空军
kōngjūn · HSK 6
force
✈️🎖️ empty sky + army = the military force that rules the skies
空 means empty/sky/air, 军 means army/military. Together they literally say sky-army, which is exactly what an air force is — soldiers who operate in the air domain.
空 — cave radical 穴 + 工 (phonetic)
Originally a phono-semantic compound: 穴 (cave, hole) gives the meaning of emptiness or hollow space, while 工 supplies the sound. The earliest sense was 'hole' or 'cavity,' which extended to 'empty,' 'sky,' and 'in vain.' The reading kòng (4th tone) specifically means 'free time' or 'vacancy.'