# Is planetary angular momentum

1. Apr 6, 2015

### quincy harman

Essentially just a consequence of inertia and gravity?

2. Apr 6, 2015

### phinds

Not sure what you mean by bringing gravity into it. Angular momentum is a consequence of rotation and mass, not rotation and weight.

3. Apr 6, 2015

### quincy harman

Then where does rotation come from? Gravity is the accelerating force right?

4. Apr 6, 2015

### phinds

The original gravitation attraction that cause matter to clump to form a planet or star brought in matter that was not all uniformly headed for the center of the mass accumulation, thus there was initial angular momentum, but I still think it's a stretch to say that gravity is directly responsible. It's the initial offset from pure radial infalling that causes the rotation / angular momentum. Gravity is quite capable of drawing in matter that is not offset from pure radial infalling, in which case there is no angular momentum, so how can you say it's gravity that is the cause?

5. Apr 6, 2015

### quincy harman

because momentum at this point can only be transferred right? then once the particles are bonded by gravity they wont have anywhere else to move but around Each other creating angular momentum. The same reason that gravity is the force that allows the Earth to be in constant acceleration. Inertia insures constant velocity while gravity changes direction.

6. Apr 6, 2015

### phinds

If I understand what you mean, then yes.

I think your wording is weird but I think you get the idea

.I'm not at all clear what you mean by all this.

Gravity keeps the Earth from flying apart, angular momentum tries to get it to fly apart. Constant velocity is just because there is no force opposing constant velocity (forgetting for the moment the rather trivial contribution of the moon). I'm not sure where you think inertia comes in. Inertial is a consequence of the angular momentum, not a cause of the angular momentum.

7. Apr 6, 2015

### quincy harman

The law of inertia states that an object in motion stays at constant velocity unless a force acts on it and change in direction is a change in velocity is acceleration so in essence a spinning object is in constant acceleration. nd change in velocity is change in direction I wouldn't know how else to word it other than once bonded by gravity they would have no other place to transfer momentum thus producing angular momentum.

Last edited: Apr 6, 2015
8. Apr 7, 2015

### jbriggs444

Yes. Perfectly correct
A change in velocity does not necessarily involve a change in direction.
Linear momentum is conserved. There is no need to "transfer" linear momentum into anything else. Angular momentum is conserved. It cannot be produced at all. If it is discovered to exist, it was always present. What gravity can do is bring moving bits of matter closer to one another so that the angular momentum that had always existed manifests as a higher spin rate -- like a figure skater bringing her arms in close.

Last edited: Apr 7, 2015