I'm guessing you either want to do some kind of vertical scrolling game, in which case what you have in mind would work, or you're trying to do a gravity script, in which case what you're trying to do will miserable fail.
Anyway, I guess for what you planned, this would be the (lazy, but working ^^" ) code for it:
class Scene_Map
alias movedown_upate update
def update
movedown_update
$game_player.move_down
end
end