AZING SPACE: Understanding Orbital Motion

[Andrew Mason]

Well the fastest way a mechanical impuls travels is with the speed of the P-waves, about 5 km/sec in granite. The average for the Earth is about 3,5 km/sec (source not available at the moment).

If a impuls would tend to slow the spinning it would be no good if it went the shortest way, though the middle of the Earth because it would not have a effective component that would change the spinning. So our impuls has to follow the surface of the Earth to change the speed over there. To the other side of the Earth, that's 20,000 km or 4000 seconds (5 km/sec) or 5714 seconds (3,5 km/sec), or 67 mins resp 95 minutes, okay?

Okay. I see what you are saying. But I think what would happen is that there would be an enormous matter wave that would keep going around the Earth until it died out eventually.

The Earth would still receive the angular momentum impulse from the asteroid. It is just that the angular momentum would be in the matter wave (ie. in the momentum x radius of all the matter in the wave). The increase in rotational speed of the Earth would occur gradually as that matter wave was absorbed.

I still don't see why the Earth would break apart. The moon is thought to have come from a large asteroid impact on the Earth about 4 billion years ago, sending up a chunk of the Earth into space. That is one theory, at least. Obviously, the Earth did not disintegrate then.

Am

 

[Andre]

Would be interesting to model that hypothetical event. Perhaps the Earth may have been shattered initially by that event due to the mentioned inertia but gravity may have rebuild the heap of big chunks into the present Earth - Moon system.

Only a thought

 

[Andrew Mason]

Would be interesting to model that hypothetical event. Perhaps the Earth may have been shattered initially by that event due to the mentioned inertia but gravity may have rebuild the heap of big chunks into the present Earth - Moon system.

If the moon is the result of the Earth being struck by a giant asteroid, where do you think it hit? ie. where is the largest expanse of low lying ground on earth? What shape (generally) is it? Just a thought.

AM

 

[Andre]

Well if you suggest the pacific, there is a small problem. Plate tectonic has send the continents drifting over the globe the last couple of billion years, forming super continents and breaking them up again roughly with 500 Ma intervals.

 

[russ_watters]

The question was "What would the Earth be like if it did not rotate?". That could be if the Earth never rotated in the first place or if it suddenly stopped rotating.

I agree, if the Earth did not rotate to begin with. But not if a rotating Earth stopped rotating.

Ok, fair enough: I was trying to conform this question to physical reality. Ie, it doesn't break any laws of the universe to speculate on if the Earth hadn't been rotating when it formed, but to speculate on it spontaneously stopping does break the laws of the universe. I don't consider such speculation useful, since (for example), if the Earth suddenly stopped rotating, earthquakes wouldn't be your first worry: stopping yourself before hitting your wall at 1,000 mph would be...

If it hit vertically, the Earth would acquire all of the asteroid's momentum in the time it takes for the asteroid to stop. Why would it be any different for angular momentum? In other words, where does the momentum "go" in that 90 minutes?

That momentum distributes itself through the Earth for 90 minutes. Consider a tennis ball hitting a wall - the point where the ball hits the wall stops immediately on hitting the wall, but the other side of the ball is still moving toward the wall and the ball compresses. Consider a nail: when you hit it with a hammer, the far end of the nail does not start moving instanly: the nail compresses and the far end starts moving when the pressure wave reaches it at the speed of sound.

So too for the earth.

I still don't see why the Earth would break apart. The moon is thought to have come from a large asteroid impact on the Earth about 4 billion years ago, sending up a chunk of the Earth into space. That is one theory, at least. Obviously, the Earth did not disintegrate then.

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).

If the moon is the result of the Earth being struck by a giant asteroid, where do you think it hit? ie. where is the largest expanse of low lying ground on earth? What shape (generally) is it? Just a thought.

Any such impact would be far too big to leave a crater. The Earth would have essentially been pulverized and reformed spherically.

 

[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