# Free Energy from the Earth's Rotation

1. Sep 2, 2014

### aeroseek

Is it possible to generate energy from the earth's rotation? Imagine a Foucault Pendulum attached to a dynamo shaft?

2. Sep 2, 2014

### A.T.

In a sense, tidal power plants use the difference between Earth's rotation and the Moon orbit rates. This energy source will be dissipated when Earth and Moon become tidally locked.

3. Sep 2, 2014

### Staff: Mentor

In principle, yes. But at one rotation every 24 hours the rate of power generation is too small to be useful in practice. A more promising approach is tidal power generation.

4. Sep 2, 2014

### aeroseek

Well, that's interesting, because if you use a weight that is sufficiently heavy, it may just be able to generate significant power.

I wonder what the mathematics looks like for deriving the energy stored in the cable of such a pendulum if it is suspended from a rigid point attached using a cable that twists as the earth rotates?

I guess you would need a practical experiment - and frictionless (magnetic?) bearings.

5. Sep 2, 2014

### jbriggs444

A Foucault pendulum can never be a source of power, no matter how big you make the mass and how perfect you make the bearings. It does not act like an fixed rotation-free anchor against which you can apply a continuing torque. If it did act like such an anchor, then angular momentum would not be conserved.

Tidal power generation requires the presence of the moon which acts as an anchor against which you can apply a continuing torque. Angular momentum is conserved because the Earth's angular momentum is dumped into the moon.

6. Sep 2, 2014

### aeroseek

Theoretically after x number of years the tides will slow the moon down and it will crash into the Earth?

7. Sep 2, 2014

### Staff: Mentor

No. The moon is getting further away as tidal effects slow the rotation of earth.

8. Sep 2, 2014

### Staff: Mentor

No, you don't need a practical experiment - the back of the envelope calculation is sufficient to tell us what the math looks like. (and this is after setting aside jbrigg's objection).

9. Sep 3, 2014

### aeroseek

10. Sep 3, 2014

### Buckleymanor

It swings in one direction only and therefore force or torque can be applied against the direction of swing.
A moveing body will continue to travell in same direction unless a force is used to change it's direction.Depending on the magnitude and direction of the force it will either change it's direction or add to it's swing.

11. Sep 3, 2014

### jbriggs444

Think about that a bit, please. You have a Foucault pendulum subjected to an unbalanced net torque. You claim that it will not rotate under this torque. Where does the angular momentum go?

Can you prove that a Foucault pendulum will not rotate under an external applied torque.

12. Sep 3, 2014

### Buckleymanor

Actualy your original statement with regards the size of the mass and no matter how large, or how perfect the bearings will never be a source of power gets to the nub of the question.Well hurracaines seem to have all the attributes of stringless pendulumes with large masses and perfect bearings.Powered by the speed of the Earths rotation the seem to have plenty of power.Also if you fired a cannon ball from south to north across the equator of the earth would it's impact on the ground be purely that of the ball's velocity or would there allso be the component of the turning of the Earth and it's speed that would have to be added to the speed of collision and hence a source of extra power.These are just extensions of the Foucault pendulum though not so obviouse.

13. Sep 3, 2014

### jbriggs444

The above is completely incorrect. The Coriolis pseudo-force acts at right angles to the direction of travel, does no work and accordingly adds no energy.

No. That's completely incorrect as well.

14. Sep 4, 2014

### Buckleymanor

An unorganised group of thunderstorms (caused by or powered by the sun) needs the coriolis force to turn them into a rotating mass of thunderstorms and ultimately a tropical cyclone or hurricane.
Quote.
The Coriolis force is required for a cyclone to form into a tropical cyclone or hurricane. The force causes a greater deflection of the air (right in the northern hemisphere and left in the southern) and the correct speeds for the tropical cyclone to form.
Hence why tropical cyclones do not form at or within 5 degrees of the equator, and cease to exist at around 35 degrees north, or 15 degrees south.
If the coriolis force did not add any energy the storm would remain just a non- rotateing storm or group of storms.
The practical impact of the "coriolis effect" is mostly caused by the horizontal component produced by horizontal motion.
When horizontal motion causes this effect on hurricanes "cylones" or the like it must be imparting force or they would not turn.

15. Sep 4, 2014

### jbriggs444

Yes, agreed. The original claim was that the Earth's rotation powered such storms. That is the claim that was incorrect.

[Edit: had that typoed for a few minutes]

The Coriolis force provides no power. It does no work. It cannot because it always acts at right angles to the direction of motion.

It imparts momentum (in the rotating frame). It does not impart energy.

Last edited: Sep 4, 2014
16. Sep 4, 2014

### Buckleymanor

If no energy is imparted then how come the earths rotation slows down or speeds up when there is or is not storms where has that energy come from or gone.
If you spin up a top or any device using right angled motion it requires energy to make the top turn or if the rain or wind is made to change direction it also requires a force it does not do it by magic.
The momentum has to come from somewhere.

Last edited: Sep 4, 2014
17. Sep 4, 2014

### A.T.

Is there net slowdown of Earth's rotation over time because of storms? Where would the Earth's angular momentum go? Into the storms that spin faster and faster and never die?

18. Sep 4, 2014

### jbriggs444

Reference, please. (Not that I disbelieve this, mind you).

