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Point C is at rest at the origin.

Object A is moving east with speed 200,000 km/s, relative to Point C, and starts at coordinate (0, 200 km).

Object B is moving west with speed 200,000 km/s, relative to Point C, and starts at coordinate (0, -200 km).

So in normal newtonian system, their relative speed would be 400,000 km/s. But that's impossible because nothing can go faster than the speed of light (~300,000 km/s).

As an observer on Object A, what would you observe is the relative speed of Object B?

Edit: This isn't my homework. This is me trying to understand relativity. I could ask general conceptual questions but they would be too vague and broad. Why not start with specific examples as concrete as possible? At least now I know what the right formula is for this type of example.

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You can use the relativistic velocity addition formula

$$V=\frac{v_1+v_2}{1+\frac{v_1v_2}{c^2}}$$

Where $v_1$/$v_2$ are the individual velocities, $c$ is the speed of light and $V$ is the added velocity. Since this formula includes relativistic effects, one will get $V<c$.

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    $\begingroup$ Please do not provide complete answers to homework-like questions. $\endgroup$
    – G. Smith
    Commented Apr 11, 2021 at 23:52

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