10. Wearing the face mask can provide a critical barrier, reducing most infectious viruses during coughing, especially of asymptomatic people and those with mild symptoms of COVID-19. Please calculate the force of cough flow acting on the mask. Assume the mask perfectly covers the mouth. The velocity of cough flow is 10 m/s and the opening of the mouth is 1 cm in diameter.
(A)0.001 N
(B) 0.01 N
(C) 0.1 N
(D)1N
(E) 10 N

6. If we try to introduce the kinetic force Ft from question I, the correlation between fand Re can be: 24 32 (A)(B) (C) (D) (E)

7. Please calculate the falling time of the droplet using the equation from question 5. The initial height of the droplet is set as 1.5 m. Please choose the closest answer. (A) l min, (B) 2 min, (C) 10 min, (D) 30 min, (E) 60 min

8. The falling velocity of the droplet may or may not be the same when using the force balance method or friction factor method. If not the same, there should be a reason behind that. Please choose the CORRECT answer. (A) The falling velocities are the same, so there is no reason behind that. (B) The falling velocities are not the same because the airflow is the creeping flow. (C) The falling velocities are not the same because the air friction is dominant. (D) The falling velocities are not the same because the density of the air can be ignored. (E) The falling velocities are not the same because the viscosity of the air is not considered when using the friction factor method.

9. Cough can increase the initial horizontal velocity of the droplets, allowing the longer travel distance of the droplets. Usually, the average velocity of cough flow is 10 m/s. Please calculate the Reynolds number of the cough flow. Assume the opening of the mouth is 1 cm in diameter. Choose the closest answer. (A)500,(B) 1000, (C) 3000, (D) 5000, (E) 6000

11. What is the SI unit of mass flux? (A) Kg/s, (B) g((s/ cm2), (C) Kg(s /m2), (D) g/s

12. Concerning the SI units of kinematic viscosity, thermal diffusivity, and mass diffusivity, which of the following statements is true? (A) All three are different. (B) All three are the same as m2/s. (C) All three are the same as s/m2. (D) All three are the same as s/cm2.

13. For the following mixtures, (a) H2-Oz at 1 atm and 273 K, (b) N2-O2 at I atm and 273 K, (c) dilute aqucous ethanol solution at 298 K, and (d) Cd in Cu at 293 K, place them in the order of increasing mass diffusivity. (A) (a)(b)(c)(d), (B) (c)(d)(a)(b), (C) (d)(c)(a)(b), (D) (d)(c)(b)(a)

14. For flow of a binary gas mixture over a flat plate, boundary layers of velocity and concentration develop. Which of the following statements concerning the thicknesses of the velocity (δ) and concentration boundary layers is true? (A) (B) (C) (D) none of the above

15. Consider a spherical particle A of radius R dissolving in a stagnant liquid B. Once dissolved, A and B react to form C with an irreversible first-order reaction reaction rate constant k). The solubility of A in B, CAo, is small and the system can be approximated as a dilute binary system. The diffusivity of A in B is DAB. The system can be treated as in a pseudo-steady state. Let b2 denote (k,R/DAB). What is the molar flux of A at the particle surface? (A) (B) (C) (D)

16. Continued from the above problem, what is dmaldr? (-molecular weight of A, ma-mass of particle (A) (B) (C) (D)