# Back and forward orbit [around gravitating objects]

by Edi
Tags: levitate, orbit
P: 173
 Quote by Drakkith ...but that isn't what you said...
Well, context is important..

More precisely, object A would not as much push object B up while accelerated/ moving at orbital speeds, but it will hold object B from falling down for the time being or, at least, slow down B's decent? (assumed both started with no orbital speed at all.)
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P: 11,056
 Quote by Edi Well, context is important.. More precisely, object A would not as much push object B up while accelerated/ moving at orbital speeds, but it will hold object B from falling down for the time being or, at least, slow down B's decent? (assumed both started with no orbital speed at all.)
No, it takes time to accelerate object A, during which both A and B are falling at the same rate. A is not slowing B down at all.
P: 173
 Quote by Drakkith No, it takes time to accelerate object A, during which both A and B are falling at the same rate. A is not slowing B down at all.
OK.
Then what about this:
Object A is already orbiting the planet and object B is orbiting the same planet in a orbit just slightly above A's orbit. Object B hits another object, object B1, that is the just like object B, orbiting the same path, but in the opposite direction, so when they hit each other, they both come to a full stop. At that exact point in time, object A just happens to be in the spot of collision, just a bit, bit lower so it is sliding beneath object B and B1 - will this delay their [object B and B1] descent ?
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P: 11,056
 Quote by Edi OK. Then what about this: Object A is already orbiting the planet and object B is orbiting the same planet in a orbit just slightly above A's orbit. Object B hits another object, object B1, that is the just like object B, orbiting the same path, but in the opposite direction, so when they hit each other, they both come to a full stop. At that exact point in time, object A just happens to be in the spot of collision, just a bit, bit lower so it is sliding beneath object B and B1 - will this delay their [object B and B1] descent ?
Hmm, I'm not sure. Gravity is pulling them both down at this point, so I can't say. And I think I'm off to bed. Hopefully someone else can answer this for you while I'm asleep. If not I'll try to give it another shot after I get up.
 P: 173 Ok, so im just going to continue anyway. Exactly - gravity is pulling them both down, but as object A comes in with its angular momentum and is sliding beneath them, it kicks them both up a bit. (?)
 P: 735 Hi Edi. I think I know what you're trying to do. I hate to admit that use to spend way too much time thinking about stuff like this. But I don't really regret it so much because I learned from it. You need to ask yourself if any of Newton's laws are being violated. Even if your answer is "no, it only appears that way", then you still need to rethink it. Even though your original concept of the back and forth orbit may be theoretically possible, you need to think about why it cannot work in a self contained unit. I don't have much time so I don't know if I will be able to get back to this thread anytime soon. So good luck with your learning project. :)
 P: 173 But what about the "object in a string" with the rotation plane being parallel to the ground?
P: 3,551
 Quote by Edi As I figure [and have consulted with some physicists], there no reason why this would not work: object orbiting a planet in one direction, then, half way trough, hits something that propels it in the other direction [at orbital speed, of course], orbits the planet the other way around and hits something that propels it in the other direction again and then the cycle continues back and forward, keeping the object in orbit, but not in a full orbit, but, in this case, just orbiting, essentially, one side of the planet.
Take two identical balls, each doing just half of the orbit, bouncing elastically at two opposite points. An orbital Newton's cradle.

 Quote by Edi Now, that (if) this works, there should be no reason why it has to be the whole half of a planet - it can be any distance at the orbits circumference. Even 100 meters. Right?
Sure, you can have a ball bouncing back and forth between the walls of an evacuated spaceship, that is in orbit. But you loose energy on each bounce.
 P: 21 I think it will fall while in the process of bouncing back as it cannot do this instantaneously without an infinite force. During bouncing back it will have a velocity less than required to maintain orbit
P: 173
 Quote by Adeste I think it will fall while in the process of bouncing back as it cannot do this instantaneously without an infinite force. During bouncing back it will have a velocity less than required to maintain orbit
.. unless the nenergy is provided from a power source.
Sci Advisor
Thanks
P: 2,960
Backing up to the original question here.....

