# Instantaneous stop

I've seen a few posts about what happens if the Earth stopped moving completely. Most seem to assume a scenario in which the Earth stops rapidly. In such scenarios, I have read that there would be tidal waves and extremely fast tornados.

Other posts (such as this one) state there will be no more hurricanes and the climate would obviously be different; with half the Earth overheating.

# More time

At dinner today we got onto the generic discussion of how there "isn't enough time in a day". It was this question which led to me saying we should develop a way to slow down the Earth's rotation.

## A giant parachute

Obviously this isn't a good solution but one I quickly came out with.

I said that if we could slow the Earth's rotation then we'd have more time in our days. I couldn't see a flaw in this plan. Simply erect a giant parachute (or sail or fin like on a plane) to encounter air resistance ergo slowing the velocity of the Earth.

There are many flaws to this (naturally), such as the lack of air resistance in space meaning that there won't be any counter acting force to slow the Earth down.

Then the topic of gravity came up. Now I'm no expert when it comes to Physics (I love the idea of it all (but am too stupid) and I watched every episode of The Big Bang Theory), so correct anything that's wrong here. But, my understanding of gravity is that it's to do with mass and velocity. Now, I don't know why I think this or where I'd have heard it but it's what I think.

# All things gravity

I don't really know how gravity works. I understand it that the more mass and high velocity then gravity will be stronger.

## Slow, slow, fly off

One of our party stated that if the Earth were to slow down then we'd "fly off" the Earth...

I found this hilarious but it might be true. Here's what I think:

## Jump high

I thought that if we slowed down then gravity would be weaker and we'd be able to jump higher!

# Finally

All things considered. I'll get to my point.

## Too long, didn't read (tl;dr)

If the Earth's rotation were to slow down, what effects would take place? (By "slow down" I mean if it span at about half the rotations)

## Questions

1. Would the Earth's path around the sun change?
2. What would happen to our gravity?
3. Would the Earth spinning at half speed mean our days are twice as long?
4. A comical question: could we slow down the Earth with some sort of parachute like a dragster has?
4.1. What about adding rockets to make us spin faster? (I'm sorry these are from the point, just find it a funny thought)

### My attempts

All my answers to these questions are based on my lack of knowledge of physics.

1. Yes. I believe that we'd drift further from the Sun. After that I'm not sure whether we'd maintain a constant path around the sun or just continue away from the Sun.
2. As stated above, I think we'd have weaker gravity.
3. Yes. I feel like this is simple maths. Right? Right?!
4. Making it big enough and keeping it within our atmosphere then yeah, I suppose we could.
4.1. As long as we don't go too fast, I don't want a headache.

Disclaimer: I am new here, I'm sorry if I haven't followed the question asking convention.

## closed as off-topic by John Rennie, sammy gerbil, ZeroTheHero, Yashas, Kyle KanosMar 29 '17 at 10:08

This question appears to be off-topic. The users who voted to close gave this specific reason:

• "We deal with mainstream physics here. Questions about the general correctness of unpublished personal theories are off topic, although specific questions evaluating new theories in the context of established science are usually allowed. For more information, see Is non mainstream physics appropriate for this site?." – John Rennie, ZeroTheHero, Yashas
If this question can be reworded to fit the rules in the help center, please edit the question.

• I'm voting to close this question as off-topic because it is a broad, open-ended, hypothetical, "What would happen if...?" type of question, to which there are many possible answers. – sammy gerbil Mar 29 '17 at 6:47
• Yeesh, what's wrong with interesting hypothetical questions? It leads to a beneficial discussion. Why do you mods have to be so strict and uptight on what people are allowed to say or ask on this site... – Inertial Ignorance Mar 29 '17 at 7:07
• @sammygerbil I didn't want to start an argument. The thing with physics (as I understand) is that it's all theory? No one knows the truth about most things it's all really, really good assumptions. If you wish to close this then very well. I just thought someone might have a good answer to what I've stated above – JustCarty Mar 29 '17 at 7:09
• If there was more time in a day, society would put more tasks on us per day and we would be off where we started. – gerrit Mar 29 '17 at 9:33
• I believe that questions of the "What if type...?" are really interesting. They make us think about the possible ramifications of theories and understand them better. No wander great physicists entertained themselves answering them... – George Smyridis Mar 29 '17 at 10:44

1) No, the Earth's path around the sun would not change much. The Earth's speed of rotation around its own axis does not affect the speed at which it is orbited around the Sun. A "year" would still be the same amount of time for us, it's just that the days would be longer and there would be less days.

2) Gravity would also not change noticeably. Gravity is a force between two objects that depends on two factors only: Their mass and the distance between the two objects. In our case, gravity is the attraction between Earth and people. The speed of rotation of the Earth does not change the mass of Earth or people, nor does it change the distance between people and the center of Earth.

3) Yes, days would be twice as long. A "day" is when the Sun shines on a part of the Earth. "Night" is when the Sun does not shine on the Earth. The Earth rotating half as fast would mean "day" and "night" would be twice as long.

4) I doubt you could slow down the Earth's rotation with parachutes. The thing is, Earth does not rotate through its own atmosphere - it rotates around an imaginary axis in space. As for adding rockets, I suppose you could cause the Earth to rotate slightly faster, although you would need rockets far more powerful than what we currently have in order to make a negligible difference at all.

• I hadn't thought about the less days but I suppose that's obvious. Also what about F = ma? Couldn't that be rearranged to m = F / a and therefore during the slowing period wouldn't mass change and therefore gravity too? And yeah point 4 was silly, I suppose rockets could slow the Earth if we turned them around;) – JustCarty Mar 29 '17 at 7:29
• Regarding point number 2, is there a connection between knot theory and spacetime? If so, is it plausible to assume that a change in rotational velocity could somehow contribute to forming such a knot and having a measurable effect on gravitational fields? If the answer to that is yes, could that affect the Moon and consequently tides? – MPath Mar 29 '17 at 11:23
• @Carty - The key here is understanding what that "a" variable means. It stands for the acceleration that Earth pulls objects towards it. Earth pulls all objects towards it at an acceleration of 9.8 m/s^2 and this is the "a" variable. Its speed of rotation would not affect this, only its gravitational field strength affects "a". And gravitational field strength only depends on two things - the mass of the planet and how far away an object is from the planet's center. The Earth's rotation slowing down wouldn't change its mass or our distance from its center. – Inertial Ignorance Mar 29 '17 at 18:44
• @mikey - Sorry, I'm not too familiar on knot theory. I'm sure the speed of rotation changing would have some effect on Earth's gravity, although I doubt it would be a lot. From a basic newtonian perspective, I don't see how that would affect Earth's gravity on the moon, but it could very well from a different aspect of physics. – Inertial Ignorance Mar 29 '17 at 18:46

Some thoughts:

• Lengthening days does not necessarily change the duration of our life. It would first of all change the partitioning of our time. So if our days were twice as long, most likely we would only see half of them in total. However, this is a medical question rather than a physics question, and probably the day length has an effect on our life span. I don't know if there are any studies on the subject, but I doubt it.

• The idea of parachutes does not work, because our atmosphere is rotating together with the Earth. See for example the gas giants like Jupiter in our solar system. They consist mainly of gases (call it their atmosphere), and they also rotate. Don't let yourself mislead from the fact that our atmosphere is comparatively thin.

• Rockets could in principle be used to change the rotation speed of the Earth. This would require a vast amount of mass being ejected at very high velocities (at least Earth's escape velocity) away from Earth.

• One minor addition to @Inertial Ignorance's answer: The rotation causes the Earth to be flattened. A slower rotation would slightly change this flattening, resulting in Earth's gravity to change slightly. This would be measurable, but not directly noticeable in everyday life.