Vis-viva equation (Orbital Velocity) with massive satellite?

Click For Summary

Discussion Overview

The discussion revolves around the application of the vis-viva equation in scenarios where the mass of the orbiting body is not negligible compared to the central body, particularly in binary star systems. Participants explore how orbital velocities are determined when both bodies have comparable masses.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the validity of the vis-viva equation when applied to a binary star system where both stars have significant mass, suggesting that the equation may not be accurate in such cases.
  • Another participant proposes that if the masses of the two bodies are comparable, the relative speed can be defined as the sum of their speeds, but expresses uncertainty about the correctness of this approach due to a lack of references.
  • A later reply clarifies that the relative speed of the two bodies is distinct from the speed of one body about the common center of mass, indicating that the speeds will differ if the masses are unequal.
  • One participant acknowledges a misunderstanding regarding the definition of velocity in the context of the vis-viva equation, leading to a revised understanding of how to calculate the speed of each body relative to the barycenter.
  • Another participant suggests that when applying their method, the total mass cancels out, which may simplify the calculations for the speeds of the bodies relative to their barycenter.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of the vis-viva equation for massive satellites, with some uncertainty about the correct interpretation of relative speeds and the implications of mass on orbital dynamics. The discussion remains unresolved regarding the accuracy of the equation in these contexts.

Contextual Notes

Participants note limitations in their understanding of the equation's application, particularly regarding the definitions of speed and mass relationships in binary systems. There is also mention of a lack of references to support certain claims.

taylorules
Messages
5
Reaction score
0
Wikipedia states that "In the vis-viva equation the mass m of the orbiting body (e.g., a spacecraft ) is taken to be negligible in comparison to the mass M of the central body."
I'm wondering how the velocity is determined if the satellite's mass is non-negligible. For example, in a binary star system where both stars have comparable masses, would the vis-viva equation be accurate?
Thanks.
 
Physics news on Phys.org
Have you found an answer to your question yet? I was waiting for someone to reply to you because I found the vis-viva equation interesting. So don't take my post as an authoritative answer.

v is defined as the relative speed of the two bodies. If the difference in mass of the two bodies is not significant then simply adding the two masses should work.

gif.latex?v_o=\sqrt{G(M_1+M_2)\left(\frac{2}{r}-\frac{1}{a}&space;\right&space;)}.gif


But since I could not find a reference of the equation being used this way, I am not sure that it is correct.

Keep in mind that the relative speed of the two bodies is not the same thing as the speed of one body about the common center of mass of the two bodies. In the latter case, the speed of one body will be different from the speed of the other body if there masses are different.
 
TurtleMeister said:
v is defined as the relative speed of the two bodies.
Thank you! That was the problem.
Because Body1's speed = Body2's speed * (Mass2/Mass1), and v = Body1's speed + Body2's speed, I found Body2's speed = (Mass1 * v) / (Mass1 + Mass2).
The problem was that I thought v represented Body2's speed, not the combined speed of both.
Thanks
 
Glad I could help. If you write out the equation using your method, you will find that the (M1+M2) cancels out:

gif.latex?v_1=\sqrt{GM_2\left(\frac{2}{r}-\frac{1}{a}&space;\right&space;)}.gif


gif.latex?v_2=\sqrt{GM_1\left(\frac{2}{r}-\frac{1}{a}&space;\right&space;)}.gif


This is the speed of each body relative to the barycentre of the two bodies.
 

Similar threads

  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 7 ·
Replies
7
Views
4K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 13 ·
Replies
13
Views
6K
  • · Replies 7 ·
Replies
7
Views
6K
  • · Replies 6 ·
Replies
6
Views
3K
  • · Replies 5 ·
Replies
5
Views
3K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 10 ·
Replies
10
Views
2K
  • · Replies 6 ·
Replies
6
Views
2K