An astronomer on earth observes two galaxies moving away from each other along a line that passes through the earth. The astronomer finds that each is moving with a speed of 2.1 × 10⁸ m/s relative to the earth. At what speed are the galaxies moving apart relative to each other?

Question

Quizplus · Accepted Answer

The relative speed between the galaxies can be found by subtracting their velocities (since they are moving away from each other):

Relative speed = velocity of galaxy 1 - velocity of galaxy 2

Relative speed = (2.1 × 10^18 m/s) - (2.1 × 10^18 m/s)

Relative speed = 0 m/s

Therefore, the galaxies are not moving apart relative to each other.

An Astronomer on Earth Observes Two Galaxies Moving Away from Each