 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 $r_2=\sqrt{2}{r}_1$. Including the leading film-weight estimate gives a cancellation radius near $2g{r}_1^2/v^2$. The printed inequality does not establish negligible force at every larger radius; the shared derivation explains this source qualification.

Full derivation: solution.