Exam-style question
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A spaceship has a proper length of 120 m. If it moves at 0.8c relative to Earth, what is its proper length?.
- A.120 m
- B.96 m
- C.0.8c
- D.0.6c
Model answer
What a good answer should say
- 120 m
Explanation
Why this works
Formula: L = L0 √(1 - v²/c²). Substitution: L0 is the proper length.
Working: L = L0 √(1 - v²/c²). Since L0 is given, the proper length remains 120 m.
Units: m. Conclusion: The proper length is 120 m.
Common mistake
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