So, I’m aware that tides are caused by the Moon’s gravity acting on the surface of the earth. That said, tidal forces move an obscene amount of matter from point A to point B and back to point A again. Where does that energy come from that doesn’t break the first law of thermodynamics? Followup question: If we harvest energy from the system using turbines or something similar, what effect does that have on the system?
In: Physics
The energy comes from the rotational movement of the earth. This causes the earths rotation to slow, the length of a day increases by about 1.7 milliseconds per century.
The ultimate energy source is the kinetic energy of the Earth’s spin. Harvesting tidal energy will in fact cause the Earth to turn slower. But not enough to worry about.
The energy comes from kinetic energy in the earth’s rotation. A day 600 million years ago was 21 hours long.
The current slowdown is that a day gets shorter by 1.7 milliseconds per century, so it is not a lot on the time scale we are used to but a lot on earth’s geological time scale.
The same has occurred for the moon but faster because it is smaller then earth. The moon has slowed down so only one side point to earth. That mean the orbital time is the same as the rotational time. This is called today locking and why we can only see one side of the moon.
If left undisturbed earth would become today locked with the moon. It would take around 50 billion years and the rotation time for earth would be 47 current days.
The problem is that our sun will expand to a red giant in roughly 5 billion years. It will expand and might swallow the earth and the moon, Venus will certainly be swallowed. So if the earth and the moon survive the surfaces will have been remelted and the lightest molecule like water would be below away, So the earth will no longer support life.
The moon exerts a gravitational force as you’ve noted, and the tides move as a result of (rotational) kinetic energy, kinetic energy and gravitational potential energy are direct equivalents, so the potential energy of Earth’s total ocean mass under a gravitational influence is equal to its maximum kinetic energy, energy is entirely conserved in this system, in other words the amount of kinetic energy a body can experience is entirely dependent on the force of gravity acting on it, as for harvesting energy, only a very small fraction of a fraction of that kinetic energy is transformed into electricity, so the energy loss in the entire system is negligible
The moon and the earth are really really big and unusually close together. There is enough gravity for all sorts of things.
It would have almost no effect.
Imagine that the moon was a big magnet and the water on earth was iron. Now imagine the moon orbiting around the earth, pulling all that water into peaks with it as it rotates (the gravity of earth’s core, of course, keeps it on the surface still) and as the earth and moon orbit the sun. When the water hits land somewhere the magnet just lets go and the tide recedes, and as it finds water again it pulls. All these forces are easier to imagine when you think of them on a small scale and then realize how big the scale truly is, and that it’s all moving, sometimes in opposite of each other.
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