The relative velocity of two small spheres subjected to a constant force in a liquid is given by u = uo/f where f is the drag correction factor and u0 is the relative velocity of the spheres when they are far apart. The drag coefficient factor, f, is a function of (r/a) where a is the radius of each sphere and r is the center-to-center distance. Given the following data for the correction factor estimate the time at which it takes the spheres to travel apart from r=22 μm to r=32 μm when u0=1 mμ/s and a=10 μm

using an appropriate open integration formula. (Hint: dt=dr/u )

r/a: 2.1 2.5 3.0 3.5

f: 4.03 1.72 1.37 1.24

I don't understand the problem, can you explain what i should do firstly?