the nature of game development is that sometimes you will need to multiply a physics number by -1 to fix it. you won't know why. you can either agonize over it all day and still not know why, or just surf that sweet, sweet -1 wave into the sunset
what they don't tell you is that this is also true in physics. if you ever get an answer that doesn't make sense, try multiplying by -1, 1/2, or both.
there's concrete reasons for this, like the fact that electrons are defined to have negative charge, or that Schrodinger's equation has a minus sign in most conventions, or that the minkowski metric is either (+++-) or (---+) depending on whether an author is more comfortable with time or space corresponding to negative distance. off-by-half errors come from whether you include spin or not, whether you're working in natural units, and so forth.
almost no one can get physics numbers right, including actual physicists.
