= Solution
\b[Covered by the official notes:] definition of a pure force via constant rest mass (PDF p. 13; handwritten p. 8).<Symmetry and Relativity 2011 Exam Solution Notes>[PDF p. 13; handwritten p. 8: definition of a pure force via constant rest mass]{ref}
The page labels $dp^\mu/d\tau$ as a proper force; that is the <four-force>. The requested <proper force> is the \b[three-force measured in the instantaneous rest frame of the object]. For a <pure force> its magnitude is $m_0$ times the <proper acceleration>, and in that frame the <four-force> has components $(0,\mathbf f_0)$.
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