Why did America switch from silver money to medal money in an almost identical format compared to the greeks??

Answer 1

Back in about 1937 America went to the "gold standard" this was to insure that for every dollar out, there was a dollar of gold in the treasury. At this time the bill was made smaller also, all old gold standard bills and large bills were to be turned in or be deemed "no good" after a date.
Americans never did like "metal" money, it was hard to carry, to handle, until it was made to be unconterfitable, no one trusted it.

Answer 2

its cheaper, and there is more if it than silver