system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If a spaceship had a flag, would it wave as the spaceship moved through space?
gpt
If the ship was accelerating, it would "hang" toward the back of the ship, as if it was hanging down from a horizontal pole on Earth, with no wind.  When the ship stopped accelerating, it would spring forward a bit, from released tension, but would then float freely until it reached a point where no forces were acting on it, where it would stay.

It's also possible that it might wave a bit from the impact of random particles, but that would be almost undetectable.  The density of gas in a very, very dense nebula is about 1,000,000,000,000 times less than our atmosphere, and the vast majority of space is far more empty than that.