A spaceship is moving in a straight line at a speed of 0.8 c (relative to Earth).It launches a missile that moves away from the spaceship (traveling in the same direction as the spaceship)at a speed of 0.6 c.Relative to Earth,the speed of the missile is
A)greater than 0.8 c and less than c.
B)1.4 c.
C)c.
D)greater than c and less than 1.4 c.

Question

Quizplus · Accepted Answer

The velocity addition formula gives the relative velocity of the missile with respect to Earth as: 
v_m = (v_s + v_r) / (1 + v_s*v_r/c^2), 
where v_s = 0.8c is the velocity of the spaceship and v_r = 0.6c is the velocity of the missile relative to the spaceship. 
Plugging in the values, we get: 
v_m = (0.8c + 0.6c) / (1 + 0.8c*0.6c/c^2) = 1.4c/(1+0.48) = 0.944c. 
Therefore, the correct choice is A as the missile moves away from the spaceship at a speed that is greater than 0.8c (velocity of the spaceship relative to Earth) but less than c (speed of light in vacuum).

A Spaceship Is Moving in a Straight Line at a Speed