Star S1 is moving away from us at a speed of 0.8c. Star S2 is moving away from us in the opposite direction at a speed of 0.5c. The speed of S1 as measured by an observer on S2 is:
A) 0.21c
B) 0.5c
C) 0.93c
D) 1.3c
E) 2.17c

Question

Quizplus · Accepted Answer

We can use the relativistic velocity addition formula to determine the speed of S1 as measured by an observer on S2:

v = (v1 + v2)/(1 + v1*v2/c^2)

where v1 is the speed of S1 relative to the observer on Earth (0.8c), v2 is the speed of S2 relative to the observer on Earth (-0.5c, since it is moving away in the opposite direction), and c is the speed of light.

Plugging in the values, we get:

v = (0.8c - 0.5c)/(1 - (0.8c)(-0.5c)/c^2) = 0.93c

Therefore, the best choice is C, 0.93c.

Star S1 Is Moving Away from Us at a Speed