# Does the Earth spin because its surface has less mass than its core as lighter things travel faster in space?

Would heavy elements from space hitting the Earth slow down the earth's rotation as the difference between the masses of the core and surface would be less?

• More on reasons for initial spin: physics.stackexchange.com/q/23104/2451 , physics.stackexchange.com/q/12140/2451 and links therein. Commented Aug 18, 2013 at 10:59
• You should give information about yourself on your user page. Apparently, you know nearly nothing about physics, which is a bit short for this site. And you should explain your assertions. It may only be me, but I found your (wrong) explanation of greater speed for lighter bodies interesting as it is actually based on observation. Trying to base your knowledge on observation is good, but will not get you very far in one lifetime. So, as Newton would say, you should also see more and further by sitting on the shoulders of scientists who came before you, which means some formal study of physics. Commented Aug 18, 2013 at 13:03

The Earth spin because of the collisions it withstood at its early begining. One theory stipulates that it actually collided with another planet form the solar system which was going opposite direction. The collision gave birth to the moon. It also explains why the moon is always facing earth the same way.

Nevertheless, it certainly has nothing to do with the difference of mass. You may also note that heavier elements do not travel slower, they are simply require more energy to accelerate.

Finaly, elements hitting Earth from space could slow Earth rotation if they had the right collision course, but the probability for any given object to hit Earth in a way that would slow it down is virtually the same as the probability for a collision that would accelerate it.

• Doesn't the earth spin because when it was made the particles that formed it was rotating around each other due to gravity?
– Reds
Commented Aug 18, 2013 at 10:45
• Of course, but that rotation didn't originate form nowhere. A certain side of rotation must have had more kinetic energy than the other thus taking the Earth to rotate there way. It's a bit like the water in a sink. It's going to start spinning the side the more particle are moving or with the more energy. Theoretically, perfectly still water would not spin when drained. It's the same with the formation of Earth. Now if another planet collided it certainly bring a lot of kinetic energy in it's direction making Earth spin its way, but as I said, the other planet is an unconfirmed theory. Commented Aug 18, 2013 at 10:53
• Doesn't the Moon face Earth the same way because of tidal locking? Most major moons in the Solar system are tidally locked to their planets. Commented Aug 18, 2013 at 13:09
• Maybe it's the only reason, but I'm just saying in that theory it plays an important role. I remember reading an article which explained some other characteristics about moon orbit which were uncommon, I'll see if I can find it, but in the meanwhile, you can read about this hypothesis if you are interested: en.wikipedia.org/wiki/… Commented Aug 18, 2013 at 13:18
• cheers guys as i now realise that if nothing else i can ask a decent question! Commented Aug 21, 2013 at 8:04

Rotation generally comes from existing angular momentum in the initial cloud that increases in angular speed as the collapse into a denser body lowers the moment of inertia.

I do not understand what you mean by "lighter things travel faster in space". It is a strange statement. It is known that light thinking travels faster than heavy thoughts, but I do not think that the properties of the noosphere extend to matter in the physical universe.

However, that is certainly not the reason for the Earth spin as, according to information you could have found yourself on the internet, the core of the earth spins a bit faster (a very tiny wee bit faster) than the outer part (example).

By the way, the surface of the planet does have less density than the core. To state that it has less mass, you should first make precise how deep you consider the surface to extend.

Change at the surface of the planet due to heavy matter hitting Earth from space is however relevant. This matter does hit the planet with some speed that could increase or decrease (very minutely) the rotation speed over time if it were always acting in the same way. However, increases and decreases probably average to nothing over time. Still, by increasing the mass at the suface of the planet, it does increase the moment of inertia (the resistance to changes of rotation speed of a spinning body), and hence it slows down extremely minutely over the millions of years the Earth rotation speed (exactly for the same reasons that the initial cloud rotated faster when collapsing into a planet). Note that this is dependent only on the total mass falling on the planet (and where it falls), it is not dependent on the heaviness of each chunk, or its density.

• a bit deep for me, but was thinking that from the piont of view from our own solar system the sun appears stationary. the earthe travels around the sun and the moon also travels around the earth. the distance travelled by the moon is much greater than the distance travelled by the earth and the reason it is greater is its mass is less. no? surly the same thinking can explain why bodies in space rotate. or am i missing something completely? sorry Commented Aug 18, 2013 at 11:38
• That is an amusing observation you made. But it is a bit like saying that people move faster than trains because, as they travel in the train, they can move inside (say, from side to side) and hence cover more distance in the same time. The Earth rotates (on itself) around its center of mass. Similarly Moon and Earth rotate both around a point that is their common center of mass, which is closer to the heavier body in proportion of masses. Hence the Moon has to travel a circle with a longer radius. They are both passengers of the Earth-Moon system (train) that travels around the sun. Commented Aug 18, 2013 at 12:26
• If you were replacing the Eath-Moon system by the moon alone on the same orbit around the sun, it would simply follow the current path of the Earth-Moon system exactly at the same speed (up to very tiny approximations). The same is true for Earth alone. So the speed and distance covered by bodies in a given time is a much more complex issue than an inverse proportion to weight (or rather mass) of the concerned bodies. It is however true that, for the same given impulse, a lighter body will travel faster than a massive one, actually twice as fast if it is half the mass. Commented Aug 18, 2013 at 13:19
• The Earth-Moon system is like a seesaw, with a big guy on one side and a small guy on the other side. To balance the seesaw, the big guy must sit closer to the axis (some seesaws actually have 2 seats on each side) so that their common center of mass is on the axis. When balancing, the small guy far from the axis will go further up and down than the big guy close to the axis. It is exactly the same for Earth and Moon: the small guy (Moon) is far from the axis (center of mass) and thus moves more when balancing the big guy (Earth). It works like levers. Commented Aug 18, 2013 at 16:02
• i like your "seesaw". would the fictional "heavy elements" alter the axis altering the centrifugal force? thanks Commented Aug 19, 2013 at 10:17

angular momentum of particles causes the body to spin. think of ice skaters. as they pull their arms in they speed up. same goes for particles sucked in by gravity. earth's collision with the body that formed the moon may have impacted the rotation speed. but it was already spinning.

• thanks, i suppose that everything spins as there are infinatly more ways for a body to spin than not. Commented Aug 19, 2013 at 10:00