The force pointing towards the center of the circle (i.e. down) is
Fnet = W – N = mg – N
The maximum value this can have occurs when N = 0, in which case for circular
motion we have
Fnet = mg – 0 = mg
so since the car is moving in a circle,
Fnet = mg = mv2/r
Hence the maximum speed is given when the force is this maximum:
v2 = gr
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