The head lost in 30 m of 15 cm-diameter pipe is known to be 5 m when a liquid of specific gravity of 0.9 flows at 0.05 m/s. Find the shear stress at the walls of the pipe.

The head lost in 30 m of 15 cm-diameter pipe is known to be 5 m when a liquid of specific gravity of 0.9 flows at 0.05 m/s. Find the shear stress at the walls of the pipe.