1. The outbreak of coronavirus disease 2019 (COVID-19) has caused a global pandemicand 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 ofCOVID-19. However, it was still broadly debated regarding "what is the safe socialdistance". Some studies recommended that at least 2 m of physical distance should bekept from others, while others suggested that 2 m may not be adequate. To answer thisquestion, it is vital to fully understand the mechanism of virus spreading. Therespiratory infections occur usually through the transmission of virus-containingdroplets and aerosols exhaled from infected individuals during breathing, speaking,coughing, and sneezing. Therefore, we can assume the spreading of the droplet issimilar to a falling particle in the air. Please identify the kinetic force (F.) acting on thedroplet. Assume u is the dynamic viscosity of airflow, R is the radius of the droplet, nis the falling (terminal) velocity.
(A)