# Centripetal force of a roller coaster car

I got all the other centripetal force questions but this one has me stumped...

As a roller coaster car crosses the top of a 30m-diameter loop-the-loop, its apparent weight is the same as its true weight. What is the car's speed at the top?

The formula I used for all of the other questions was F=mv^2/r. The problem is the only number I have is the diameter (therefore radius). I'm guessing because the apparent weight is the same as true weight the net force is zero? There's also no way to get mass. Maybe there's another formula? Any assistance with this problem would be much appreciated!

The true weight of something is $$F=mg$$, so then you don't need to figure out the mass.