Discussion
Terminal velocity of a grain in an air stream is the velocity at which
Answer: B
Correct answer: Drag exactly balances the weight
A particle released in an upward air stream accelerates until the aerodynamic drag equals its net weight, after which it moves with the air at constant relative velocity:
\[ C_d\,A\,\frac{\rho_a v_t^{2}}{2} = mg \]
That balance velocity is the terminal velocity. It depends on the particle's mass, projected area and drag coefficient, so different components of a harvested mixture have different values - which is precisely what an aspirator exploits. Setting the air velocity between the terminal velocity of the chaff and that of the sound grain lifts one and leaves the other behind.
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