- #1
Neolight
As we know that most planets has two motions around the sun , one revolving and one rotation on its own axis. So what exactly is the reason for this motions to come into play.
There is an infinite number of values their angular momentum could have, so the probability of it being exactly zero goes towards zero. And even if a planet would end up with fixed orientation somehow, the tidal torque would make it spin until it reaches the tidally locked state:Neolight said:As we know that most planets has two motions around the sun , one revolving and one rotation on its own axis. So what exactly is the reason for this motions to come into play.
sophiecentaur said:It can be very hard to accept that angular momentum is conserved in ALL situations. It's easy enough with two large solid bodies colliding but it's harder when thinking of a cloud of widely spaced dust particles which are part of a spinning cloud and never touching. But the principle has been shown to apply without any exceptions, when measurable.
Neolight said:As we know that most planets has two motions around the sun , one revolving and one rotation on its own axis. So what exactly is the reason for this motions to come into play.
But the angular momentum value is, in fact, finite. And the probability of it being exactly zero is nonzero (but small). After all, everybody, including planets and stars, possesses an angular momentum which is either half-integer or integer... the latter including zero.A.T. said:There is an infinite number of values their angular momentum could have, so the probability of it being exactly zero goes towards zero.
So how small is it, given an infinite number of possible values?snorkack said:And the probability of it being exactly zero is nonzero (but small).
Why integer?snorkack said:angular momentum which is either half-integer or integer.
Finite number of possible values, for a finite value.A.T. said:So how small is it, given an infinite number of possible values?
Integer or half-integer. Because spin is quantized, in steps of half Planck constant.A.T. said:Why integer?
The angular momentum of a planet cannot be obtained by adding the spins of each of its constituent fundamental particles. You'd get a number that is not even in the right ballpark.snorkack said:Finite number of possible values, for a finite value.
Earth consists of finite and countable number of protons, neutrons and electrons. Obviously the total number of protons, neutrons and electrons in Earth must necessarily be either even (in which case Earth is a boson) or odd (in which case Earth is a fermion).
Integer or half-integer. Because spin is quantized, in steps of half Planck constant.
Yes. If the meteorite came from the West, the day would shorten. If from the East, it would lengthen.Timvanhoomissen said:Does this mean that if a massive meteoroid were to strike Earth, the length of a day on Earth would change?
I guess that an impact big enough to cause a significant change would produce such changes in the atmosphere as to make a minute or two of day length the least of our probs.Timvanhoomissen said:Does this mean that if a massive meteoroid were to strike Earth, the length of a day on Earth would change?
As has been pointed out previously, this is not even wrong.snorkack said:Simple note about the chance of not revolving:
The angular momentum of Earth orbit around Sun is about 1074 [no units]
andrewkirk said:Planets start small and grow bigger by other matter - gas, dust, rocks - being attracted to them by gravity.
Consider a rock - a meteorite - hitting a planet. Unless it hits the planet while its velocity points directly towards the planet's centre of mass (which will almost never happen), its velocity will have a component of momentum perpendicular to the line connecting the collision point to the centre of mass (circumferential momentum). That momentum has an associated angular momentum, equal to the circumferential momentum times the distance from the centre of mass. That angular momentum is transferred to the planet by the collision.
The spin of a planet reflects its angular momentum around its axis of rotation, which is the sum of the angular momentum received from all collisions of matter with the planet.
rootone said:A star and the planets which form with it are formed from a collapsing nebula of dust and gas.
That nebula has an intrinsic angular momentum.
That momentum (it can't go away) ends up being mainly distributed in the star and the other large bodies.
Hence the Star and the largest planets end up spinning in a similar way.
Although that nice arrangement can easily be disturbed by gravity of other stars which could pass nearby.
jbriggs444 said:As has been pointed out previously, this is not even wrong.
This assumes that angular momentum is always quantized. If linear momentum and position are not quantized then there is no reason to expect that angular momentum is quantized. As @andrewkirk pointed out:snorkack said:Derivation of Earth angular momentum around Sun:
Mass of Earth - 6*1024 kg
Speed of Earth on orbit - 3*104 m/s
Lever arm of the speed - 1,5*1011 m
So angular momentum: 2,7*1040 J*s
Planck constant: 1,05*10-34 J*s
Thus the quantum number of Earth orbit - about 2,5*1074, as stated before.
andrewkirk said:To put it crudely, all spin is angular momentum, but not all angular momentum is spin.
Planets rotate because of the conservation of angular momentum. When the solar system formed, the cloud of gas and dust that eventually became the planets was rotating. As the cloud condensed, it spun faster and faster, and this rotation was passed on to the planets.
The planets revolve around their own axis because of the same forces that cause them to rotate. As the planets formed, the material that made them up was already rotating. This rotation continues to this day, causing the planets to revolve around their own axis.
The speed at which planets rotate varies depending on their size and distance from the sun. For example, the Earth rotates at a speed of about 1,000 miles per hour at the equator, while Mercury rotates at a speed of about 800 miles per hour. Jupiter, being much larger, rotates at a much faster speed of about 29,000 miles per hour.
No, not all planets rotate in the same direction. Most planets in our solar system rotate in a counter-clockwise direction, but Venus and Uranus actually rotate in a clockwise direction. This is due to the way they were formed and the collisions they experienced during their formation.
Yes, the rotation of planets affects their orbits. This is because the rotation creates a centrifugal force that counteracts the force of gravity pulling the planet towards the sun. This results in a slightly elliptical orbit rather than a perfectly circular one. Additionally, the rotation also affects the tilt and stability of a planet's axis, which can impact its orbit over time.