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)0.50c
C)0.93c
D)1.3c
E)2.2c

Question

Quizplus · Accepted Answer

The formula for velocity addition in relativity is v = (u + v)/(1+uv/c^2), where u and v are the velocities of the two objects and c is the speed of light. So, to find the velocity of S1 as measured by an observer on S2, we plug in u = 0.8c and v = -0.5c (since they are moving in opposite directions) and get v = (0.8c - 0.5c)/(1 - 0.4) = 0.93c. Therefore, the correct answer is C.

Star S1 Is Moving Away from Us at a Speed