There is a sense in which the Earth's rotation can be seen to impart energy to, for instance, a rotating top. However, to make that interpretation work, you must first adopt a non-rotating frame of reference. Which means you must discard the Coriolis force entirely.

If you spin a top in a same direction that the Earth is spinning then it obtains a total spin that is equal to its apparent spin (in the Earth-centric rotating frame) plus an extra contribution of approximately one rotation per day. Since the rotational energy of a spinning object scales as the square of the rotation rate, the increase in energy as you spin up such a top is greater than if you had spun it up in the opposite direction. The excess comes from the Earth's rotation.

This is not a source of free energy. As the top comes to a stop, the excess energy bleeds back into the Earth. You cannot harvest it with an Earth-anchored device.

In the same way, even if a storm did pick up extra energy from the Earth's rotation, you can't harvest the excess using an Earth-anchored device.

The same effect applies if you hold a 100 meter race at the equator. If you set up the track running from west to east, each runner gets a massive energy boost from the rotational velocity of the earth as they jump off the starting blocks. If you set up the track running from east to west the runners actually lose kinetic energy as they jump off the starting blocks. But this is just an artifact of the choice of coordinates. Again, it is not a useful free energy source. You can't harvest it using an earth-anchored device.

If you want to launch a rocket, the effect becomes important, of course.

19. Sep 5, 2014

### Buckleymanor

http://www.bbc.co.uk/blogs/legacy/23degrees/2011/03/can_an_earthquake_shift_the_ea.html
When you say you can't harvest extra energy from storms what exactly do you imply because windmills obviously harvest energy and slow down the earth's rotation.

20. Sep 5, 2014

### A.T.

The Earth and the atmosphere exchange angular momentum back and forth. But you cannot continuously slow down both to extract energy, without external torques.

Where is the "extra energy from Earth's rotation" here? Does the windmill produce less shaft power if the wind turns 180° keeping the same speed, so the windmill now accelerates the earth's rotation?

21. Sep 5, 2014

### bahamagreen

Extracting energy from Earth's rotation will result in slowing down the rotation (very slightly).

The most efficient power producing method I can think of is by doing it indirectly through Earth's magnetic field by using a pair of satellites, one in a high orbit one in a low orbit, with a set of cables between them. The cables will complete a full cut through magnetic lines of force around Earth each orbital period - about every 90 minutes or so.

Since the high orbit end will lag and the low orbit end will lead, the angle will increase the amount of cable in the field... already a lot since the ends can be 10-100 or so miles apart and you can use lots of parallel cables...

A bonus is that the orbital altitude may be adjusted by discharges from the ends of the affair, allowing one to move the whole thing higher or lower as desired.

22. Sep 6, 2014

### jbriggs444

Windmills harvest energy that originated from the sun. They do not slow down the earth's rotation.

There is an elementary problem that you are up against when you try to harvest "free" energy from the rotational kinetic energy of an isolated system.

[Waving my hands a bit here and using formulas that apply for rigid planar rotation] (*)

Energy is conserved. If you want to get energy out, you have to reduce rotational kinetic energy. That's Iω2/2. Angular momentum is conserved. That's Iω. So how do you reduce Iω2/2 without changing Iω? Obviously you need to reduce ω. But angular momentum is still conserved. How do you reduce ω without reducing Iω? Obviously you need to increase I.

And there it is. The basic rule is that you can only extract energy from the rotation of an isolated system by increasing its moment of inertia. The only way you can keep extracting more energy is to keep increasing the moment of inertia further. Storms won't do it. Windmills won't do it. Cannonballs fired across the equator won't do it. Satellites in orbit dangling wires into the magnetic field won't do it either. (Unless I'm missing something). No transient process that leaves the size and shape of the isolated system more or less unchanged can ever work.

There are a couple of approaches that will do the job. One way is to launch a moon and use the tides. You harvest tidal energy and the resulting torque on the moon moves it farther and farther away. As expected, this results in a continual increase in the moment of inertia of the system. Another way is to use a beanstalk, paying energy to lift dirt and rocks up to geosynchronous orbit and then harvesting more energy as the soil and rocks are dropped down the cable on the far side. Again, this has the general effect of continually increasing the moment of inertia of the system.

(*) A non-rigid system may have different pieces rotating at different rates. It will normally be possible to harvest energy from those differences. For instance, it is easy to harvest the energy in a pair of counter-rotating rings. Once that energy is harvested or if one starts with a system that is rotating at one consistent rate in one plane, I maintain that the above argument holds water and that the general result applies, even for non-rigid systems.

Last edited: Sep 6, 2014
23. Sep 6, 2014

### Staff: Mentor

No, they don't. If you count the atmosphere as part of the earth then there is no change in angular momentum whatsoever. If you separate the earth from the atmosphere then at most they put back angular momentum that was exchanged by the formation of the weather.

24. Sep 6, 2014

### Buckleymanor

They put back some angular momentum that was exchanged but not all of it .The prime objective of a modern windmill is to produce electrical power which dissipates eventualy as heat.

25. Sep 6, 2014

### jbriggs444

Angular momentum is conserved. Even if you use rotational kinetic energy (which is not the same thing as angular momentum) to produce electricity, the amount of angular momentum lost in the process is exactly zero.