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Centripetal question

  1. Mar 23, 2007 #1
    1. The problem statement, all variables and given/known data
    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.

    2. Relevant equations
    Using newton's second lay, and centripetal force formula
    3. The attempt at a solution

    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 ?
  2. jcsd
  3. Mar 23, 2007 #2
    looks okay to me.
  4. Mar 23, 2007 #3
    i don't quite get it


    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. :'(
  5. Mar 23, 2007 #4
    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.
  6. Mar 23, 2007 #5
    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?
  7. Mar 23, 2007 #6
    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..
    I think drawing free body diagram would help
  8. Mar 23, 2007 #7
    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
  9. Mar 23, 2007 #8
    k. thnx a lot. i think i kinda understand it now...
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