# B If the Earth stopped spinning? (orbiting the Sun)

1. Sep 4, 2015

### avito009

Why does the Earth not fall into the sun? Is it because Earth has angular momentum? or is it because the Earth has orbital momentum?

2. Sep 4, 2015

### Drakkith

Staff Emeritus
Both. It has angular momentum about an axis passing through the Sun due to its orbital motion. You could call this orbital angular momentum. It also spins around its own axis, but that is unrelated to its orbital motion.

3. Sep 26, 2015

### Neon

why angular momentum.The moon is tidal locked so no angular momentum is applied. I believe that the moon get pulled in and swings out to the other side and get further away and repeats. Picture this, a cut rubber band attached to a tennis ball and swing it over your head. The rubber band acts like the gravity while the tennis is the moon and the hand is the earth.You see, the moon did not get pulled in by the rubber band but as it pulls in, it moves forward and continue to fall away from the earth but the earth grab it and so on and so on.Orbital motion i think is the reason. Correct me if i am wrong as it is my observation.

4. Sep 26, 2015

### 256bits

Interesting, but a few small details could be refined.

Angular momentum is not a torque.

The rubber band does have a tension force when stretched, and the force will increase with more of the more the stretch.
Gravity on the other hand decreases with distance. Gravity is entirely not like a rubber band.

5. Sep 26, 2015

### Drakkith

Staff Emeritus
Angular momentum is not applied. It is like linear momentum in that it is a conserved quantity associated with movement.

6. Sep 26, 2015

### phinds

The moon rotates around its axis. Anything that rotates around its center of mass has angular momentum

7. Sep 26, 2015

### Neon

Nothing else acts like gravity.

8. Sep 30, 2015

### Neon

9. Sep 30, 2015

### davenn

OK I will bite .... in what way do you think nothing else act like gravity ?
how about expanding on your comment and we can see if you are correct or if you just misunderstand

D

10. Oct 1, 2015

### Neon

string does not act like gravity because well you should know.Basically nothing that is elastic and the tension gets weaker the more it gets pulled.

11. Oct 1, 2015

### sophiecentaur

There is no reason why you couldn't make a device that would follow ISL for force / extension. It would be very possible if the device had 'active' control (power supplied from the central pivot) . Of course, it would not be 'elastic' but that wouldn't matter if you just wanted to produce an elliptical orbit with it. Come to think of it. I'm surprised I haven't seen this at some Science Exhibition; they all seem to make do with the conventional tapered bowl with balls orbiting round.
To hard for a School Demo piece of equipment, though.

12. Oct 1, 2015

### Buckleymanor

Let us suppose the moon did not rotate around it's own axis even though from Earth it would look like it does.This property would not cause the moon to fall into the Earth any more than the Earth would fall into the sun if it behaved in a similar manner.
It is the orbital rotation of the Earth around the sun irrespective of it's angular momentum about it's own axis which stops it falling into the sun.

13. Oct 1, 2015

### sophiecentaur

The fact that the Moon and Earth are not homogenious spheres means that the angular momentum of each of them becomes relevant. The tidal effects between the two is causing the angular momentum of their individual rotations to be coupled to the angular orbital momentum. The effect is that the Moon's orbit around the Earth is very slowly increasing in radius. So you cannot completely separate the rotational and orbital momenta.

14. Oct 1, 2015

### Buckleymanor

I agree but as far as the op is concerned it's about a decrease in radius rather than an increase.
Would the Earth eventualy fall into the sun if it became locked in it's orbit.

15. Oct 1, 2015

### phinds

I was simply responding to the issue of whether or not the moon has angular momentum. Do you argue that it does not? I was not addressing anything about orbits.

16. Oct 1, 2015

### DaveC426913

It is moving at 60,000mph perpendicular to the direction of the sun. How would you cancel this momentum?

17. Oct 2, 2015

### Buckleymanor

Obviously from my reply I regard it does have angular momentum .Mt reply was just an attempt to bring the thread back on topic as far as all the op questions.

18. Mar 8, 2016

### john george pa

the earth is falling into the sun - but it is falling in a straight line in curved space so it never gets there - try imaging it from the side you would see a corkscrew path