# Tidal forces and thermodynamics

## Main Question or Discussion Point

If the gravitational effect of the moon influences tidal forces on earth, doesn't that violate the 2nd law of thermodynamics?
If gravity is a force and not energy, and only forms of energy can convert into other forms of energy. Then how can gravity create tidal forces, which are kinetic energy???
Kinetic energy from tidal forces can then be converted into electrical energy here on earth.
So you have a situation where gravity is being converted into "work" and dare I say "perpetual motion".
Remember according to physics only energy can convert energy. Oh I forgot, the planets revolving is kinetic energy of corse, but what made them revolve? Oh the force of gravity, right.
So we are back to gravity again. If there was no gravity there would be no "kinetic" energy. Yet how can a force create or convert energy?
If someone mentions the sun in all this, then it's just another example of gravity acting on the planets and not solar energy.
Tidal forces have little to do with solar energy, it's gravity that are responsible for high and low tides.

Related Other Physics Topics News on Phys.org
russ_watters
Mentor
Welcome to PF!

The tides dissipate kinetic energy in earth's rotation, slowing it down.

Nugatory
Mentor
There is an enormous amount of kinetic energy in the earth and the moon rotating about their axes. Tidal power generation works because the gravitational forces between the earth and the moon work in such a way that some of this kinetic energy is transferred to the kinetic energy of water sloshing back and forth in the earth's ocean basins. Googling for "tidal lock" will find some good explanations, but the at a hand wavy level it's pretty simple - neither the earth nor the moon are ideal point particles so the distances between different points on the earth and the moon, and hence the gravitational forces between the moon and the water at those points on earth, are different. The different gravitational forces add up in a way that opposes the rotation of both bodies, so slows them.

We can extract energy from the water sloshing back and forth, but it's not free. It's coming from the kinetic energy of the earth's rotation. If we set up a perfectly efficient tidal power system and ran for long enough, we would find that the earth's rotation would gradually slow down until there were no more tides and our power station would stop generating power.

SteamKing
Staff Emeritus
Homework Helper
So you have a situation where gravity is being converted into "work" and dare I say "perpetual motion".
Gravity is used to convert the potential energy of a mass of water stored behind a dam into electricity by letting the flow of water turn a turbine. There's no violation of the Second Law here, and there is no creation of perpetual motion, because once all the water is let out from behind the dam, no more electricity is generated.

Perpetual motion is just that. Once the PM mechanism has been started, it runs forever, supposedly, without any further intervention or energy input.

Drakkith
Staff Emeritus
Yet how can a force create or convert energy?
Look at the basic definitions of work and energy. Work is defined as the application of a force over a distance, W=FD. So applying a force, which then causes an object to move in the direction of that force, means we are performing work on that object. Since energy is the potential to perform work, when we perform work on an object we are accelerating it and giving it kinetic energy that it can then give up to something else by performing work on it.

There is an enormous amount of kinetic energy in the earth and the moon rotating about their axes. Tidal power generation works because the gravitational forces between the earth and the moon work in such a way that some of this kinetic energy is transferred to the kinetic energy of water sloshing back and forth in the earth's ocean basins. Googling for "tidal lock" will find some good explanations, but the at a hand wavy level it's pretty simple - neither the earth nor the moon are ideal point particles so the distances between different points on the earth and the moon, and hence the gravitational forces between the moon and the water at those points on earth, are different. The different gravitational forces add up in a way that opposes the rotation of both bodies, so slows them.

We can extract energy from the water sloshing back and forth, but it's not free. It's coming from the kinetic energy of the earth's rotation. If we set up a perfectly efficient tidal power system and ran for long enough, we would find that the earth's rotation would gradually slow down until there were no more tides and our power station would stop generating power.
A couple of question. If this is correct, can you answer this.
If the earth slows down over time due to kinetic energy being diverted from it's spin into tidal forces... Then by that reasoning the earth must be rotating slower now than it did when the dinosaurs lived? Maybe it is.

However if the solar system is based around the mass of the sun, causing a gravitational pull on the rest of the planets, and if the sun is a virtual constant in the system (atleast for a billions of years etcetera) then even if tidal forces suck kinetic energy from the earths rotation or even from the moons rotation, then the fact that the sun exerts a constant pull on the planets, wouldn't the rotation of the earth and moon be unaffected overall due to tidal forces, as the mass of the sun is the engine to the solar systems kinetic energy and the planets are basically cogs. To say tidal forces would slow a cog down would imply that in turn the cog slows the engine down?

