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In the equation p = gamma m u for relativistic momentum, how is the rest mass m measured?
AIt is measured when the object is at rest, without any gamma factor
BIt is measured only while the object moves at the speed of light
CIt is measured by dividing momentum by gamma alone
DIt changes constantly depending on the observer's own speed
Answer & Solution
Correct answer: A. It is measured when the object is at rest, without any gamma factor
1. In the momentum equation p = gamma m u, m stands for the object's rest mass.
2. Rest mass is the mass measured while the object is at rest, without any gamma factor applied.
3. It cannot be measured only at the speed of light, since a massive object never reaches that speed.
4. Rest mass is a fixed property of the object, not something that changes with the observer's own speed.
5. So mass measured at rest, with no gamma factor, is correct.
_Source: OpenStax Physics (CC BY 4.0), Ch 10 "Special Relativity", section 10.2 Consequences of Special Relativity_
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