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Energy-Work Problem

  1. Nov 23, 2013 #1
    1. The problem statement, all variables and given/known data

    A cyclist is competing in a race and decides to pass some fellow cyclists who are getting tired. The initial speed of the cyclist is 21 km/h and he has 3.0 * 10^2 W of power. If the bicycle is able to convert 92% of input energy into kinetic energy, how fast will cyclist be travelling after 4.0 s. The combined mass of the cyclist and the bicycle is 78 kg.

    The answer is 7.9 m/s. But I dont know how.

    2. Relevant equations

    p=e/t
    Ek=1/2mv^2



    3. The attempt at a solution
    energy= Pt= 3x10^2 * 4 = 1.2 x10^3 J
    Useful energy = 1.2 x10^3 *0.92 = 1.104 x10^3
    KE= 1/2m( v)^2
    root(1.104 x10^3 *2 /78) = change in v = +/-5.32 m/s (3 S.F.). He is accelerating hence +5.32m/s
    21km/h = 21000m/60x60s= 5.83 m/s
     
  2. jcsd
  3. Nov 23, 2013 #2
    why are you putting the same post every time???
     
  4. Nov 23, 2013 #3
    go back to your previous post......!!!!
    i have posted solution over there
     
  5. Nov 23, 2013 #4
    initial kinetic energy + useful energy given= final kinetic energy
     
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