A spaceship moves at a speed of 0.95 c away from the Earth. It shoots a star wars torpedo toward the Earth at a speed of 0.90 c relative to the ship. What is the velocity of the torpedo relative to the Earth? (The direction in which the spaceship moves is the positive direction.)
A) −0.34 c
B) 0.34 c
C) 0.060 c
D) −0.060 c
E) 0

Question

Quizplus · Accepted Answer

The velocity of the torpedo relative to the Earth is the sum of the velocity of the torpedo relative to the ship and the velocity of the ship relative to the Earth. The velocity of the torpedo relative to the ship is 0.90 c, and the velocity of the ship relative to the Earth is 0.95 c. Therefore, the velocity of the torpedo relative to the Earth is 0.90 c + 0.95 c = 1.85 c. However, since the speed of light is the maximum possible speed, the velocity of the torpedo relative to the Earth must be less than c. Therefore, the velocity of the torpedo relative to the Earth is 1.85 c - c = 0.85 c. Since the torpedo is moving in the opposite direction of the spaceship, the velocity of the torpedo relative to the Earth is -0.85 c.

A Spaceship Moves at a Speed of 0