PHYSICS! Maximum Height for a car to make a loop, hot wheels track.

What is the drop height required so that the roller coaster has the minimum speed required to successfully complete the loop. (So the riders feel weightless at the top of the loop.)
mass= 0.0040kg
radius= 0.10m
diameter=0.20m
Fg= 0.03924N
I do not know how to start this question, at all. Please help.

The first step is to find when the centripetal force is equal to the force of gravity.

Fc= (mv^2)/r
Fg=ma


Fc = Fg ---> (mv^2)/r = ma ----> v^2 = ar ---> v=0.31305 m/s This is the minimum speed required for the roller coaster to make it to the top of the loop.

Next we want to use conservation of energy to figure out the initial height required to make it to the top of the loop.

E = PE + KE

Ei is the initial energy
Ef is the final energy (at the top of the loop, so we use 0.2 m for the height of Ef)

PE = mgh
KE= (1/2)mv^2

Ei = mgh+0(because the car has zero velocity at the start)--->(0.004 kg)(9.8 m/s^2)(h)
Ef = (0.004 kg)(9.8 m/s^2)(0.2 m) + (1/2)(0.004 kg)(0.31305 m/s) ---> 0.008036 J

Now we set Ei=Ef because the energies must be equal, it's the law.

(0.004 kg)(9.8 m/s^2)(h) = 0.008036 J

Solve for h

h = 0.205 m (This is the drop height so that the roller coaster will successfully complete the loop)