= Solution
<Bernoulli's theorem> and <conservation of mass> give $t=r_1^2/(2r)$. Ignoring weight, the stated stagnant-rear-pressure model gives zero resultant at $r_2=\sqrt2r_1$. Including the leading film-weight estimate gives a cancellation radius near $2gr_1^2/v^2$. The printed inequality does not establish negligible force at every larger radius; the shared derivation explains this source qualification.
Full derivation: <past-exam-of-the-physics-b-course-of-the-university-of-cambridge/2010/paper-2a/section-c/c10/solution>.
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