3. Assume the diameter, density, viscosity of the droplet is 5 um, 0.997 g/cm³, and kg/m s, respectively. The density of the air is 1.184 kg/m³ and gravity is 9.8 m/s². If the initial height of the droplet is set as 1.5 m, please calculate the falling time of the droplet. Choose the closest answer.
(A) l min,
(B) 2 min,
(C) 10 min,
(D) 30 min,
(E) 60 min

49 What message can we infer from the final sentence, "We don't just use our smartphones for everything—we rely on them."? (A) New brands of cellphones will soon be on the markets. (B) We should not be controlled by cellphones. (C) More functions should be added to cellphones. (D) Smartphone addiction should not be a problem.

50 What is the author's attitude toward smartphone addiction? (A) Critical. (B) Supportive. (C) Indifferent. (D) Humorous.

1. The outbreak of coronavirus disease 2019 (COVID-19) has caused a global pandemic and significant impact on society, economy, and human life. To combat the virus, wearing the mask and keep a safe social distance can help prevent the spread of COVID-19. However, it was still broadly debated regarding "what is the safe social distance". Some studies recommended that at least 2 m of physical distance should be kept from others, while others suggested that 2 m may not be adequate. To answer this question, it is vital to fully understand the mechanism of virus spreading. The respiratory infections occur usually through the transmission of virus-containing droplets and aerosols exhaled from infected individuals during breathing, speaking, coughing, and sneezing. Therefore, we can assume the spreading of the droplet is similar to a falling particle in the air. Please identify the kinetic force (F.) acting on the droplet. Assume u is the dynamic viscosity of airflow, R is the radius of the droplet, n is the falling (terminal) velocity. (A) (B)(C)(D) (E)

2. Please obtain the falling velocity of the droplet. [Hint] Use the force balance of gravity force, buoyance force, and kinetic force. pu is the density of the droplet and p is the density of the air. (A) (B)  (C)  (D) (E)

4. If the evaporation of the droplet is considered, obviously, the small droplets would evaporate first before falling on the ground. However, the evidence demonstrates that large droplets are not affected by evaporation. Please calculate the minimum size (in diamcter) of the droplet before it falls on the groud. Assume the initial horizontal velocity of the droplet is 2 m/s and the initial height of the droplet is set as 1.5 m. The transmission distance is 2 m. (A) 134 um, (B) 226 um, (C) 474 um, (D) 508 um, (E) 668 um

5. As we know, the falling velocity of the particle can also be determined by introducing the friction factor fconcept when we assume kinetic force F& = AKf. A is a characteristic area and K is a characteristic kinetic energy. D is the diameter of the particle. Please identify the correct answer. (A) (B) (C) (D)(E)

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