Force and Motion - Car going over a hill

[Shatzkinator]

Homework Statement

In Figure 6-58, a stuntman drives a car (without negative lift) over the top of a hill, the cross section of which can be approximated by a circle of radius R = 211 m. What is the greatest speed at which he can drive without the car leaving the road at the top of the hill?

Homework Equations

Ac = mv^2/R
fnet = ma

The Attempt at a Solution

I honestly have no idea how to tackle this problem. Help?

 

[Delphi51]

It is circular motion. Recall that a "centripetal force" is required to hold an object in circular motion. You must use that formula and be aware of what force is providing the centripetal force. That force will become too small when the speed gets too high. Set them equal and solve for the speed.

 

[Shatzkinator]

It is circular motion. Recall that a "centripetal force" is required to hold an object in circular motion. You must use that formula and be aware of what force is providing the centripetal force. That force will become too small when the speed gets too high. Set them equal and solve for the speed.

I don't understand. First of all, the force causing the motion is the car itself moving up the hill. If the formula you're talking about is Fc = mv^2/R, then I don't see how the force will become small when the speed is high. They are directly proportional so the force would increase as the speed increases. Please clarify... thanks =)

edit: got the answer, Fg is the force = mg = mv^2/R

 

[Delphi51]

Congrats! Yes, gravity provides the centripetal force right at the top of the hill.