Wù / tǐ / zài / zhēn / kōng / zhōng / zuò / zì / yóu / luò / tǐ / yùn / dòng / shí / zhǐ / shòu / zhòng / lì / zuò / yòng

物体在真空中做自由落体运动时，只受重力作用。

When an object undergoes free fall in a vacuum, it is acted on only by gravity.