How much force to stop planetary rotation?

[daenku32]

I suppose this would be a simple question using earth's mass and diameter. But I was wondering if we could get everyone on earth to 'shift' at the same at the same direction, could the rotational speed be affected?

Or how many rockets would we need to tie to the ground to slow down or increase the rotation significantly (>10%)... I just think that would be pretty cool, from an experimental point of view of course.

Now that I think about it the crust would probably just shift and absorb the force since the liquid hot mag-ma underground is where a lot of the momentum actually resides.

[Molinaro]

Whatever shift we all made would eventualy come to a stop and result in the earth getting just enough of a kick in the other direction to avoid disaster.

Then again maybe it should work to ensure Darwining culling on a societal scale.

Since this is a crazy physics thread I'll ask the next one :)


Could any kind of impact against the earth result in the crust being compressed to the point of impacting the solid core and thereby gaining some impulse of momentum that would alter the orientation of the crust relative the rotating core?

[Mercutio]

My favorite cartoon ever. (http://xkcd.com/c162.html)

(Don't forget to place your curser over the cartoon for the hidden message.)

[Davo]

Correct me if my Physics is wrong.

Momentum of the earth would be a product of the Earths mass and its angular velocity.

Compare the mass of the planet with the mass of the human population. Its a minute fraction of a percentage.

Trying to move sideways to slow momentum wouldn`t work. It would be like trying to shift your body around in a car to slow the vehicle down. Don` t bother trying.

Rockets would work (sorry can` t give quantity), but you would problaby have to have a lot of them and have them running continuously to make even a dent in the momentum.

Please correct me if any of the above is not correct.

[Roboramma]

I have no idea about this topic, but something sprung into my head concerning rockets:
Would the fact that the rocket fuel would stay in the earth's atmosphere and eventually come back to earth (or at least transfer it's momentum to the earth - even if in the form of heat tranfered to the air?) mean that the effect would be canceled out?*
Can a body's rotation be affected without something apart from that body being affected?

*I guess that if the rocket fuel acheived escape velocity this point would no longer matter.

Note: I have no idea, just wondering.

[rjh01]

There are four ways of changing the rotational speed of a planet.
1. An object hits the edge of the planet, giving some or all of it's energy to the planet. This is how the Earth got its spin in the first place. A Mars sized planet hit the Earth. It also produced the Moon.
2.The reverse of the above. Does not happen except as part of 1 above.
3. A body has a flies past the planet. This may also impact on the distance of the planet from the Sun
4. Tidal means. The moon is slowly further further away from the Earth. It is getting the energy from slowing the Earth's spin. If we were around in the early days then we would have 16 hour days.

[Dave1001]

There are four ways of changing the rotational speed of a planet.
1. An object hits the edge of the planet, giving some or all of it's energy to the planet. This is how the Earth got its spin in the first place. A Mars sized planet hit the Earth. It also produced the Moon.
2.The reverse of the above. Does not happen except as part of 1 above.
3. A body has a flies past the planet. This may also impact on the distance of the planet from the Sun
4. Tidal means. The moon is slowly further further away from the Earth. It is getting the energy from slowing the Earth's spin. If we were around in the early days then we would have 16 hour days.

Fascinating. Sound like the easiest way would be to alter the course of a large meteor so that it hits the edge of the planet in just the right way, correct?

[rjh01]

Er ... Anyone planning on doing this anytime soon? I would hate to hear that a terrorist had got hold of an idea from me or this forum.

Also it would also have massive other consequences, depending on the size of the meteor. A very bad winter lasting a year or two would be the most minor of these.

Or a near miss on a large body the size of the Moon would give massive tidal effects.

