Exam-style question
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When two observers in relative motion compare the speed of a light pulse, what does the second postulate guarantee?.
- A.The speed measured depends on the observer's motion.
- B.The speed of light is always c regardless of observer.
- C.The speed of light is zero in the moving frame.
- D.The speed of light is infinite in the moving frame.
Model answer
What a good answer should say
- The speed of light is always c regardless of observer.
Explanation
Why this works
Formula/Rule: Second postulate – the speed of light in vacuum is constant (c) for all inertial observers. Substitution: observer A (stationary) and observer B (moving).
Working: Both observers measure the same value of c for the light pulse, independent of their relative motion. Answer: The speed of light is always c regardless of observer.
Units/Conclusion: c (≈3.0×10^8 m/s).
Common mistake
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