A spherical balloon to be used in hydrogen is tested by towing a 3:1 model in a wind tunnel (the 3:1 ration means that the model is bigger than the prototype). When the model is 1.0 m in diameter and it is in airflow at a speed of 7.8 m/s, its drag coefficient is measured to be 0.4. This drag coefficient includes LaTeX: \frac{1}{2}\rho V^21 2 rho V 2 in its definition. What is the drag force on the prototype balloon in N? The density and viscosity of hydrogen are 0.09 kg/m3 and 9x10-6 Pa.s, respectively. The density and viscosity of air are 1.2 kg/m3 and 1.8x10-5 Pa.s, respectively.
