A small sphere is moving horizontally through a viscous liquid.
(a) Stokes' law can be used to calculate the drag force on an object.
State the conditions that must apply for Stokes' law to be valid.
(b)There is a constant force of 2.3 × 10−5N acting horizontally on the sphere.
diameter of sphere = 4.5 × 10−3m
viscosity of liquid = 7.1 × 10−2Pa s
(i) At one instant, the speed of the sphere is 5.2 × 10−3ms−1.
Calculate the resultant horizontal force on the sphere.
Part a of this question was easy,but I'm confused for part (b).In mark scheme,they used Stokes' law formula,F=F(pi)(n)rv.But Stokes' law needs constant velocity,but how do we know that the speed we are given is a constant velocity?