So does that mean the sun is put under more pressure to burn energy because of kinetic energy being syphened off due to things like tides??? Seems highly unlikely.

Also if force over distance = work, and if "work" is just another term for kinetic energy then why can't energy be harvested directly from gravity???

I'm not talking about hydro electric because we know water only accumulates in a dam due to solar radiation evaporating water from one place and then rain dropping it elsewhere.

Basically I think there is a huge and obvious flaw in the practical application of the 2nd law of thermo dynamics. Practical as in the sense that even if tidal forces can slow down planets to a slight degree after millions of years, then it would mean the 2nd law is "literally" correct, but an effect that takes millions of years to show up as even a small difference is hardly significant when it comes to the idea of using gravity to propel an engine on earth for an indefinite period.

For instance if the tides are a result of gravity, but their movement isn't "free" because it slows down the rotation of the earth etc...but only slightly over millions of years etc....then why can't it be considered that a man made device could simply use the same reasoning and use the kinetic energy of the earth or moon to operate on the same principals. Therefore lasting for millions of years and to all "appearances" being in "perpetual motion"? Not literally but just as practically as the tidal forces are continuous over millions, or billions of years.

It's ridiculous comparing how long planets or the sun will last when talking about practical man made devices that may last thousands of years or more and be practical perpetual motion with no obvious input energy.

So isn't it clear that all someone has to say in order to justify a machine that doesn't use oil, gas, electricity, hydro, solar, wind, nuclear or any other conventional form of energy to power a machine, the machine would have no "obvious" input power source, but that the machine could work "practically" in perpetual motion and "appear" to create energy over thousands of years, just as how the tidal forces work. Then the machine for all "practical " reasoning would syphen off kinetic energy from the earths rotation in relation to the moon, as it's power source.

So I believe the dogmatic application of the 2nd law can fudge thigs like tidal forces (which by the way have been in continual and predictable motion for millions of years) which also lose energy due to friction but it hasnt stopped the tide, has it.
Based on this id like to hear the rational for saying a machine can't work based on the same principals as how tidal forces work, without piggy backing off the motion of the tide but in some other manner using the kinetic energy of the earth...
????

Last edited:
Sorry, I meant to say...
Is there a legitimate reason why a device can't be designed that functions using the same principals as the tide? Without simply having to piggy back on tidal energy.

For instance isn't it reasonable to assume there must be more than one way to use the same principals that allow the kinetic energy of the tides to function, to propel some, as yet, unknown engine, which is independent of the tides. Even a fully land based system.

An engine, which like the tides, workes based on the motion of the moon and earth, which would require no additional input fuel or energy. And because it syphons a small amount of the kinetic energy of the earth it would last almost indefinitely, in practical terms. Surely this is possible? But if so why are such ideas rubbished as "literal" overunity devices or perpetual motion machines, when input energy can indeed come from the earths rotation as proven by tidal forces.

Drakkith
Staff Emeritus
If the earth slows down over time due to kinetic energy being diverted from it's spin into tidal forces... Then by that reasoning the earth must be rotating slower now than it did when the dinosaurs lived? Maybe it is.
That's correct. One day was several hours less back when the dinosaurs lived than it is now.

However if the solar system is based around the mass of the sun, causing a gravitation pull on the rest of the planets, and if the sun is a virtual constant in the system (atleast for a billion years etcetera) then even if tidal forces suck kinetic energy from the earths rotation or even from the moons rotation, then the fact that the sun exerts a constant pull on the planets, wouldn't the rotation of the earth and moon be unaffected overall due to tidal forces, as the mass of the sun is the engine to the solar systems kinetic energy and the planets are basically cogs. To say tidal forces would slow a cog down would imply that in turn the cog slows the engine down?
The tidal force from the Sun is much less than the Moon even though the Sun's gravity is much greater than the Moon's. The reason is that as the difference in the strength of the Sun's gravity on one side of the Earth compared to the other is much less than the difference in the Moon's gravity on opposite sides. It is this difference in strength that gives rise to tidal effects. Since the tidal effect from the Sun is so much smaller than the Moon, we don't notice it as much, even though it is there.

