# How gravity make planet to rotate?

1. Feb 26, 2009

### freeman1879

how gravity make planet to rotate???

If you asked any body why things rotate;they will answer you if you mean in astronomy the reson is gravity and if you mean at the level of atom the resone is electracal force but that is not what i want .
i want to know how gravity do this and what realy it is ,and tha same in atome
we say that electron is negative and proton is positive(+ve)
but what is the meaning of those sines
please help me.

2. Feb 26, 2009

### Phrak

Re: how gravity make planet to rotate???

Gravity, as you mean it, doesn't make things rotate.

3. Feb 26, 2009

### Blenton

Re: how gravity make planet to rotate???

Well technically, didn't it? The cloud of dust started attracting one another due to gravity as it formed the earth which caused the spin the earth has today did it not?

4. Feb 26, 2009

### D H

Staff Emeritus
Re: how gravity make planet to rotate???

I'll assume you meant orbit, or revolve, rather than rotate, freeman1879. The Earth rotates about its own axis once every sidereal day (about 4 minutes shy of 24 hours) and orbits, or revolves, around the Sun once every sidereal year (about 20 minutes longer than the basis of the calendar, the tropical year).

The Earth's orbital velocity around the Sun is about 30 kilometers per second. The velocity vector's direction is very close to perpendicular to the line between the Sun and the Earth. The acceleration of the Earth toward the Sun is about 5.9 millimeters per second squared. So, in one second the Earth has moved about 30 kilometers in a direction roughly perpendicular to the Sun-Earth line. That small acceleration means a tiny change in velocity of about 5.9 millimeters per second, directed normal to the velocity vector. This tiny change doesn't change the magnitude of the velocity vector; it instead makes it point in a slightly different direction. The next second, the Earth moves another 30 kilometers, but in a slightly different direction, and the velocity vector takes another very tiny turn. Do this 31+ million times (the number of seconds in a year), and the Earth will return to the same spot it started, always staying about 1 astronomical unit from the Sun.

5. Feb 26, 2009

### Nabeshin

Re: how gravity make planet to rotate???

DH Has well covered revolution, but on the off chance that you did mean rotate I'll provide a partial explanation:

The rotation of celestial bodies (Earth, sun, galaxy) is a result of the conservation of angular momentum. When a large cloud of gas, say, collapses, not all of the material falls directly into the middle of the cloud. In fact, most of the gas "misses" the center and enters into something of an orbit around the center. We can characterize these orbits with a certain amount of angular momentum which must be conserved. As a structure forms out of the gas (i.e the sun is formed), angular momentum is conserved and the resulting object (sun) is spinning (rotating).

6. Feb 26, 2009

### Staff: Mentor

Re: how gravity make planet to rotate???

To answer this one more directly: the fact that the cloud of dust had a net angular momentum means it was rotating. What gravity did by making it collapse was decrease the moment of inertia, which increases the rotation rate, while keeping the angular momentum the same. Like a spinning figure skater pulling her arms in.

7. Feb 27, 2009

### Chronos

Re: how gravity make planet to rotate???

When matter coallesces into a graviationally bound body, rotation is the remnant of any such object. In Newtonian physics its called conservation of angular momentum.

8. Mar 3, 2009

### Saturni

Re: how gravity make planet to rotate???

Gravity in fact, started planets, bringing new matter into the atmosphere of that matter.
And by rotate you mean orbit, am I right?

Chronos: You might want to stop consuming your offspring, it's disgusting! <:D.

1. Kronos
2. Khronos
3. Kronus
4. Cronos
5. Cronous
6. Chronus
7. Chronos

Spelled many ways. :P
Blame it on the Greeks!

9. Mar 4, 2009

### Chronos

Re: how gravity make planet to rotate???

No, i meant rotate. Gravity imparts rotation on matter as it collasces to form a planet [or star]. Rotation is the remnant of that action [conservation of angular momentum thing]. Physics 101. Chronos is my only moniker. Only one identity per user is permitted here. I do, however, plead guilty to inspiring imitators.

Last edited: Mar 4, 2009
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook