A bacterium swims through a fluid of viscosity $\eta$ (units $\text{kg}/(\text{m}\cdot\text{s})$). Its viscous drag force depends only on its length scale $R$, swimming speed $v$, and $\eta$. Unlike the simplistic model in which all cells produce the same flagellar force, careful biophysical measurements indicate that the rotary motors of cells of all sizes deliver the same mechanical power $P$ to the fluid. A cell of length $1\ \mu\text{m}$ is observed to swim at $20\ \mu\text{m/s}$. Estimate the swimming speed of a cell of length $0.5\ \mu\text{m}$.