Solve Centripetal Force Question: Find Vehicle Speed

[harmeet_angel]

Homework Statement

An automobile moves at constant speed over a hill with a radius of 18 m. At the top of the hill, the drivers notices that she barely remains in contact with the seat. find the speed of the vehicle.

Homework Equations

Using Newton's second lay, and centripetal force formula

The Attempt at a Solution

N=0
W-N = Fc <centripetal force>

therefoce, (mv^2)/r = mg

and v= (rg)^0.5

I do not have the answer, so wondering if I have made any mistake somewhere above ?

 

[robb_]

looks okay to me.

 

[a seeker]

i don't quite get it

"N=0"

what does N rep?
and where did you get the equation "W-N = Fc <centripetal force>"
isn't the formula for centripetal force, (mv^2)/r?
and how do know if that equals mg?

ugh. this stuff is pretty new to me and it's confusing me. :'(

 

[Harmony]

N represents normal reaction. Draw a free body diagram of normal reaction and weight. The centripetal force is the resultant force of weight and normal reaction.

 

[a seeker]

oooooooooooh, i get it.- so FBD is the answer!

but why is Fn zero again? is it because she "barely remains in contact with the seat"? then, it's not the same as when a person feels weightless, is it?
in what other situations is like this?

 

[harmeet_angel]

thanks a lot to all!

and, as for how did i get there<assuming that me's right>
centripetal force is the resultant, and
there must be something that provides it...
in this case,
the car is pushed up by the road, down by the gravity
and the net force is keeping the car in the circle
as normal force is against or negative..
so...
I think drawing free body diagram would help

 

[harmeet_angel]

oooooooooooh, i get it.- so FBD is the answer!

but why is Fn zero again? is it because she "barely remains in contact with the seat"? then, it's not the same as when a person feels weightless, is it?
in what other situations is like this?

yea, because she is not being pushed by her seat so shez weightless..
other situation..a free falling elevator, with some thing in it, and that thing would be weightless

 

[a seeker]

k. thnx a lot. i think i kinda understand it now...