# Tag Info

49

Hiroshima exploded 67 terajoules of energy i.e. $6.7\times 10^{13}$ joules. We may calculate the mass of the Russian meteor assuming that the speed is $v=20,000$ m/s: $$\frac{1}{2} mv^2 = 6.7 \times 10^{13}$$ We obtain 335 tons. The numbers aren't precise but they're in the ballpark and reasonable. The Russian academy of sciences actually estimates 10,000 ...

36

The moon does not fall to Earth because it is in an orbit. One of the most difficult things to learn about physics is the concept of force. Just because there is a force on something does not mean it will be moving in the direction of the force. Instead, the force influences the motion to be a bit more in the direction of the force than it was before. ...

35

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 ...

28

Briefly: Because the moon's orbit "wobbles" up and down, so it isn't always in the plane of the earth's orbit around the sun. There's a 2D plane you can form from the ellipse of the earth's orbit and the sun. This plane is known as the ecliptic. The moon's orbit is not exactly in the ecliptic at all times; see this (slightly overcomplicated) picture from ...

21

Correct. If you split the earth up into spherical shells, then the gravity from the shells "above" you cancels out, and you only feel the shells "below" you. When you are in the middle there is nothing "below" you. Refrence from Wikipedia Gauss & Shell Theorem. {I am using some simplistic terms, but I don't want to break out surface integrals and ...

19

I suspect the question may be unanswerable, and possibly even meaningless. As I understand it, the giant impact that resulted in the formation of the Moon would almost certainly have also completely liquefied whatever crust the Earth had at the time, producing a global "magma ocean". Thus, there would've been no traces of the impact left — or rather, ...

18

Those code names all come from certain catalog. For example, NGC means 'New General Catalogue'. There are various catalogs aiming at different objects, like stars, nebulae, galaxies, etc, but not for the Earth, at least not yet. You can find almost all known astronomical catalogs and tables at CDS

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 ...

16

The simplest way to think about it is that there is mass all around you in the center of the Earth so you get an equal gravitational "pull" from all directions. The pulls cancel out so you get no acceleration. If one assumes constant density for the Earth (which isn't strictly speaking true but it is close enough for this illustration) the gravitational ...

14

On the earth-sheltering question, the answer is yes, using material to increase the thermal mass of structures would work just as well on the Moon as on Earth. There might be minor differences due to different materials and lack of water in Moon soils but the general principal would still apply. As for the Moon's core still containing significant heat, ...

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 ...

13

The Moon's orbit is inclined with respect to the Earth's orbit. In other words, if you imagine a Sun, Earth, and Moon model sitting on a tabletop, the Sun would sit approximately still and the Earth might slide around the desktop, while the Moon would orbit the Earth, hopping up off the table, and sinking back down into it. (I used to do this demonstration ...

12

Because you were also in orbit around the sun with the Earth and still have that velocity. You may be imagining this in terms of stepping off of a slow moving vehicle on the Earth: you jump off, you come to a stop relative the ground and watch the trolley car go it's merry way. But that is a feature of friction between you and the ground. There is no such ...

12

The Moon moves at about a thousand metres per second, but it's a long way away so it only appears to move slowly. Most of the apparent movement of the Moon is actually due to the rotation of the Earth. We see it appearing to go round the Earth once a day, but it actually takes about 28 days to complete an orbit. The Wikipedia article on the Moon's orbit has ...

11

Since the Earth is much bigger than the Moon, (by about a factor of 4 in radius) its shadow at the distance of the lunar orbit is much larger than the Moon itself. So it is possible for the moon to spend considerable time in the Earth's shadow depending on the geometry of the eclipse. Since the Moon's orbit is inclined relative to the Earth's orbit around ...

10

The actual effects of a Gamma-ray Burst on the earth have been thought about a great deal. There is some thought that the Ordovician extinction was caused by a GRB (PDF Paper). This is also detailed in a couple of astrophysical journals. Astrophysical Journal, 2007: Gamma-ray bursts (GRBs) directed at Earth from within a few kiloparsecs may have ...

9

Jupiter has over 60 moons, and the dozens of man-made satellites (along with the thousands of pieces of space-trash) orbiting the Earth could be considered tiny moons. Earth's main Moon would disrupt the orbit of anything smaller at certain radii. The particular disrupted orbits are called resonances, and occur where the orbital period in question divided ...

9

An orbit isn't a fragile balance, where some small deviation can knock an object out of orbit. For an object at a given distance from a central body (the sun for the Earth, the Earth for an Earth satellite), any velocity less than the escape velocity results in an elliptical orbit...Earth won't spiral in or out because it's moving too slow or fast. If the ...

9

The most obvious interpretation of the phrase blow up the Earth is to dismantle it into tiny particles headed off to infinity. If you're prepared to accept this definition then the calculation is easy because it's (approximately) the gravitational binding energy for matter with the mass of the Earth falling into a sphere the size of the Earth. I say ...

8

The page to which you linked suggests that the Earth will explode due to global warming. What about Venus? Not only is it closer to the sun, but its greenhouse effect is an order of magnitude stronger than ours. Yet Venus has not exploded. The page also suggests that the Earth's core is some kind of nuclear reactor. It can't be a fusion reactor because the ...

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 ...

7

You're probably perceiving it incorrectly. The sky is not a flat surface; it looks like the inside of a sphere, and we tend to perceive it as a flattened sphere, with the zenith "closer" than the horizon. This perception is reinforced by the appearance of the daytime sky, in which overhead clouds really are much closer than clouds near the horizon. That ...

7

Imagine that you're sitting in a car that's sitting on a wide flatbed truck. The truck is moving down the freeway at, say, 65 miles per hour. You step out of the car onto the bed of the truck. You've just left an object that's moving at 65 mph, but it doesn't move away from you -- because you, the car, and the truck are all still moving at 65 mph relative ...

7

For an object in low earth orbit (at 100+ miles above the earth's surface) the speed needed is about 17,000 miles per hour. Even if a trebuchet could achieve that speed on the earth's surface, you would have at least three problems: The object would IMMEDIATELY burn up in our dense atmosphere. Think about the space shuttle which is going at orbital speed ...

7

Yes they do, and this is a great way to introduce perturbation theory. Exact solutions to n-body problems where n is greater than 2 are hard to find. It has been accomplished in the case of 3 and 4 bodies, and a general solution where bodies do not collide has also been found, but these problems are extremely complex. One way to get around this complexity ...

7

In the restricted three body problem, where you consider two objects orbiting each other, such as the sun and earth, and the motion of a third object that does not affect the movement of the first two, but is affected by their gravity, you can sort of figure out how far/fast from one object you have to be to not be orbiting it anymore. The picture above ...

7

No, the "skipping" effect does not work like this. It is not like a stone skipping on water. The problem is that before re-entry the capsule is travelling at a high velocity relative to the Earth and this means it is in a highly elliptical orbit. Actually it's in something approximating a Hohmann transfer orbit. If you did nothing to change this velocity ...

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