Exam-style question
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A train of proper length 200 m moves at 0.6c relative to the ground. Which of the following is the proper length?.
- A.200 m
- B.120 m
- C.0.6c
- D.0.4c
Model answer
What a good answer should say
- 200 m
Explanation
Why this works
Formula: L = L0 √(1 - v²/c²). Substitution: L0 is the proper length.
Working: L = L0 √(1 - v²/c²). The proper length is given as 200 m.
Units: m. Conclusion: The proper length is 200 m.
Common mistake
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