# Where does the energy to create tides come from?

1. Dec 15, 2009

### Jahn89

Hi, I wonder if any of you can help, probably a very simple answer, but here goes...

The gravitational pull of the sun and the moon cause the tides, correct? I also understand that energy cannot be created or destroyed, also correct?
Since the orbit of both the sun and the moon are constant, i.e not decreasing in radius or speed,and the mass of both could be assumed to be fairly constant I would assume no energy is being taken from these systems.
Therefore my question is, where does the energy come from to move the seas and oceans?

Thanks

2. Dec 15, 2009

### uart

The energy is coming from the Earths rotation. Tidal friction is causing the Earths rotation (about it's own axis) to slow and hence for the length of a "day" to gradually increase.

Last edited: Dec 15, 2009
3. Dec 15, 2009

### Staff: Mentor

Right answer to the wrong question. The energy is being dissipated by slowing the earths rotation but the op is asking where the energy is coming from. And the answer is that it is stolen from the moon's orbital energy.

4. Dec 16, 2009

### uart

No. The action of the tides actually causes energy to be transferred from the Earths rotational energy to the moons orbital energy. Because the earth rotates faster (lower period) than the moon orbits it causes the tidal bulge to always move slightly ahead of the line joining the earth and moon centers. This in turn causes the gravitational pull to be between the earth and moon to have both a radially directed and also a small orbitally directed component (due to the slight "dumbell" shape of the earth with tides). The orbitally direct component provides a drag on the earth's rotaton and at the same time actually imparts orbital energy to the moon.

So the net effect is that the earth is losing rotational energy and some of that energy is being transferred into the moons orbit, but only a relatively small proportion as the tidal system is a very lossy process so that most of the energy is just lost as heat in the tidal system.

5. Dec 18, 2009

### Cleonis

There is one other scenario that is interesting to explore. What if both the primary and the satellite spin at the same rate as the orbit around the common center of mass?
In that scenario there is no dissipation of energy, so no source of energy is involved.

The state of spin rate identical to orbit rate is called 'tidal lock'. And in fact our Moon is in tidal lock. (More precisely: the Moon orbit is somewhat eccentric, so the Moon orbit angular velocity fluctuates accordingly. The Moon spin rate is constant, so as seen from the Earth the Moon is kind of "shaking its head".)

Finally, a thought demonstration. What if you bring two celestial bodies from great distance to each other, setting them up in orbit around each other, with both celestial bodies in tidal lock. In the final state the two celestial bodies are tidally distorted. What provided the energy for that distortion away from spherical shape? The answer must be that when two celestial bodies distort each other tidally (while in tidal lock) then gravitational potential energy is the energy source. In distorting each other tidally the two celestial bodies must move a little closer to each other, thus releasing gravitational potential energy.

Cleonis

Last edited: Dec 18, 2009
6. Dec 18, 2009

### Staff: Mentor

Dang, I always get that part wrong. Sorry. So energy is transferred from the earth's rotation to the Moon's orbit and in the process some (29/30ths according to the wiki) is lost to friction.

Still, I think what the OP is wondering is where the energy to create/move the tidal bulge itself comes from (and I still didn't answer it right...). In other words, the friction doesn't cause the bulge to move, it causes the bulge to slow down. So what causes it to move? Answer is that there is no energy associated with causing the moving of the bulge. It is a essentially a standing wave and it was created when the moon was created/entered into orbit and would move around the earth forever the same way if there were no friction.

Last edited: Dec 18, 2009