There is no gravity - the earth sucks.
Still, if 100 or us in this very forum compete for the top 10 spots on a single keyword WE CAN'T ALL GET THE TOP 10 SPOTS! - end of story. Anything below 30 is a waste of time so I would think that the more popular the keyword, and the more competition, the more difficulty.
Some sites seem to break through to muck now and then and hit the top 10 easily but I've always found it much easier to get a bunch of secondary terms in the SERPs opposed to the popular keyword. Eventually, if you develop a natural network of links moving up in the primary keyword itself happens, but more likely than not that single word is the tough nut to crack IMO.