$. We might consider another ...

8

Your question will eventually lead you to Mach's Principle. It is an old, yet unsolved question, that still remains at the stage of "philosophical idea". I understand that your question is equivalent to "What would be found if we could measure all effects on the pendulum with infinite accuracy?", what if even the tiniest contributions could be registered? ...

7

First of all, let's calculate the rotational velocity of the Earth, at the equator. The diameter of the earth is 12,756 km. Therefor, the speed is ${12,756km*\pi}/{24 hrs}=531.5 km/hr$. Given that we know the rate of spin of the Earth, what else would be required to see this change? The point of reference would need to be relative to the Sun, or possible ...

6

There are actually several different ways to interpret that question, depending on what you mean by "vector" and "rotation". But here's a sense that I've often wondered about myself: in introductory physics, the velocity vector is defined as the time derivative of the position vector (relative to some fixed point). Why is the same not true of angular ...

6

Defining properties of vectors are that you can add them and multiply them by constants. These both make sense for angular velocities. On the other hand, adding rotations doesn't make sense. What you can do with two rotations is compose them: first rotate one way, then rotate another. This operation doesn't look like addition of any sort. For one thing, it ...

6

Your argument is actually more or less right, but some of the details are wrong. First you have to realize that Newtonian mechanics and general relativity have different definitions of an inertial frame. According to Newtonian mechanics, the coffee cup sitting on my desk right now defines a (very nearly) inertial frame, but a falling rock is extremely ...

6

One way to explain it that makes sense to me is that the backwards spin on the ball is a force pushing the ball toward you. The one time applied force of your finger that creates the backward spin also pushes the ball forward, but the backspin stays almost constant. At the moment you spin the ball, the forward force is greater then that of the backspin, so ...

6

According to this article, most spiral galaxies rotate with the spiral arms trailing. NGC 4622 appears to be an exceptional case, probably because of a past interaction with another galaxy. Interestingly, the spiral arms seem to be transient phenomena, like density waves. A star in an arm doesn't necessarily move along with the arm; instead, the arm is a ...

6

There is a substantial literature on this positivist point of view. The key to looking it up is "Mach's principle". Wikipedia has a page on it and the Stanford Encyclopedia of Philosophy discusses the question amongst other issues on a page on "Early Philosophical Interpretations of General Relativity". If you want to go to the opera on the question the SEP ...

6

According to this Scientific American article, the Earth's rotation rate just after the collision that formed the Moon was about once every 6 hours. At that time, the Moon would have been about 25,000 kilometers away. The tidal effect of the Moon is the major reason the day has been lengthening, and the Moon's orbit has been widening. The collision is ...

6

It appears that both existing answers refer to the cumulative effect of rotations on the orientation of the ball and not to the total angular momentum achieved. As has been pointed out in comments, it's impossible to achieve vertical angular momentum by pushing around a horizontal axis. This is because pushing around a horizontal axis applies a torque around ...

5

During the flight, you need to get up to use the restroom. There's one 10 rows in front of you, and another 10 rows behind you. Does it take longer to walk to the one that's moving away from you at 600 mph than the one that's moving towards you at 600 mph? No, because you're moving at 600 mph right along with it -- in the ground-based frame of reference. ...

5

In a frame of reference attached to the surface of the planet, everything far away (other planets, stars, distant galaxies...) follows a circular (or nearly) path with a period of 24 hours. These paths pose two problems They involve observed accelerations with no obvious forces causing them Any of these bodies more than $24/2\pi$ light hours away are ...

5

The effect on a Foucault pendulum due to the earth's orbital motion is not just small, it's zero. You can't analyze the orbital motion the same way as the rotation of the earth about its axis. The earth free falls around the sun. A Foucault pendulum isn't free-falling around the center of the earth. One way of stating the equivalence principle is that in a ...

5

There's a nice subtlety that hasn't been addressed yet. First of all, a Foucault pendulum is influenced by The rotation of the Earth around its axis. The change in the orientation of the Earth's rotation axis: precession, nutation and long-term changes in the obliquity of the axis. In principle, it is also influenced by changes in the local gravitational ...

4

The boiled egg wins hands down when there is friction. The internal degrees of freedom of the liquid in the raw egg will affect its rotational motion and increase its friction and absorb part of the energy into internal motion. Two sliding eggs will retain the same velocity if there is no friction and initial rotation, they will not roll so it is a tie. If ...

4

I'm not sure if you want an object that doesn't spin at all, or one that somehow doesn't spin on its axis. In the former, any structure large enough (e.g. superclusters of galaxies) that its dynamical time is longer than the age of the universe is effectively not rotating. For chaotic spinning (often called tumbling), one example is Hyperion, a small moon ...

4

The Wikipedia page on Sun gives these three velocities, ~220 km/s (orbit around the center of the Galaxy) ~20 km/s (relative to average velocity of other stars in stellar neighbourhood) ~370 km/s(relative to the cosmic microwave background) So my inference is that 220km/s is the estimated orbital velocity. It is not constant velocity because the orbital ...

4

Well it is quite expensive to go into space and so taking a video camera for some holiday snaps is not exactly a high priority. However weather sattelites do exactly this with images of cloud formations and movements. Obviously these aren't typically very glamorous so rarely make into the public domain. However they did recently take a camera on board the ...

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