why dosen't the earth slow down?

[OrganizedChaos]

http://seattlepi.nwsource.com/local/20841_blue30.shtml

ocean tides can power turbines right? tides are caused by the rotation of the earth/moon and gravity? gravity is a constant based on mass? to do work you need energy? so by converting the kinetic energy of the earth/moon into electrical energy shouldn't it slow down?

 

[boran]

as far as I know, the earth does slow down... albeit at a very very slow rate. (the slowing down that is, not the rotation)

 

[Cogman]

Yes it is slowing down. But as I remember its like 0.00000002 seconds are added to each day after each year.

 

[cquark]

Originally posted by: OrganizedChaos
http://seattlepi.nwsource.com/local/20841_blue30.shtml

ocean tides can power turbines right? tides are caused by the rotation of the earth/moon and gravity? gravity is a constant based on mass? to do work you need energy? so by converting the kinetic energy of the earth/moon into electrical energy shouldn't it slow down?

The Earth's rotation is slowing down because of tidal friction, but whether you use tides to produce electrical power or not doesn't increase the output of the system so it's basically free energy since we can't do anything to prevent angular momentum of the Earth/moon system from being converted into tidal motion.

 

[glugglug]

There are a lot of planetary systems that HAVE stopped rotating --

For example, the moon is no longer rotating, it is just revolving around the earth, so the same side of the moon is always facing the earth. Also, Mercury always has the same side facing the sun. This suggests the moon and Mercury used to have oceans for tidal friction to eventually stop their rotation.

 

[boran]

Originally posted by: glugglug
There are a lot of planetary systems that HAVE stopped rotating --

For example, the moon is no longer rotating, it is just revolving around the earth, so the same side of the moon is always facing the earth. Also, Mercury always has the same side facing the sun. This suggests the moon and Mercury used to have oceans for tidal friction to eventually stop their rotation.

hmmh, now am I wrong that if the moon is not rotating that we would see always a different part of the moon in a different place (on the other side of the world you would see the other side of the moon) ? and that it now still rotates as fast as it rotates around the earth, causing us to see the same side all time. (I might be wrong about this tho).

 

[cquark]

Originally posted by: glugglug
There are a lot of planetary systems that HAVE stopped rotating --

For example, the moon is no longer rotating, it is just revolving around the earth, so the same side of the moon is always facing the earth. Also, Mercury always has the same side facing the sun. This suggests the moon and Mercury used to have oceans for tidal friction to eventually stop their rotation.

Actually, the moon is rotating, but its rotational period and the period of its orbit around the Earth are the same, so that we always see the same face. However, Mercury has a 3:2 orbital resonance with the Sun, not a 1:1 resonance that would always have the same side facing the Sun.

There are sources of tidal friction other than surface oceans, so tidal locking isn't indicative of former oceans. For example, Io's volcanoes are a result of tidal disturbances in that moon's core arising from Jupiter and its companion moons Ganymede and Europa.

 

[Calin]

The moon also has a oscillating movement, so the visible part of the moon is (a bit more) than half - something around 55% of the surface is visible.
Energy from the tide? The Earth' rotational energy is so big that there is very little energy that the human race could muster from that, so the relative effect is hardly accountable (imagine the energy needed to rise half an ocean to 6 feets - but in some places the tidal wave is much higher)

Calin

 

[silverpig]

Originally posted by: Calin
The moon also has a oscillating movement, so the visible part of the moon is (a bit more) than half - something around 55% of the surface is visible.
Energy from the tide? The Earth' rotational energy is so big that there is very little energy that the human race could muster from that, so the relative effect is hardly accountable (imagine the energy needed to rise half an ocean to 6 feets - but in some places the tidal wave is much higher)

Calin

Pfft, Superman can just fly around the earth really really fast to make it spin backwards

 

[unipidity]

Re Saying that the moon and Mercury had oceans... no.

They do each rotate about their axis- the interesting fact is that the period of axial rotation is the same as their preiod of orbit around the Earth and Sun respectivly. They have a captured rotation caused I think by irregular mass distribution, although why it is a factor of 1 and not, say, 2/3rd I dont know. You should be able to google this easily.

 

[unipidity]

An interesting side note on this topic (sort of)... since you can extract energy from the Earth's magnetic field by trailing a wire from a satellite, what exactly is this getting the energy from? Is it slowing some structure within the Earth that produces the field?

 

[Armitage]

Originally posted by: unipidity
An interesting side note on this topic (sort of)... since you can extract energy from the Earth's magnetic field by trailing a wire from a satellite, what exactly is this getting the energy from? Is it slowing some structure within the Earth that produces the field?

It's getting the energy from the orbital energy of the satelite. So it's not really a power solution for satellites, because it degrades the orbit

 

[Jeff7]

What we regard as a lot of energy is almost nothing compared to a planet. When we sent Cassini to Saturn, it stole some orbital momentum from Venus, Earth, and Jupiter to help send it to Saturn quicker. But the 5.5 megagrams of spacecraft won't make much of a dent in the momentum of a planet that has a mass of over 5,974,200,000,000,000,000,000 megagrams (Earth). Take that much mass, and move it at a little less than 30km/s, and you've got a LOT of energy.

