NOTES
Gravity is proportional to mass - and gravity can be removed from a problem (locally) by a suitable reference frame selection. By that standard, gravity is a fictitious force, only included in the family of real forces by being "grandfathered in" so I feel no hesitation in allowing gravity's cousins like the centrifugal force and Coriolis force into the club. XKCD, the geeky webcomic (whose author, Randall Munroe, is up for a fan artist Hugo award this year) makes a similar point in issue #123.