空く
あく · aku · JLPT N4 · verb-godan
to become empty
🪑 aku: a KUshion-less seat = the spot is vacant, it opened up
空 is the kanji for empty/sky (also in 空気 air, 空港 airport). With く it becomes the verb for a seat, room or slot becoming vacant. Same sound as 開く (to open), so one aku covers both: something opens up and is free.
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.'
すく · suku · JLPT N4 · verb-godan
to be less crowded
🚃 suku sounds like scoot: people scoot off and the train empties out
空 means empty or sky; read すく it is about space freeing up as people leave. A train that is 空いている has seats to spare. Same kanji as 空く (あく, to become vacant), but すく is the less-crowded reading, also used in お腹が空く, to get hungry.
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.'