[wollery]

1. An object hits the edge of the planet, giving some or all of it's energy to the planet. This is how the Earth got its spin in the first place. A Mars sized planet hit the Earth. It also produced the Moon.Umm, no. Or at least, rather misleading. Are you suggesting that ALL of the planets in the Solar system were hit with other bodies of comparable size to themselves? Because they all spin (although Mercury is tidally locked to the Sun, Venus' spin is retrograde and Uranus' is on it's side). The impact may have increased the Earth's spin rate, but it didn't start it spinning. The spin originates from the angular momentum gained in accreting material from the rotating Solar disk.

[bobdroege7]

have fun

http://www.lpl.arizona.edu/impacteffects/

[RecoveringYuppy]

I'd second posts 5, 6 and the minor correct in 9 to 6 and add:

Most of Earth's angular momentum is going to be in the crust near the equator. AM is proportional to both mass and the distance of the mass from the radius. The distance of the crust from the radius vs the core's shorter distance from the radius outweighs the density difference by a wide margin.

Someone already pointed out that the mass of humanity is nothing compared to the mass of either the Earth as a whole or the crust. Consider that each human is easily matched in weight by a small volume of Earth directly under their feet. Even if humans covered every square inch of the land and seas we would only be matching the top few feet of Earth at best.

And any change in the angular momentum of Earth due to humanity moving in synch would only last as long as they continued moving. To make a lasting change to Earth's AM, AM has to leave the Earth through one of the mechanisms noted earlier.

[fuelair]

Umm, no. Or at least, rather misleading. Are you suggesting that ALL of the planets in the Solar system were hit with other bodies of comparable size to themselves? Because they all spin (although Mercury is tidally locked to the Sun, Venus' spin is retrograde and Uranus' is on it's side). The impact may have increased the Earth's spin rate, but it didn't start it spinning. The spin originates from the angular momentum gained in accreting material from the rotating Solar disk.
Well said - I just got to this thread and seeing the got hit thing hoped someone would catch that earlier than me!!:) :) :jaw-dropp

[Dave1001]

Umm, no. Or at least, rather misleading. Are you suggesting that ALL of the planets in the Solar system were hit with other bodies of comparable size to themselves? Because they all spin (although Mercury is tidally locked to the Sun, Venus' spin is retrograde and Uranus' is on it's side). The impact may have increased the Earth's spin rate, but it didn't start it spinning. The spin originates from the angular momentum gained in accreting material from the rotating Solar disk.

It sounds like you're saying the spin started when the planets broke off from the sun in the first place? Maybe so, but wouldn't we still be able to stop the Earth's spin with the right type of collision?

[wollery]

It sounds like you're saying the spin started when the sun threw off the planets in the first place? Maybe so, but wouldn't we still be able to stop the Earth's spin with the right type of collision?Oh, where to start.

The Sun didn't "throw off" the planets. The Sun formed from a massive rotating ball of gas, mostly Hydrogen, but with some heavier elements and some molecules (mostly Silicates, Oxides, Hydrides, CO2, H2O and various simple Iron compounds). As self gravity dragged the material in towards the centre the rotation speeded up due to conservation of angular momentum and the ball flattened into a disk. Due to this speeding up, and the subsequent centripetal force the heavier particles fell in more slowly. After the Sun had finished its formation it had left a large disk of heavy material orbiting it in a flat disk. This material clumped together and formed the planets, but it was still rotating about the Sun, carrying angular momentum, and as it collected into smaller spaces conservation of angular momentum again made it spin faster. This is a highly simplified explanation.

And yes. A large enough collision at the right angle would stop the Earth's spin. And it would start spinning again due to tidal forces from the Moon and Sun, assuming that the collision hadn't completely obliterated the Earth-Moon system, which it probably would have.

ETA you edited your post while I was replying, but it doesn't make any difference, the Sun is a huge ball of plasma and gas. Nothing "broke off" either.

[RecoveringYuppy]

By current models the planets didn't break off from the sun originally. Both the sun and the planets accreted from the same original cloud of dust.

Are you familiar with the skater example of angular momentum? As a skater pulls in their arms they spin faster and as they push out their arms they spin slower? Same thing happened with the planets. As stuff fell on to the planets it's like pulling in the arms. The spin rate has to go up to compensate for the decreased radius, otherwise the angular momentum of the incoming stuff would not be preserved.

