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Work Energy Theorem and Circular Motion

  1. Feb 10, 2009 #1
    1. The problem statement, all variables and given/known data
    A car is coasting without friction toward a hill of height 'h' and radius of curvature 'r'.
    What initial speed will result in the car's wheels just losing contact with the roadway as the car crests the hill?

    2. Relevant equations
    Kinetic Energy = (1/2)(m)(v^2)
    Potential Energy = mgy

    3. The attempt at a solution
    Because the acceleration will vary with time, constant energy kinematics can't be used to solve the question. Without friction, there are no nonconservative forces acting on the system, so there is no energy lost. Therefore the kinetic energy of the car at the bottom of the hill must be equal(?) to the potential energy of the car at the top of the hill. But I can't seem to work the radius of curvature into the theory at all. There's can't be a centripidal force because the only force holding the car to the curvature of the hill is the force of gravity. I'm a little stuck at this point. Thank you in advance for any help someone can give.
  2. jcsd
  3. Feb 10, 2009 #2


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    Homework Helper

    Welcome to PF.

    When the mv2/r is greater than mg contact force won't that mean that the car will lose contact with the road? So when v2 = g*r.

    So your initial mV2 must be greater than m*g*h +mv2/2 where v2 can be substituted with g*r ?
  4. Feb 10, 2009 #3
    Thank you very much for the kind welcome.

    I see. So if the formula a=(v^2)/r is usually used for centripidal force, and the only force holding the car to the curve is gravity, then this formula has to be used as a minimum where a=g? I think that makes sense.
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