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## Homework Statement

Oil of Density 888kg/m^3 and viscosity 0.8Pa*s is flowing in a horizontal pipe of diameter 0.15m. The local flow velocity at the centerline is 10m/s. find the shear stress exerted by the oil on the wall of the pipe.

## Homework Equations

tau = -eta*(dv/dy)

## The Attempt at a Solution

= -0.8*(10/0.075) = 106.7 Pa

but the given solution is 213Pa, exactly twice my answer, what am I doing wrong? thanks