Example sentences
zuò坐jǐ几lù路gōng公gòng共qì汽chē车？
Which bus do I take?
gōng公gòng共qì汽chē车wǎn晚diǎn点le了。
The bus is late.
About this word
公 is "public" and 共 is "shared", while 汽车 is the everyday word for a car, so 公共汽车 is literally "public automobile", which is a bus. 汽车 itself was coined when vehicles ran on steam and gas engines, and that is where 汽, "vapor", comes from. 公共汽车 is the city bus that stops every few blocks; a long-distance coach is a 长途汽车. In speech people shorten it to 公交车. The English word bus is hiding in 巴士, which entered Chinese through Cantonese and still names the smaller shuttle buses in the south.
The story behind the character
公
公 is 八 (to divide, to turn away) above 厶 (private). Han Feizi put it in four words: 背厶為公. Turn your back on the private and you get public. Xu Shen agreed, glossing 公 as 平分也, to divide evenly. Fair shares. That is what the shape is arguing for, and the word has meant public ever since.
共
共 is two hands holding something up. The bottom is 廾 (two hands) and the top is the object being lifted, an offering. Xu Shen wrote 共, 同也: the same, together. Two hands raised with one thing between them. The idea of sharing does not get much plainer than that.
汽
汽 is 氵 (water) plus 气, which gives the sound. It is a made character with one job: steam, the vapor water turns into. 气 by itself covers air and gas. Add the water radical and you have the hot stuff coming off a kettle. 汽车 uses it too, and reads literally as steam vehicle, which is what early cars ran on.
车
The earliest 车 is a war chariot: two wheels, a body in the middle, a shaft out front. The traditional 車 still shows the outline; the simplified 车 lost the wheels. Xu Shen called it "the general name for cart and wheel."