SAT Subject Physics Practice Question 229: Answer and Explanation

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Question: 229

75. A spaceship is moving directly toward a planet at a speed of . When the spaceship is 4.5 × 108 m from the planet (as measured by someone on the spaceship), a pulse of light is emitted by someone on the planet. As measured by someone on the spaceship, how long does it take the light pulse to travel from the planet to the ship?

A. 0.5 sec
B. 1.0 sec
C. 1.5 sec
D. 2.0 sec
E. 2.5 sec

Correct Answer: C

Explanation:

The second postulate of Einstein’s theory of special relativity states that the speed of light is a universal constant (c = 3 × 108 m/s), regardless of the motion of the source or the detector. So despite the fact that the spaceship is approaching the planet at a speed of c/2, occupants of the ship will still measure the speed of light to be c. The time it takes the light pulse to travel a distance of 4.5 × 108 m is

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