centripetal vs centrifugal again.

If the moon's force on the earths water (molecule) is exactly equal to the centrifugal force (on that molecule) then there should be no movement (tide) so why is there a tide. During spring tide, why do both ends of the earths water bulge (high tide)

 

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If you draw a line through both the Moon and Earth, there is only one point where the pull of the Moon's gravity exactly cancels out the centrifugal effect of the Earth revolving around the Moon-Earth barycenter, and that would be at the center of the Earth.

At the Point of that line which intersects with the surface of the Earth facing the Moon, a water molecule is tracing out a slightly smaller circle, and has slightly smalled centrifugal effect, while at the same time is closer to the Moon and is pulled on more strongly by the Moon. This results in a net pull towards the Moon and the moon facing tidal bulge.

At the point of the line that intersects the surface of the Earth facing away from the Moon, a water molecule is tracing out a larger circle and is undergoes a larger centrifugal effect. At the same time, it is further from the Moon and is pulled on more weakly by the Moon's gravity. The centrifugal effect dominates, causing a net pull away from the Moon, and resulting in the tidal bulge on the side opposite from the Moon.

 

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To Janus58:
That is so clear. Much better than the typical explaination that discusses the gradient of the moon's gravity.

 

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