moon's gravity

[johnmeds]

IF the moon causes the tides by it gravity pull on the ocean.....then why does it not pull lighter items such as dust, feathers etc towards it too?

[gen2]

It only works in fluids(liquids and gases) which can reach a hydrostatic equilibrium. The moon effectively cancels out a portion of the pull of the Earth's gravity on the water in one part of the ocean. The greater pull elsewhere pulls the water closer to the Earth and it bulges up where the pull is less. A bit like squashing a ballon in at one part and watching it bulge out elsewhere.

The hydrostatic force is given by mgh. So, if g is reduced, h must increase to maintain equilibrium.

In actual fact, the tides are much smaller than we experience. The movement is anplified by the resonant sloshing from side to side of the Atlantic and the tides are actually smaller in the Pacific. Estuaries and channels also amplify tides.

[heathfield]

The moon does pull these lighter items towards it. It's just that the effect is so small that it's not noticeable.

[JudgeJ]

All objects on the earth (and indeed the earth itself) are affected by the gravitational force of the moon. However, two things need to be borne mind:

1. The earth has a mass of almost 100 times that of the moon, and this means that its total gravitational effect is 100 times greater.

2. (Far more important) is that the effect of gravity decreases as the square of the distance from the centre of the body. An object on the surface of the Earth is about 4,000 miles from the earth�s centre but about 240,000 miles from the centre of the moon (about 60 times as far). So of the two gravitational forces felt by an object on the surface of the earth, the force imposed by the Earth is about 360,000 (60 squared x 100) times as great as that imposed by the moon.

All bodies which are subject to the gravitational effect of another body are prone to tidal forces. This is a stretching effect which elongates the body in line with the gravitational force. It is caused by the difference in the gravitational effect apparent on the near side and the far side of the effected body. This causes the effected body to distort from its spherical shape. It is easily seen on earth because the oceans are fluid and so more easily distorted.

[Zevon]

Gravity has an infinite range but quickly loses its power, there is an equation for the effect of two bodies on one another:-

The force due to gravity (F) between 2 masses (M1 and M2) is related to the universal gravitational constant (G) spearated by a distance (d) is given by:-

F=G*(M1*M2)/d-squared

Where the product of the masses is divided by d-squared and then multiplied by G.

Therefore although the moon's gravity and the sun's gravity act on Earth they are further away than the centre of the Earth therefore the distance is greater and if M2 is the mass of earth then it is constant. This is why the sun has a stronger effect on earth than the moon M1 ius much greater despite the increased distance.

However for small bodies on earth the distance is not from body to the centre of earth but distance from the body to the smaller object. M1 is the same as before for the sun or moon. This is why the sun and moon have a small effect on Earth locally, e.g. us and footballs because the mass (M2) is less. But the moon has larger effect on the oceans, M2 is greater for oceans so the force is greater.

Also during eclipses the tides are stronger as both the sun and moon's gravitational forces combine.