Consider a roller coaster with a loop-the-loop. The loop is not circular (that would be too dangerous) but has a radius of curvature which decreases with height. A roller coaster car starts from rest from the top of a hill which is 5 m higher than the top of the loop. It rolls down the hill and through the loop. What must the radius of curvature at the top of the loop be so that the passengers of the car will feel, at that point, as if they have their normal weight?
(a) 5 m
(b) 10 m
(c) 15 m
(d) 20 m