Also if force over distance = work, and if "work" is just another term for kinetic energy then why can't energy be harvested directly from gravity???
I'm not talking about hydro electric because we know water only accumulates in a dam due to solar radiation evaporating water from one place and then rain dropping it elsewhere.
It can be harvested directly from gravity. Drop a rock on he ground and energy has been given to the rock due to the acceleration provided by gravitation. You can recapture this energy in various ways when the rock hits the ground or your energy generating device. The problem is that you have to expend energy to get the rock back up in the air in order to drop it again. If you repeatedly pick up the rock and drop it, you don't have any net gain in energy. You've expended just as much picking the rock up as you've gotten back when it drops.

Basically I think there is a huge and obvious flaw in the practical application of the 2nd law of thermo dynamics. Practical as in the sense that even if tidal forces can slow down planets to a slight degree after millions of years, then it would mean the 2nd law is "literally" correct, but an effect that takes millions of years to show up as even a small difference is hardly significant when it comes to the idea of using gravity to propel an engine on earth for an indefinite period.
There's no flaw in the 2nd law. There's only a flaw in your understanding of how it applies.

For instance if the tides are a result of gravity, but their movement isn't "free" because it slows down the rotation of the earth etc...but only slightly over millions of years etc....then why can't it be considered that a man made device could simply use the same reasoning and use the kinetic energy of the earth or moon to operate on the same principals. Therefore lasting for millions of years and to all "appearances" being in "perpetual motion"? Not literally but just as practically as the tidal forces are continuous over millions, or billions of years.
We can do this. We just need something to move back and forth under the influence of the Moon's gravity. The very best object we have is the ocean itself, and there are tidal generators that generate power using the tides. Unfortunately, the rate of change between maximum and minimum tide is very slow, so it's very difficult to generate lots of power this way unless we have a LOT of tidal generators, which becomes expensive.

Based on this id like to hear the rational for saying a machine can't work based on the same principals as how tidal forces work, with piggy backing off the motion of the tide but in some other manner using the kinetic energy of the earth...
The main problem is that tidal effects are only dominant on very, very large objects, such as the Earth as a whole. Small objects have very little change in gravitational force from one side to the other, so we can't simply throw a generator together unless you want it to be several hundred or thousand miles across. The other option is to use the ocean, which we are already doing.

For instance isn't it reasonable to assume there must be more than one way to use the same principals that allow the kinetic energy of the tides to function, to propel some, as yet, unknown engine, which is independent of the tides. Even a fully land based system.
I'd say it's more reasonable to not assume anything.

anorlunda
Mentor
I recall reading that at the time of the moon's creation, the Earth day was 5 hours. Also, the newly created moon orbited much closer to the surface of the Earth than it does today. Tides draw energy both from the rotstitions of both bodies around their own axes, and from the gravitational energy of both orbiting around the common center of gravity.

The OP should also study descriptions of Saturn's moon Io as an extreme example of energy crested by tidal forces.

With over 400 active volcanoes, Io is the most geologically active object in the Solar System. This extreme geologic activity is the result of tidal heating from friction generated within Io's interior as it is pulled between Jupiter and the other Galilean satellites—Europa, Ganymede and Callisto.

PeterDonis
Mentor
2019 Award
Since the tidal effect from the Sun is so much smaller than the Moon, we don't notice it as much, even though it is there.
We don't notice it "as much" as the Moon, but we definitely notice it; there's a significant difference between spring tides and neap tides. If you run the numbers, the tides due to the Sun are about half as large as the tides due to the Moon, so the Sun's tides are smaller, but not "much smaller".

CWatters
Homework Helper
Gold Member
An engine, which like the tides, workes based on the motion of the moon and earth, which would require no additional input fuel or energy. And because it syphons a small amount of the kinetic energy of the earth it would last almost indefinitely, in practical terms. Surely this is possible?
It certainly is. There is no fundamental law of physics that stops you building such a machine that runs "almost indefinitely".

But if so why are such ideas rubbished as "literal" overunity devices or perpetual motion machines, when input energy can indeed come from the earths rotation as proven by tidal forces.
It depends entirely on the proposal. I've not seen anyone rubbish tidal power as a "literal over unity device" or "perpetual motion machine". Tidal power does not break any laws of physics.

However a machine that claims to work by extracting energy from gravity alone would break known laws of physics.