Car Driving over a hill

1. The problem statement, all variables and given/known data
A car is driven at constant speed over a circular hill and then into a circular valley with the same radius. At the top of the hill the normal force on the driver from the car seat is 0. The driver's mass is 70.0 kg. What is the magnitude of the normal force on the driver from the seat when the car passes through the bottom of the valley??


2. Relevant equations
Fg - Fn = 0 ?
F = mv^2/r


3. The attempt at a solution
Stumped. I looked at this question and was like... wtf?

Only thing I can think of is Fg - Fn cannot equal zero because there is always Fg and if Fn is 0, then that equation doesn't make sense. But I don't know any better than this equation. I'm so confused!! =S
 

LowlyPion

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At the top of the hill if his normal force is 0, then what must his centripetal acceleration be?
 
At the top of the hill if his normal force is 0, then what must his centripetal acceleration be?
Fg = 9.8 * 70 = 686
 

LowlyPion

Homework Helper
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How is the direction of the centripetal acceleration different at the top as opposed to the valley?
 
How is the direction of the centripetal acceleration different at the top as opposed to the valley?
At the top, it is directed downwards toward the centre, at the bottom is directed up toward the centre...
 

LowlyPion

Homework Helper
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At the top, it is directed downwards toward the centre, at the bottom is directed up toward the centre...
... and you still can't answer the original question?
 
Fn = Fg .. which would be al;so 686 at the bottom of the hill.

The answer should be 1.37 x 10^3 N

=P

No I still can't answer the original question.

Edit: and yes I see the answer is 2* 686... I have no idea why.
 

LowlyPion

Homework Helper
3,055
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In the first case Fn = 0 = mg - mv2/r

That means that mv2/r = mg.

In the valley

Fn = mg + mv2/r

But you just figured out that mg = mv2/r so

Fn = 2*mg
 

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