Exam-style question
Try this first
Which statement correctly defines proper length?.
- A.It is the length measured in a frame where the object is moving
- B.It is the length measured in a frame where the object is at rest
- C.It is the contracted length observed by a moving observer
- D.It is the length measured in any inertial frame
Model answer
What a good answer should say
- It is the length measured in a frame where the object is at rest
Explanation
Why this works
Formula/rule: Proper length L0 is defined as the length of an object in its own rest frame. Substitution: In any other inertial frame where the object moves, the measured length is L = L0 sqrt(1 - v^2/c^2) < L0.
Working: Therefore the only frame that measures L0 is the rest frame. Answer: It is the length measured in a frame where the object is at rest.
Units/conclusion: Proper length is expressed in metres (m).
Common mistake
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