## College Physics (4th Edition)

We can write the expression for the magnitude of the drag force for a sphere falling through a viscous fluid: $F_d = 6\pi \eta ~r~v$ A falling object reaches terminal velocity when the opposing drag forces are equal in magnitude to the gravitational force: $mg = F_d$ $mg = 6\pi \eta ~r~v$ $v = \frac{mg}{6\pi \eta ~r}$ From this equation, we can see that for spheres with the same radius, the sphere with a larger mass will have a larger terminal velocity. Since A has a larger mass, A has a larger terminal velocity. The correct answer is: (a) A has the larger terminal velocity.