1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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


    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted