Viscosity and Shear Stress: Example Problem

Try to solve this problem before watching the solution in the screencast.

Example Problem

A toothpick is placed inside a 5 mm diameter straw full of a mysterious fluid with a gap of 1 mm between the toothpick and the straw. If 50 mm of the toothpick is pulled from the straw with a force of 0.1 N such that the toothpick moves with a constant velocity of 0.1 m/s, what is the viscosity of the fluid? Assume the fluid is Newtonian and velocity profile is linear.