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Total kinetic energy of a car

  1. Dec 14, 2008 #1
    1. The problem statement, all variables and given/known data
    The 4 wheels of a car each weigh 25kg and have a radius of 0,3m. Without its wheels, the car weighs 10^3 kg. The wheels are homogenous cylinders. What is the total kinetic energy of the car and the wheels if the car is moving at a speed of 30 m/s?


    2. Relevant equations
    [tex]K_{tot}=4(0.5I_{CM}\omega^{2}+0.5MV_{CM}^{2})+K_{car}[/tex]
    the first to terms of the last question concern the wheels
    [tex]I_{cylinder}=\frac{MR^{2}}{2}[/tex]
    [tex]\omega=\frac{V_{CM}}{R}[/tex]

    3. The attempt at a solution
    replacing omega by V/R, I by MR^2/2 and distributing the 4:
    [tex]K_{tot}=MV^{2}+2MV^{2}+\frac{10^{3}\times30}{2}[/tex]
    [tex]K_{tot}=25\times30^{2}+50\times30^{2}+\frac{10^{3}\times30}{2}=82,2 \mathrm{kJ}[/tex]
    The right answer is 518 kJ

    where is the mistake? :(
    thank you, sorry for my english
     
  2. jcsd
  3. Dec 14, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    You forgot to square the speed in that last term.
     
  4. Dec 14, 2008 #3
    Thank you very much
    sorry for the waste
     
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