AZING SPACE: Understanding Orbital Motion

[Andrew Mason]

So too for the earth. The fact that such a large chunk of the Earth out in space implies that it did disintegrate. Because of gravity, this disintegration was not necessarily permanent (as Andre said). Any such impact would be far too big to leave a crater. The Earth would have essentially been pulverized and reformed spherically.

I don't see why the Earth would have necessarily been pulverized. The moon is "only" 1/81 part of the Earth in mass.

Is it not possible that an asteroid of considerable energy but even smaller could have struck and tossed a lot of molten matter high up into space which then formed the moon.

AM

 

[Moneer81]

Doesn't every celestial object (planets, stars, etc.) rotate? isn't that a property of all celestial objects?

 

[Andrew Mason]

Doesn't every celestial object (planets, stars, etc.) rotate? isn't that a property of all celestial objects?

I would agree with you. While it is theoretically possible for an object to have 0 rotation, it is a very 'unstable' state.

This is how I would explain it:

Every celestial object has to be in some form of obital motion about some other mass. It may not be very fast (as in the sun's orbital motion about the center of gravity of the Milky way galaxy. A galaxy has to rotate or it will collapse into a huge ball of matter (ie. an unstable state).

So, if every object is in 'orbit' around an axis through some distant point, then it will tend to a rotation about a parallel axis through its centre of mass (as in the moon, which has a rotation that is synchronous with its orbital rotation). If successfully resists the gravitational torques that tend to make objects rotate in synchronous orbit (so that it does not rotate about that axis at all - ie. such as the earth), that resistance must be due to gyroscopic forces: ie because it is rotating about another axis (ie. the Earth rotating about its polar axis). It seems to me that there is nothing the object in space can 'push against' to generate counter-torque unless it has angular momentum.

In summary:
1. all objects are in orbit of some kind.
2. all orbiting objects experience torques that tend to cause the object to face the centre of rotation (which gives the object rotation about its own centre of mass)
3. These torques can only be resisted by gyroscopic forces, which means that the object must be rotating about some other axis.

AM

 

[Moneer81]

Every celestial object has to be in some form of obital motion about some other mass
AM

why is that the case?

 

[Andrew Mason]

why is that the case?

Because of gravity. It may not be a closed orbit, but it is still experiencing gravitational acceleration from something.

AM

 

[Moneer81]

Because of gravity. It may not be a closed orbit, but it is still experiencing gravitational acceleration from something.

AM

ok that's fair enough...but what confuses me is why does gravity cause the orbit although it is a central force? why don't these objects gravitate towards each other till they collide?

 

[Andrew Mason]

ok that's fair enough...but what confuses me is why does gravity cause the orbit although it is a central force? why don't these objects gravitate towards each other till they collide?

Well they do. But the amount that one accelerates toward the other is in inverse proportion to their respective masses.

[correction:]I see that you were asking 'why don't they collide' (rather than why don't they accelerate toward each other). The answer has to do with the velocity relative to the gravitating object. Sometimes they do collide, of course. But if the object has sufficient tangential speed, it will fall toward the gravitating object but keep missing it - hence orbit.

AM