An approximation for the boundary-layer shape in Figs.1.5b and P1.51 is the formula u(y) \approx U \sin \left(\frac{\pi y}{2 \delta}\right), \quad 0 \leq y \leq \delta where U is the

stream velocity far from the wall and d is the boundary layer thickness, as in Fig. P1.51. If the fluid is helium at 20°C and 1 atm, and if U = 10.8 m/s and o = 3 mm,use the formula to (a) estimate the wall shear stress T in Pa, and (b) find the position in the boundary layer where Tis one-half of Tw.