 

[Calin]

It is just that the rotational energy of the Earth is less than the figure you are telling (what you are telling is the cinetic energy of movement, the rotational energy is just the same mass, but with some kind of 450 m/s - rotational velocity at equator - and a constant to compensate for the spherical mass, uneven mass distribution inside the Earth, and other factors.
While a several orders of magnitude smaller (a million times less, maybe even less than that) it still remains a LOT of energy

Calin

 

[unipidity]

Disagree about the satellite- NASA was invstigating it as a possible propulsion source- get a satellite into low orbit and use this to boost it to geostationary, for example.

*Thinks*

We have a moving wire in a magnetic field. Thus a current. And a force acting against the movement. Hmmm. And your not going to be extracting more electrical energy than is required to boost the orbit, so your correct.

Damn.
Im sure popular science outlets were going on about the economic consequences of this.

 

[redhate]

I've heard that something like every 10 years the earth slows down by 2 seconds (in complete rotation I guess). Whats funny is... according to the evolutionary theory... if the earth really was 4.7 billion years old. It would have been spinning so fast it would have never been able to support life. And if the earth was 4.7 billion years old the sun would have touched the earth after like 10 million years (according to scientist the sun shrinks some figure a day.

 

[jay75]

Does this whole Earth, Moon, Gravity system really slow down the earth at all? As we can see from the tides, they move in one direction for 12 hours and then in the opposite direction for the next 12 hours. Doesn't this indicate that the net effect of the moons pull on the earth is being cancelled out?

About the use of tide generators, it is the rotation of the earth and the pull of the moon that influences the tides, not the other way round, i feel! even if the tide does push the earth in its direction of flow (by banging on the continental shelves), the point illustrated in the first para comes into play. when judged from a single point on the earth, the generators would speed up the earths rotation when the tide opposes the earths rotation, as the generators would oppose the tide. and then vice versa when the tide moved the other way therefore going with the direction of the earth.

 

[cquark]

Double post.

 

[cquark]

Originally posted by: redhate
I've heard that something like every 10 years the earth slows down by 2 seconds (in complete rotation I guess). Whats funny is... according to the evolutionary theory... if the earth really was 4.7 billion years old. It would have been spinning so fast it would have never been able to support life.

Your numbers are off by several orders of magnitude. Tidal friction slowdown is measured in milliseconds, not seconds.

And if the earth was 4.7 billion years old the sun would have touched the earth after like 10 million years (according to scientist the sun shrinks some figure a day.

In order to reach your absurd conclusion, you'd have to assume that the Sun was shrinking at a constant rate for all that time, which no scientist will tell you is the case. Stars do change in size over their life cycle, and the Sun in particular, will likely enter a red giant phase and grow enormously in the future instead of shrinking away to nothing as your argument above would claim.

 

[JHutch]

Originally posted by: redhate
I've heard that something like every 10 years the earth slows down by 2 seconds (in complete rotation I guess). Whats funny is... according to the evolutionary theory... if the earth really was 4.7 billion years old. It would have been spinning so fast it would have never been able to support life. And if the earth was 4.7 billion years old the sun would have touched the earth after like 10 million years (according to scientist the sun shrinks some figure a day.

Quick google search (I'm not this smart, but I can find quotes from people who are )

http://www.talkorigins.org/indexcc/CE/CE011.html

The earth's rotation is slowing at a rate of 0.005 seconds per year per year. This extrapolates to the earth having a 14-hour day 4.6 billion years ago, which is entirely possible.

The rate at which the earth slows today is higher than average because the present rate of spin is in resonance with the back-and-forth movement of the oceans.

Fossil rugose corals preserve daily and yearly growth patterns and show that the day was about 22 hours long 370 million years ago, in rough agreement with the 22.7 hours predicted from a constant rate of slowing [Scrutton 1965; Wells 1963].

So, no, the Earth is not slowing by 2 seconds every 10 years. More like 1 second every 1000 years.

JHutch

 

[cquark]

Originally posted by: jay75
Does this whole Earth, Moon, Gravity system really slow down the earth at all? As we can see from the tides, they move in one direction for 12 hours and then in the opposite direction for the next 12 hours. Doesn't this indicate that the net effect of the moons pull on the earth is being cancelled out?

You're forgetting about energy lost due to friction.

 

[Atomicus]

If you remember from your physics class, work is relative to the distance travelled. However, if the object travels and goes back to the point of origin, you have generated 0 work. Since the earth is travelling in a circular path, it will eventually come back to the same spot, thus 0 work.
As for the spinning.... you'll need to get into the mechanics and dynamics of relative motion and all that un-fun stuff.

 

[jay75]

You're forgetting about energy lost due to friction.[/quote]

I dont think were talking about energy lost due to natural friction here, but about it being drawn out due to manual application of an opposing force.

 

[jay75]

You're forgetting about energy lost due to friction.[/quote]

if it was tidal friction you're talking about, the explanation(tides one way then the other) cancel the effect of friction out. the sea is of the same enertial reference frame as the earth and so otherwise cannot be taken as a separate object than the earth is rubbing against, as it is moving with the momentum of the earth before we take tides into consideration.

 

[Chu]

EDIT : Wrong Thread, oops