I am working on a project for physics that involves tides. This is my current mind set when thinking about tides:

The earth applies a gravitational force on some mass M. The moon & sun apply a gravitational forces on the mass M away from the earth. The bodies of water on the earth have some acceleration towards the moon & sun due to their forces (F=ma). The bodies of water accelerate till they reach a sort of equilibrium between forces.

I would like you to poke holes in my understanding of tides and try and answer my naive questions bellow.

How would you describe tides effected by the moon using forces? How can that be used to calculate the height of tides?

  • $\begingroup$ Don't ask Bill O'Reilly. He was completely stumped by tides, and attributed them to an act of God. Lol! $\endgroup$ Commented May 15, 2012 at 5:34

1 Answer 1


It would be better to think in terms of water trying to flow until it reaches a equal-potential surface.

Like this. In the absence of competing forces from the sun and moon, the water doesn't rush down until the Earth stops pulling on it, it flows until there is nowhere for it to go because the other water is in the way. That happens when the surface of the water is all at a uniform value of the gravitation potential. (If it wasn't a bit of water could get "lower" by moving sideways from the "high" spot...)

The difference is that you have the Earth's gravitational potential, plus the pseudo-potential of rotation (it is only the centifugal "force" that you need worry about in this case), plus the tidal potential due to the moon and that due to the sun.


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