2-Body inverse cube problem

[The_Brain]

Hello, everyone. Here is my question and my thoughts on it:

Suppose we lived in a bizarre world in which gravitation, instead of being an inverse square law, were an inverse cube law.
(a) In this world, show that the 2-body problem can be brought to a central force problem in a plane P by an appropiate choice of coordinates.
(b) Write this central force problem in polar coordinates in P.
(c) Describe the motions. Are there any where r(t) is bounded away from 0 and infinity?

My thoughts:
(a) This has nothing to do with the fact that gravity is an inverse cube law. Just use radial coordinates and the reduced mass to write it in terms of radial vectors. My only qualm is that reduced mass comes from Newton's 3rd Law and maybe Newton's 3rd Law doesn't hold in inverse-cube gravity?

(b) Normally, the central force problem is written as: http://www.answers.com/main/ntquery;jsessionid=5tbtqmmn2rtno?method=4&dsid=2222&dekey=Two-body+problem&gwp=8&curtab=2222_1&sbid=lc03a&linktext=Two-body%20problem
but I am wondering whether the tangential component of acceleration is not equal to zero as it is here because the radius will not be constant...

(c) I'm pretty sure that the motions will be spirals and are thus unstable so there is no r(t) where the motions are stable (or not bounded away from 0 and infinity). I'm pretty sure this is the right answer and is partially what is tripping me up on (b) as now the radius won't be constant therefore there will be a tangential component to acceleration in polar coordinates.

Any thoughts or suggestions are appreciated.

 

[anathema]

Thank you for your interesting question. I would like to offer my thoughts and insights on this topic.

Firstly, I agree with your thoughts on part (a). The inverse cube law does not affect the fact that the 2-body problem can be reduced to a central force problem in a plane by using radial coordinates and the reduced mass. This is a fundamental principle of classical mechanics and remains valid regardless of the form of the force law.

Moving on to part (b), you are correct in noting that the tangential component of acceleration will not be zero in this scenario. In polar coordinates, the equations of motion would be:

r'' - r(θ')^2 = F(r)
θ'' + 2(r')θ' = 0

where F(r) is the force law in terms of the radial distance r. As you can see, the tangential acceleration is dependent on both the radial distance and the angular velocity. This means that the motion will not be a simple circular or elliptical orbit, but rather a spiral.

Finally, for part (c), you are correct in stating that the motions will be unstable and there will be no r(t) where the motion is bounded away from 0 and infinity. This is because, in this scenario, the force decreases with distance faster than in the traditional inverse square law of gravity. As a result, the objects will spiral towards each other or away from each other, depending on the direction of the force.

I hope this helps to clarify your thoughts on the matter. If you have any further questions, please do not hesitate to ask. Keep exploring and questioning the world around you!

 

[quicknote]

Hello! I would like to provide some clarification and additional insights on the 2-body inverse cube problem.

(a) You are correct in saying that the inverse cube law does not affect the use of radial coordinates and reduced mass in the 2-body problem. This is because the reduced mass is a result of the two bodies interacting with each other, regardless of the type of force acting between them. As for Newton's third law, it still holds in inverse cube gravity as it is a fundamental principle of classical mechanics.

(b) In the central force problem, the tangential component of acceleration is not necessarily zero, even if the radius is constant. This is because the force acting on the body is not necessarily directed towards the center, but can have a tangential component as well. In polar coordinates, the equations of motion would be similar to those in the link provided, but with an additional term for the tangential acceleration.

(c) The motions in this scenario would indeed be unstable spirals, as you have correctly pointed out. This is due to the fact that the force acting between the two bodies increases as the distance decreases, leading to a runaway effect. In this case, there would be no stable orbits where the distance between the bodies is bounded away from 0 and infinity.

I hope this helps clarify any confusion and provides a better understanding of the 2-body inverse cube problem. As scientists, it is important to constantly question and analyze our assumptions and theories, and your initial thoughts and questions are a great example of this critical thinking process. Keep up the good work!