Air (density $=1.2 \mathrm{~kg} / \mathrm{m}^3$, kinematic viscosity $=1.5 \times 10^{-5} \mathrm{~m}^2 / \mathrm{s}$ ) flows over a flat plate with a free-stream velocity of $2 \mathrm{~m} / \mathrm{s}$. The wall shear stress at a location $15 \mathrm{~mm}$ from the leading edge is $\tau_w$. What is the wall shear stress at a location $30 \mathrm{~mm}$ from the leading edge?
- $\tau_w / 2$
- $\sqrt{2} \tau_w$
- $2 \tau_w$
- $\tau_w / \sqrt{2}$