Now it is possible that the stuff coming in could have zero net angular momentum but the odds are against. Even if the original disk had exactly zero angular momentum (itself unlikely) the planets wouldn't be likely to each get exactly zero angular momentum. It just wouldn't be expected to distribute perfectly evenly across all the planets.

Factor in the requirement that the planets have to be moving to stay in orbit and it becomes very unlikely that a planet is going to form without any spin at all.

[Just thinking]

There are four ways of changing the rotational speed of a planet.
1. An object hits the edge of the planet, giving some or all of it's energy to the planet.
2.The reverse of the above. Does not happen except as part of 1 above.
3. A body flies past the planet. This may also impact on the distance of the planet from the Sun
4. Tidal means. The moon is slowly further further away from the Earth. It is getting the energy from slowing the Earth's spin. If we were around in the early days then we would have 16 hour days.

5. A significant amount of mass is moved inward (or outward) relative to the Earth's center -- thus causing angular velocity to increase/decrease like a spinning ice skater pulling in their arms (or stretching them out). This happened with the tsunami earthquake of 2005 if I'm not mistaken.

[Orangutan]

What if every one in china took their jumpers off at once?

(Jumper is a UK word for knitted sweater, so that joke might only work if your from the UK.) ;)

[wollery]

What if every one in china took their jumpers off at once?

(Jumper is a UK word for knitted sweater, so that joke might only work if your from the UK.) ;)That's unlikely to happen. It's bloody cold here at the moment! :D

[Crossbow]

I suppose this would be a simple question using earth's mass and diameter. But I was wondering if we could get everyone on earth to 'shift' at the same at the same direction, could the rotational speed be affected?

Or how many rockets would we need to tie to the ground to slow down or increase the rotation significantly (>10%)... I just think that would be pretty cool, from an experimental point of view of course.

Now that I think about it the crust would probably just shift and absorb the force since the liquid hot mag-ma underground is where a lot of the momentum actually resides.

I did some rough calculations and here is what I have come up with.

The Kinetic Energy of a Rotating Sphere is:

KE = (1/2)*m*r2*w2

KE = Kinetic Energy of the Sphere
m = Mass of the Sphere
r = Radius of the Sphere
w = Angular Velocity of the Sphere

Using the actual values
Mass of Earth = 5.9742 * 1024 kilograms
Radius of Earth = 6,378,100 meters
w = 7.27 * 10-5 rad/s

Therefore, the Kinetic Energy of the Earth rotating is approximately:

6.42 * 10 29 Joules

So a similar amount of energy will be needed to stop the rotation of the Earth.

Now then, the engines of the Space Shuttle produce about 1,800,000 Newtons of thrust, and if they were somehow fixed to the surface of the Earth, positioned to provide thrust to the West, and located at the Equator, then one would need about 5.6 * 1016 of these rockets to stop the rotation of the Earth.

Soooo,

It is pretty unlikely that us humans could do such a thing.

[grayman]

If God supposedly created the planets, then I'm sure he could stop the rotation as well.

Ask him (you'll just get his voice mail though).

[grayman]

It seems this fellow Joshua did something like that. Is he still around?

Jos 10:12 ¶ Then spake Joshua to the LORD in the day when the LORD delivered up the Amorites before the children of Israel, and he said in the sight of Israel, Sun, stand thou still upon Gibeon; and thou, Moon, in the valley of Ajalon.


Jos 10:13 And the sun stood still, and the moon stayed, until the people had avenged themselves upon their enemies. [Is] not this written in the book of Jasher? So the sun stood still in the midst of heaven, and hasted not to go down about a whole day.


Comment from the Skeptics Annotated Bible (http://www.skepticsannotatedbible.com/): In a divine type of daylight savings time, God makes the sun stand still so that Joshua can get all his killing done before dark.

