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Homework Help: Acceleration of car and power

  1. May 21, 2010 #1

    A car o fmass 1250 kg accelerates from 0 to 100 km/h in 4 s . The average power of the car is ?

    2. Relevant equations

    3. The attempt at a solution

    Though it looks simple , but i don see my mistake.

    The average power is the change in kinetic energy over time ,

    (1/2 x 1250 x 100^2)/(4)

    is my working correct ? But that doesn't give the answer which is 121 kW
  2. jcsd
  3. May 21, 2010 #2
    Re: mechaniscs

    For the formula to yield an answer in watts, you need to convert km/h into m/s. Divide the answer you get by 1000 to get an answer in kW.
  4. May 21, 2010 #3
    Re: mechaniscs

    You know the change in velocity and the change in time so you can calculate the acceleration. Knowing the acceleration and time you can calculate the distance the car travels. Knowing the mass of the car and its acceleration you can calculate the force on the car. Knowing the force and distance you can calculate the work done. Power equals work/ change in time.
  5. May 21, 2010 #4


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    Homework Helper

    Re: mechaniscs

    This over complicates things quite a bit. The work is given by the difference of kinetic energy. thereddevils only mistake is the usage of the wrong units.