I asked a question on finding the initial velocity of two objects of equal mass in order to follow a keplerian orbit with a given eccentricity but I am still having some trouble.

I have seen the reduced mass approach where you consider a reduced mass orbiting around the centre of mass of the original configuration. And they use the vic-viva equation to obtain the velocity of the reduced mass....but how do I find the initial velocity of each of my original masses? (The two objects are placed on the x axis so the velocities will be in the y axis)


If you have found the necessary relative velocity $\mathbf v=\mathbf{v}_1-\mathbf{v}_2$, then the velocities of the two individual masses are

$$\mathbf{v}_1=\frac{m_2}{m_1+m_2}\mathbf v$$


$$\mathbf{v}_2=-\frac{m_1}{m_1+m_2}\mathbf v.$$

Note that these satisfy


which says that the center of mass isn’t moving.

These equations follow from those here.

Thank you, @MarcoCiafa, for noticing a confusing typo in the last equation.

  • $\begingroup$ Just to clarify: the relative velocity V in your equation is the velocity of the reduced mass? So if we consider two masses of 0.5 placed at +- 10 units on the x axis (on a keplerian orbit of e=0.9). What is my initial velocity of each body? $\endgroup$ – Warrenmovic Feb 29 '20 at 9:08
  • $\begingroup$ See physics.stackexchange.com/questions/533517/… $\endgroup$ – G. Smith Feb 29 '20 at 23:47

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