That objects can fall straight down to Earth under specific physical conditions is a matter of everyday observation and basic physics, requiring no formal citation.
The claim states that an object can fall straight down to Earth under specific physical conditions, which is a fundamental and trivially true physical fact governed by gravity and observable in everyday life. Per the instructions, statements that are trivially true by definition or direct everyday observation are classified as COMMON_KNOWLEDGE, requiring no papers and leaving the index arrays empty.