SAT Subject Physics Practice Question 149: Answer and Explanation

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Question: 149

12. An Imperial battle cruiser, sitting in a hanger deck, is measured to have a length of 200 m by a worker on the deck. If the cruiser travels at a speed of c past a planet, what will be the length of the cruiser, as measured by the inhabitants of the planet?

A. 0
B. Between 0 and 200 m
C. 200 m
D. Greater than 200 m
E. None of the above, since it is impossible to reach the described speed

Correct Answer: B


Special relativity dictates that length contracts for objects moving at relativistic speeds, so the length should be smaller. That’s C. (Bear in mind that the speed is attainable because the square root of three is less than 2, so the speed is less than the speed of light.) Mathematically, you could calculate the length as follows. If v = ()c, then the relativistic factor is

So, as measured by the inhabitants of the planet, the length of the cruiser as it passes by will be

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