[drkitten]

the Kinetic Energy of the Earth rotating is approximately:

6.42 * 10 29 Joules

So a similar amount of energy will be needed to stop the rotation of the Earth.

Now then, the engines of the Space Shuttle produce about 1,800,000 Newtons of thrust, and if they were somehow fixed to the surface of the Earth, positioned to provide thrust to the West, and located at the Equator, then one would need about 5.6 * 1016 of these rockets to stop the rotation of the Earth.


I think I missed a step. How'd you get from Joules to Newtons?

Could I do the job with just one space shuttle engine if I run it for "a while"?

[RecoveringYuppy]

Could I do the job with just one space shuttle engine if I run it for "a while"?
Yes, there is a step missing there and you could do it with one if you have the time. One other caveat though: I don't know how much angular momentum we might be gaining or losing from current infall of meteors. I also don't know if that might already be factored in to calculations refered to earlier about how fast the Earth and Moon are heading towards full tidal locking. If Earth is gaining AM from some outside source we haven't accounted for yet then the minimum to accomplish this in any amount of time would be any amount larger than what it takes to compensate for that gain.

[Just thinking]

I get 3.56 x 1023 rocket engines ... but that's if you wanted to stop the Earth from spinning in all of 1 second.

I strongly suggest taking a bit more time in doing that. However, if you happen to be at the North Pole holding the switch ... ;)

[Crossbow]

I think I missed a step. How'd you get from Joules to Newtons?

Could I do the job with just one space shuttle engine if I run it for "a while"?

To explain the units a bit further:

One Joule = One Newton * One Meter

So, by placing a rocket (of a known thrust in units of Newtons) at the equator (where the distance to center of the Earth is known in units of meters) ,
Therefore the product of the thrust [Newtons] and the distance [Meters] provides the units of energy [Joules].

Using a smaller number of engines and letting them run for a while would probably be better, however the Space Shuttle engines only run for about 4 minutes, so the time factor is something that would have to be seriously considered.

[Crossbow]

I get 3.56 x 1023 rocket engines ... but that's if you wanted to stop the Earth from spinning in all of 1 second.

I strongly suggest taking a bit more time in doing that. However, if you happen to be at the North Pole holding the switch ... ;)

True enough!

The case I outlined was substantially simplified, however I really do not have the time to provide a terribly realistic answer (which could take at least a couple of days to work out and I am rather busy right now). However, I still feel that the answer I did provide does essentially address the OP of this thread.

[boooeee]

Another way to do it:

Have everybody in the Northern Hemisphere countries drive on the right side of the road and everybody in the Southern Hemisphere countries drive on the left side of the road. The effect would be subtle, but if you think it through, you realize that it would decrease the rotational speed of the earth.

It would help to have really heavy cars, really wide roads, and no speed limits.

[casebro]

If everybody on earth jumped westward, there would be no difference in the AM. Because, to jump westward, the have to thrust against the earth, thereby giving it eastward AM. Then, when they land, exactly the same amount of thrust would be used to brake them, now in a westward direction. Net thrust= ZERO. Give er take some losses.

[Dave1001]

I think I missed a step. How'd you get from Joules to Newtons?

Could I do the job with just one space shuttle engine if I run it for "a while"?

I'm sure. These type things are done in man-hours (or "shuttle units of time"). Lots of men or lots of hours does the job.

[natureboy]

It would help those kids who were born on one of those "leap days" feel alot better about themselves.

[rjh01]

5. A significant amount of mass is moved inward (or outward) relative to the Earth's centre -- thus causing angular velocity to increase/decrease like a spinning ice skater pulling in their arms (or stretching them out). This happened with the tsunami earthquake of 2005 if I'm not mistaken.

5a. A significant mass moves from the poles towards the equator. This will happen if the ice melts at the poles. The mass stays at the same distance from the centre of the Earth. At the poles there is no (or little) angular momentum. This means that if the ice caps melt we get a longer day. However you would need an atomic clock to measure it.

