Two spaceships travel away from the earth.

[CarlB]

They go in opposite directions. In the rest frame of the earth, the speed of each spaceship is 0.75 c.

In the rest frame of the earth, what is the rate at which the distance between the two spaceships changes?

Carl

[HallsofIvy]

Surely you should know the formula for this. If, in a given frame, two objects, moving in opposite directions, have speeds, relative to an object motionless in that frame, of v1 and v2 then their speeds, relative to one another in that frame is \frac{v_1+ v_2}{1+ \frac{v_1v_2}{c^2}}.

[Doc Al]

In the rest frame of the earth, what is the rate at which the distance between the two spaceships changes?

1.5 c, of course.

(Halls answered a different question: What is the rate at which the distance between the ships changes in the rest frame of either ship.)

[HallsofIvy]

Oops. Thanks, Doc.