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Homework Help: Kinetic Energy etc. Please check.

  1. May 2, 2010 #1
    1. The problem statement, all variables and given/known data
    When a toy car is rapidly scooted across the floor, it stores energy in a flywheel. The car has a mass 0.180 kg, and its flywheel has a moment of inertia 4 x 10-5 kg.m². The car is 15.0cm long. An advertisement claims that the car can travel at a scale speed of up to 700 km.hr-1. The scale speed is the speed of the toy car multiplied by the ratio of the length of an actual car to the length of the toy. Assume a length of 3.0 m for a real car.
    a) For a scale speed of 700 km.hr-1, what is the actual translational speed of the toy car?
    b) If all the kinetic energy that is initially in the flywheel is converted to translational kinetic energy of the toy, how much energy is originally stored in the flywheel?
    c) What initial angular velocity of the flywheel was needed to store the amount of energy calculated in part b)?

    2. Relevant equations

    Subscript t = toy car; r = real car; f = flywheel; s = scale

    3. The attempt at a solution

    a)Vs = Vt x lr/lt
    Vt = 700/20 = 35km/hr

    b)Kf= kt= 1/2 mtVt2
    = 1/2 x 0.18 x 9.722
    = 8.51J

    c) K = 1/2Iw2 = 8.51J
    8.51 = 1/2 x 4 x 10-5 x w2
    w = 652.3 rad/s
  2. jcsd
  3. May 2, 2010 #2
    Yep all right
  4. May 2, 2010 #3
    Cheers Pat!
  5. May 2, 2010 #4
    Hey, was this for an assignment or mastering physics online crap??
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