[Just thinking]

5a. A significant mass moves from the poles towards the equator. This will happen if the ice melts at the poles. The mass stays at the same distance from the centre of the Earth. At the poles there is no (or little) angular momentum. This means that if the ice caps melt we get a longer day. However you would need an atomic clock to measure it.

Actually I should have been a bit more specific -- the mass has to be moved either to wards or away from the Earth's axis of rotation in order for the angular velocity to change. Your example of the ice caps melting would not really change the distance to the Earth's core so much as away from the rotational axis.

I wonder how much all the tall building's man has built slowed down the Earth, or how much it sped up as a result of the erosion of huge mountains. Then again, new mountain ranges form over time as older ones disappear.

[The Bad Astronomer]

Note that 6 x 10^29 Joules is about 1500 times the Sun's output every second. So capture the Sun's entire output for an hour and you've got the energy you need. You might want to use oven mitts.

[69dodge]

To explain the units a bit further:

One Joule = One Newton * One Meter

So, by placing a rocket (of a known thrust in units of Newtons) at the equator (where the distance to center of the Earth is known in units of meters) ,
Therefore the product of the thrust [Newtons] and the distance [Meters] provides the units of energy [Joules].Why is the Earth's radius the right distance to multiply the rocket's thrust by?

Maybe we're supposed to multiply the rocket's thrust by the rocket's own radius. Or its length. Or some other distance.

Like, how far the rocket travels during its burn. Just to pick a random distance. (Ok, not entirely random...)

Using a smaller number of engines and letting them run for a while would probably be better, however the Space Shuttle engines only run for about 4 minutes, so the time factor is something that would have to be seriously considered.Yes, it needs to be considered. But have you considered it? The rocket's thrust is what it is, and the Earth's radius is what it is, regardless of how much time the rocket burns for. So your method of calculation gives the same answer whether the rocket burns for a minute or for a year.

[Dylab]

Crossbow: The force times radius is the torque. I believe to consider the change of kinetic energy you are going to need to consider the moment of inertia. The momentum will be the same either way. Plus I have found that the moment of inertia of a sphere is (2/5)MR^2. Sooo...

(Force of rockets) * (radius of earth) * (time interval) = (2/5)MR^2 *(current angular speed)
Using your numbers:
N * 1.148 * 10^13 (Kg * m^2 / sec^2) * (time) = 7.067 * 10^33 (Kg * m^2 / sec)

Where N is the number of rockets.
N (time) = 6.156 * 10^20 seconds

So for one second you will need 6.156 * 10^20 rockets.
On the other hand for one rocket you will only need to run it 19.5 trillion years.

Also I can't believe I wasted time doing that when I have a two take home exams I should be working on

[RecoveringYuppy]

Why is the Earth's radius the right distance to multiply the rocket's thrust by?

Maybe we're supposed to multiply the rocket's thrust by the rocket's own radius. Or its length. Or some other distance.

Like, how far the rocket travels during its burn. Just to pick a random distance. (Ok, not entirely random...)
Guessing you're joking a bit, but yes that last distance is the right one. It's the length of distance the force acts over that is relevant to the energy change.

[Just thinking]

Only if the rockets are placed along the Equator ... we must first check to see if there is enough available space along that latitude to do the job.

BTW ... might there be a way to slow the earth down in its orbit? The only way I can see doing it (with minimal thinking) is to place rockets at the poles such that they can rotate 360o in 24 hours. Must use both poles for balance.

;)

Hmmm ... I can see the moon becoming a tricky player in this.

Wait ... this can't work; it's like trying to get a sailboat moving by having a fan on deck blow at the sails.

[Modified]

Wait ... this can't work; it's like trying to get a sailboat moving by having a fan on deck blow at the sails.

Roboramma sort-of pointed this out earlier. The rockets will need to be mounted on long poles extending far enough out into space so that the exhaust can achieve escape velocity. I suppose generating a huge counter-rotating wind would have some effect while it lasted, but it would also make a big mess.

