a roller coaster has a dip in a vertical circle of radius r. the passenger feels the seat push upward with a force equal to twice her weight. The r = 20.0 m, how fast is the roller coaster going at the bottom of the dip.

To get you started: The acceleration of an object traveling in a circular motion is v^2/r. Since F=ma, the force needed to keep the object moving in a circular motion is F = mv^2/r. So now you just need to determine how much force the girl felt that was contributed by her circular motion. Can you take it from here?