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What is proper length, L0, as defined for a moving object?
AThe length an object would have if it had no mass
BThe length measured only by an observer moving near light speed
CThe average of every observer's length measurement
DThe length measured by an observer at rest relative to the object
Answer & Solution
Correct answer: D. The length measured by an observer at rest relative to the object
1. Proper length is measured in the reference frame where the observer and the two points are both at rest.
2. This is the largest length value, since any observer moving relative to the object measures a shorter length.
3. It is not restricted to observers moving near light speed, and it is not an average of different measurements.
4. Proper length has nothing to do with an object having no mass.
5. So the length measured at rest relative to the object is correct.
_Source: OpenStax Physics (CC BY 4.0), Ch 10 "Special Relativity", section 10.2 Consequences of Special Relativity_
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