How did the "american"accent develop?
The different mixes of where the people who settled there came from. New Jersey was always a hot bed of new immigrants from Eastern Europe, and various countries (being close to the harbors where the European ships docked) Atlanta is more about English, Irish, Scottish settlers, and many of them were lower class, so you hear influences from cockney and the like. Did you ever wonder why they chose a British actress to play Scarlett O'Hara? It's because the accent of Atlanta is so close to the straight upper-class British accent. @MAtt8 You're funny! You have no idea how much closer YOUR accent is to that of Southern Spain than any of the rest of Latin America!!! ACCENTUALLY, that is. Linguistically, your language has taken on many other European influences, but the accent is ANDALUZ. (very close)
How did the English (American) accent develop?
Languages in colonies tend to be more conservative than in the mother country. A common example cited by scholars is Icelandic which is still pretty close to the Old Norse spoken by the Vikings over 1,000 years ago. However, other examples exist too. The Albanians of Southern Italy still speak Albanian almost the same way their ancestors who fled the Turks over 500 years ago did, and the Greek of Cyprus still retains a few words and forms from the ancient Greek of Thucydides's time no longer used in Greece today. English is the same way. The Englishes of the United States, Canada and Jamaica are closer to the English of the 16th and 17th centuries in many respects than the English of England both in accent and vocabulary. For example, the Middle English word "yonder" meaning 'in that place' or 'over there' is still used in many regions of the United States especially in Appalachia and the South, but is no longer used in England. 17th Century English was rhotic (or r-colored) as opposed to modern British English which is non-rhotic (or r-less). Rhotic English is still spoken in Canada and most of the U.S. except for New York, Boston and South Philadelphia where a non-rhotic English somewhat similar to British is spoken. This is why the late President John F. Kennedy always pronounced words like "Harvard" and "Cuba" like "Hah-vud" and "Kew-ber." It is not known exactly what areas of England the various American English accents spawned in although many scholars trace the English of the New England and Great Lakes regions to East Anglia on the North Sea. South Hampton was the port that most English ships heading for America came out of in colonial times, so some linguists postulate that many American accents are probably based on the South Hampton accent, but the South Hampton accent as it existed in the 17th and 18th centuries not as it exists there today. Even Samuel Johnson writing around 1750 noted that South Hampton English was a little different from his London English dialect when he made a visit to the city on one occasion. The Englishes of Australia and New Zealand are definitely British-sounding yet they still reflect more the British English of the 19th century when Charles Dickens and Lewis Carroll were writing rather than current BBC British English.
As a 21-year-old American, would I develop an accent after living in London for 10 years?
First, please understand that you do have an accent - you have a strong American acccent. People, especially those that haven't moved around a lot, are conditioned to believe that their way of speaking is normal, and every other way is an accent. I've lived abroad for 12 years and it took 3-4 years before I started to understand the distinctive qualities of my home (American) accent I left the US at 18 and have been in the UK for the past 11 years, with one year in Singapore mixed in. My accent did drift, but not fully to British. I picked up a lot/most of the British lexicon, quite a bit of British intonation, and a bit of the accent. Americans now think I sound (weakly) British, but Brits still find my accent American. Again, people think their accent sounds normal, and only pick up on the differences to their own accent. (Incidentally non US/British people I meet struggle to place my accent, but on balance probably put me as "North American")How much your accent drifts will vary, depending on who you interact with in your day. My drift towards a British accent slowed considerably once I started living with my Canadian wife. Similarly one could spend a long time in London around a mostly international or, if you wanted, American group. So whether your accent drifts will also depend on whether you want to let it drift or retain it, on your personality, etc.
How did African Americans develop a culture that combined ...?
It guess it just happened naturally. They blended their own heritage with influences from white Americans.
How would American culture have developed differently if the automobile had not been affordable in the 1950s?
We would be less developed, and when i say less, i mean LESS. All of manufacturing would be affected because a car company (Ford) invented the assembly line, the way all factories work these days because they are so much more efficient.
How long does it take for an American accent to develop?
I think that it should take a month or two. It depends though. If you have friends around you with the American accent you will eventually start talking like them. I know that Cody Simpson came to the United States and he lost his accent after about 2-4 months of being out here and Simon Cowell has a house in the United States and works with people that were born here yet he hasn't lost his accent. You should pick it up in no time.