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Mahbod|Druid
- 22
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Hello
why is moon getting away from earth about 2-3 centimeter/year ?
why is moon getting away from earth about 2-3 centimeter/year ?
can we estimate when the moon will have flown out of its orbit?
can we estimate when the moon will have flown out of its orbit?
The Moon generates tides, which involve the movement of vast quantities of water. The movement of all that water causes friction, which leads to the release of energy as heat. The energy actually comes from the Earth's rotational inertia. The tides are literally slowing down the Earth's rotation.
Because the Earth's rotation is slowing, the Moon is moving further away. It must do so to converse angular momentum.
- Warren
The friction mentioned by chroot causes the tidal bulges to move forward relative to the Moon. The gravitational force exerted on the Moon is then a bit larger because of these bulges that are propelled a bit ahead of the Moon. This causes the Moon's velocity to be a bit higher than it should be, causing the Moon to move further away from the Earth.
Note that by action is minus reaction, the Moon pulls has hard on the bulges as the bulges pull on the Moon. This means that in addition to the friction forces exerted by the Earth on the bulges there are the tidal force exerted by the Moon on the bulges as well. Because the bulges are moving at constant velocity with the Moon, there is equilibrium: The tangential component of the gravitational force exerted by the Moon on the bulges is the negative of the friction force exerted by the Earth.
The moon does not move faster in higher orbits, it moves slower. The circular orbit equation: [tex]\frac{v^2}{r}=\frac{GM_e}{r^2}[/tex] therefore [tex]v=\sqrt{\frac{GM_e}{r}[/tex] clearly shows this.
This is because of the virial theorem. The energy boost given to the moon by the tidal forces increases its total energy (remember total energy is negative). According to the virial theorem: <V>=-2<T> so that E=<V>+<T>=-<T>. As E increases (gets less negative), <T> must decrease - i.e. it gets slower. Twice the energy boost is going to the potential energy so the kinetic energy must decrease by one times the energy boost for energy to be conserved.
I suppose on short enough time scales, the moon may not be in virial equilibrium and "speed-up" a little bit before being slowed back down again. On longer time scales, though, the moon should be slowing.
That's not quite right. Yes, the Moon is moving a bit faster than it would if the bulges were not present. However, that is not what is causing the Moon to recede. Suppose the Earth was tidally locked to the Moon and was covered with an all-encompassing ocean. The tidal bulges would be perfectly aligned with the line between the Earth and Moon. The Moon would still be moving a bit faster than it would were the bulges not present -- and the Moon would not be receding.That's right, the Moon is indeed slowing down as it moves away from Earth, but that happens because of the extra energy added which leads to the Moon always moving a tiny bit faster than the speed it should move at if it were not for the tidal bulges.
Because the Martians are pulling harder!
I'll leave a better explanation to someone else, but by simply looking at the size of the craters on the moon and thinking about the size of the objects that caused them, do you think LCROSS would have a noticeable effect?
^^Here is this from the Nasa site. How can I reasonably deduce what effect the mission had on the moon by just thinking of all the other "big impacts" which created a creater?The LCROSS mission is a search for water on the moon. The LCROSS mission is going to do this by sending a rocket crashing into the moon causing a big impact and creating a crater, throwing tons of debris and potentially water ice and vapor above the lunar surface.
I started another thread on this. https://www.physicsforums.com/showthread.php?t=406050" I'm actually looking for the "better explanation" that someone else can offer. Someone else answered me in the same manner you have which tells me nothing in essence. I am looking for a more scientific answer with equations and the like.