[RecoveringYuppy]

The rockets will need to be mounted on long poles extending far enough out into space so that the exhaust can achieve escape velocity.
Yes, and we're not talking just outside the atmosphere. Most current rockets have an exhaust velocity far short of escape velocity or even orbital velocity. They'd have be to far enough up that Earth's gravity field has fallen off enough to compensate for whatever exhaust velocity they have.

Another thought: I don't think the goal described in the original post could be satisified by putting a bunch of stuff in retrograde orbit around Earth. Wouldn't necessariy have to escape Earth. It would still move angular momentum off Earth even if it was still in orbit.

[Pipirr]

My favorite cartoon ever. (http://xkcd.com/c162.html)


Mercutio, thank you, Sir. That site has made my day!

(Especially this (http://xkcd.com/c169.html))

Apologies for slight derail...

[gnome]

Force is irrelevant. What matters is having the ability to bend the world to your will. With this Darkness Device, say.

Sincerely,
The Gaunt Man

[Just thinking]

Another thought: I don't think the goal described in the original post could be satisified by putting a bunch of stuff in retrograde orbit around Earth. Wouldn't necessariy have to escape Earth. It would still move angular momentum off Earth even if it was still in orbit.

That would make for one hellova wind storm.

[fuelair]

I trust we all realize we die when this happens. Slow or fast but dead is dead.

[rjh01]

I trust we all realize we die when this happens. Slow or fast but dead is dead.

Please do not tell me you are taking this as anything but an academic exercise. The cost of doing any of these things would be huge.

[fuelair]

Please do not tell me you are taking this as anything but an academic exercise. The cost of doing any of these things would be huge.
Actually, the cost would be prohibitive - and yes I realize that it is academic. On the other hand, original questioner should have been able to work out the needed info. Basic physics.

[rjh01]

5. A significant amount of mass is moved inward (or outward) relative to the Earth's centre -- thus causing angular velocity to increase/decrease like a spinning ice skater pulling in their arms (or stretching them out). This happened with the tsunami earthquake of 2005 if I'm not mistaken.

Actually that is why neutron stars spin so fast. Take one large old star with all of its fuel used up. It will start collapsing. The star would be so big that the atoms collapse, the protons and the electrons combine to form neutrons. Hence the name. The entire mass of the star (less a little that got ejected in the nova(s) or super nova(s) is now in a very small area, with the same angular momentum as before. So it spins very quickly, several times a second.

[senorpogo]

So in a post about stopping planetary rotation no one mentions the end of the original "Superman". I'm surprised by that. Or did I just miss it?

[Soapy Sam]

Among other problems is the fact that even if we manage to stop some local bit of lithosphere moving, relative to something or other, unless we also manage to stop the more fluid asthenosphere in it's tracks, we are just spinning our wheels.
Except for the earthquakes.

[RecoveringYuppy]

Among other problems is the fact that even if we manage to stop some local bit of lithosphere moving, relative to something or other, unless we also manage to stop the more fluid asthenosphere in it's tracks, we are just spinning our wheels.
Except for the earthquakes.
Well, we're talking a large amount of energy here. Probably enough to cool the asthenosphere until it solidifies and then some. Not sure how fast it would reheat from below though.

[Soapy Sam]

Well, we're talking a large amount of energy here. Probably enough to cool the asthenosphere until it solidifies and then some. Not sure how fast it would reheat from below though.

I suspect the friction would actually cause local melting. Stopping continental crust moving over a mantle plume can also be a bad idea.

I think we should build a space elevator to the moon, nail both ends down and see if conservation of momentum will flip both bodies clear out of the solar system.

[gnome]

So in a post about stopping planetary rotation no one mentions the end of the original "Superman". I'm surprised by that. Or did I just miss it?

He didn't truly stop it, he just appeared to because external time slowed down for him and they were showing it from his point of view.