 Quote by Edi As I figure [and have consulted with some physicists], there no reason why this would not work: object orbiting a planet in one direction, then, half way trough, hits something that propels it in the other direction [at orbital speed, of course], orbits the planet the other way around and hits something that propels it in the other direction again and then the cycle continues back and forward, keeping the object in orbit, but not in a full orbit, but, in this case, just orbiting, essentially, one side of the planet.
It is true that if an orbit in one direction works, then an orbit at the same height and speed in the other direction will work as well. So yes, if you could arrange the collision to instantaneously send the object back in the opposite direction at the same speed, it would behave as you describe.

 Now, that (if) this works, there should be no reason why it has to be the whole half of a planet - it can be any distance at the orbits circumference. Even 100 meters. Right?
yep... still good....

 Now the most interesting part. If we put this object a, say, evacuated tube with magnetic system to propel the object back and forward....
You have to remember that if the evacuated tube with the magnetic system is applying a force to the object, then the object is also applying an equal and opposite force on the magnetic tube. So to analyze the situation, you have to consider the behavior of the tube+object as a system, not just the object in isolation; and both the linear and the angular momentum of the system must be conserved. Include this in your calculations and you'll find that the interesting stuff that you're hoping for won't happen.

(Imagine that you were inside a cardboard box and you were trying to levitate it by jumping - you might get it off the ground by jumping up and hitting the ceiling on your first jump, but on your next jump your feet will just shove the floor back down again. Your orbital example is basically the same problem, except with angular momentum involved as well).
 P: 173 And what about the ball in a string scenario? If it object can stay in orbit by bouncing, can it stay in orbit while rotating parallel to the ground?
P: 735
 Quote by Edi And what about the ball in a string scenario? If it object can stay in orbit by bouncing, can it stay in orbit while rotating parallel to the ground?
No. An object remains in orbit not only because of it's velocity but also because of it's trajectory. Are you familiar with Newton's cannonball? If the object has orbital velocity then it will orbit because the earth curves away faster than gravity can pull it down. If the object were to travel in a circle parallel to the surface then it could not achieve orbit no matter how fast it travels. That's because it will always be parallel to the earth's surface and thus the same distance from the source of the gravitational force - regardless of how fast it is traveling. In other words, in order to maintain orbit the object must have orbital velocity tangent to the great circle around the earth.
P: 173
 Quote by TurtleMeister No. An object remains in orbit not only because of it's velocity but also because of it's trajectory. Are you familiar with Newton's cannonball? If the object has orbital velocity then it will orbit because the earth curves away faster than gravity can pull it down. If the object were to travel in a circle parallel to the surface then it could not achieve orbit no matter how fast it travels. That's because it will always be parallel to the earth's surface and thus the same distance from the source of the gravitational force - regardless of how fast it is traveling. In other words, in order to maintain orbit the object must have orbital velocity tangent to the great circle around the earth.
What about an ellipse instead of the circle?
P: 735
 Quote by Edi What about an ellipse instead of the circle?
That's just a modification to your "tube" idea.

Your original idea is plausible if you account for the losses during the change in direction, ie., the losses that another poster pointed out previously. You could do this by using a rocket engine to give the object an extra push after each change in direction. However, I think your idea of having all of this housed in a closed self contained system will not work. Do you know why I think that? Have you asked yourself if any of Newton's laws are being violated with this idea?
P: 1,351
 Quote by Edi What about an ellipse instead of the circle?
With an object that is in a normal circular or elliptic orbit around the earth, the average force of gravity over a complete orbit is 0. If this wasn't so, the object would crash into the earth or escape from orbit.

The average force of gravity can only be 0 with a circular orbit with the center of the earth in the center, or with an elliptic orbit, with the center of the earth at one of the focii.

For any other orbit you'd need an external force to prevent falling to the earth.

If you let an object bounce between 2 walls, the walls provide this force if they are not quite parallel. Two vertical walls some distance apart won't be parallel, Since they are aligned with non-parallel lines through the center of the earth. The object will get a tiny upward push each time it bounces.
 P: 173 Ok, thatk you for your time, people. I will chew through this information now. :)

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