Why the gravititational force between Earth and moon decreasing?

Question

The distance between Earth and moon had increased over a period of time, indicating decrease in gravitational forces between heavenly bodies...

Answer 1

It's the other way around- the gravity is decreasing because the moon is getting farther away.

This orbital decay is caused by the synchronous rotation of the moon- the tidal forces between earth and moon cause an angular torque. Energy and angular momentum are transfered from the Earth to the moon, decreasing the Earth's rotation and increasing the size of the moon's orbit.

To the posters above me: Are you toying with him? Or do you mean those answers? Because if you do, it's sort of like the blind leading the blind. You shouldn't answere a science question if you don't understand it.

Answer 2

Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (i.e. a moon), and the planet (called the primary) that it orbits. It causes a gradual yet slow recession of the satellite's orbit away from the primary, and a corresponding slowdown of the primary's rotation. The process eventually leads to tidal locking of first the smaller, and later the larger body. The Earth-Moon system is the best studied case.

Answer 3

Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (i.e. a moon), and the planet (called the primary) that it orbits. It causes a gradual yet slow recession of the satellite's orbit away from the primary, and a corresponding slowdown of the primary's rotation. The process eventually leads to tidal locking of first the smaller, and later the larger body. The Earth-Moon system is the best studied case.

The similar process of tidal deceleration occurs for satellites that have an orbital period that is shorter than the primary's rotation period.

Answer 4

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Answer 5

Tidal forces will distort any body experiencing differential gravitational forces. This will normally occur for bodies of finite extent in gravitational fields because of the strong distance dependence of the gravitational force. Thus, not only the oceans, but the body of the Earth is distorted by the Lunar gravity. However, because the Earth is rigid compared with the oceans, the "tides" in the body of the Earth are much smaller than in the oceans.

There is a limiting radius for the orbit of one body around another, inside of which the tidal forces are so large that no large solid objects can exist that are held together only by gravitational forces. This radius is called the Roche Limit. Thus, solid objects put into orbit inside the Roche limit will be torn apart by tidal forces, and conversely, solid objects cannot grow by accreting into larger objects if they orbit inside the Roche limit. A famous example is the rings of Saturn: because they lie inside the Roche limit for Saturn, they cannot be solid objects held together by gravitation and must be composed of many small particles.
Obviously solid objects can exist inside the Roche limit (for example, spacecraft) but they must be held together by forces other than gravity. This is true of a spacecraft, where chemical forces between the atoms and molecules are much larger than the gravitational forces.


The tidal forces are reciprocal. Not only will the Moon induce tides in the body of the Earth and the Earth's oceans, but by the same argument the gravitational field of the Earth will induce differential forces and therefore tides in the body of the Moon. Again, because the body of the Moon is quite rigid these Lunar tides will be very small, but they occur.

This reciprocal induction of tides in the body of the Earth and the Moon leads to a complicated coupling of the rotational and orbital motions of the two objects. These tidal forces and associated couplings have the following general effects:

The interior of the Earth and Moon are heated by the tides in their bodies, just as a paper clip is heated by constant bending. This effect is very small for the Earth and Moon, but we shall see that it can be dramatic for other objects that experience much larger differential gravitational forces and therefore much larger tidal forces. For example, we shall see that the tidal forces exerted by Jupiter on its moon Io are so large that the solid surface of Io is raised and lowered by hundreds of meters twice in each rotational period. This motion so heats the interior of Io that it is probably mostly molten; as a consequence, Io is covered with active volcanos and is the geologically most active object in the Solar System.

The tidal coupling of the orbital and rotational motion tends to synchronize them. In the simplest instance, the period of rotation for the two bodies and the orbital period eventually become exactly equal because of this tidal coupling (and as a result, the size of the orbit is changed in such a way as to conserve angular momentum for the entire system). This is called gravitational (or tidal) locking, because as the two objects revolve around their common center of mass each keeps the same side turned toward the other.

Thus, the fact that the rotational period of the Moon and the orbital period of the Earth-Moon system are of the same length is not an accident. Presumably this was not always true, but over billions of years the tidal coupling of the Earth and the Moon has led to this synchronization. In the case of the Earth-Moon system the synchronization is not yet complete. The Earth is slowly decreasing its rotational period and eventually the Earth and Moon will have exactly the same rotational period, and these will also exactly equal the orbital period. At the same time, the separation between the Earth and Moon will slowly increase in just such a way as to conserve angular momentum for the entire system.
Thus, billions of years from now the Earth will always keep the same face turned toward the Moon, just as the Moon already always keeps the same face turned toward the Earth. We will encounter other examples of such tidal locking in other pairs of objects in the Solar System.

Answer 6

The earth is actually compressing slowly over millions of years. It was larger way back when. The shrinking of the core also affects the level of gravity. Look at our solar system. The planets that are bigger have more gravity. The ones that are smaller have less.

Answer 7

the rotational speeds are beginning to repell the two?
or it could be that the magnetic polarity of the earth is starting to change again--therfore causeing a sepperation of the two

Answer 8

http://www.talkorigins.org/faqs/moonrec....
http://en.wikipedia.org/wiki/tidal_accel...