I just watched Apollo 13 on tv tonight, and I noticed something that confused me.
When preparing for reentry, Mission Control informs the crew that their trajectory is still a bit shallow. This is because of the lighter weight of the craft due to not having any lunar cargo. They are told to ballast the craft by bringing items of the LEM into the Control Module (is that the correct term?)...thereby making it heavier.
But they're in space! There is no gravity. How can this possibly work? Could this be an oversight by the film's producers?