2021 F = ma Exam — Question 14 — Physics Guide

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F = ma (11–17)

Dimensional Analysis

FMA-2021-14 · 2021 F = ma (AAPT)

2021 F = ma Exam — Question 14

A bacterium cell swims by rotating its bundle of flagella to counter a viscous drag force in the medium. The drag force F (R, v, η) only depends on the typical length scale of the cell R, its speed v, and the viscosity of the fluid η, which has units of kg/(m · s). It is observed under a microscope that a cell of length 1 µm swims at about 20 µm/s. Estimate the speed of a cell of length 0.5 µm, assuming cells of all sizes generate the same amount of force from their flagella. — Source: Question 14, 2021 F = ma Exam, © 2021 American Association of Physics Teachers (AAPT) and the U.S. Physics Team. Adapted by PhysicsTrainer: reformatted into a separate question and answer choices for online multiple-choice practice; any shared setup from nearby questions is repeated here. Not affiliated with or endorsed by AAPT or the U.S. Physics Team.

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F = ma problems are © the American Association of Physics Teachers (AAPT) and the U.S. Physics Team, the original source, and are used with permission. Problems shown here are adaptations: they have been reformatted by Physics Guide into separate question and answer-choice pieces for online multiple-choice practice. Physics Guide is an independent student project and is not affiliated with, endorsed by, operated by, or supported by AAPT, F = ma, USAPhO, or the U.S. Physics Team.

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