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Homework Help: What is the average power of a boy

  1. May 23, 2016 #1
    1. The problem statement, all variables and given/known data

    A boy trample everything he can on his tricycle and manages to briefly hold
    speed 9 km / h on flat ground. When he stops pedaling cycle continues by itself 7.5m
    bout it stops due to friction. The boy and the bike weighs together 21
    kg. How large average power develops boy during the short time he keeps
    fly 9 km / h.

    2. Relevant equations
    kinetic energy = 0,5*m*v^2

    3. The attempt at a solution
    kinetic energy = 0,5*m*v^2=0,5*21*2,5^2 = 66 Joule
  2. jcsd
  3. May 23, 2016 #2


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    Hello JiJiasd, :welcome:

    But what you calculate is an energy. The exercise asks for the power , i.e. energy/time. So you'll need another approach. Any ideas ? Any relevant equations with power in them ?
  4. May 23, 2016 #3


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    Hint: The distance it takes to stop gives you a way to find out the friction.
  5. May 23, 2016 #4


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    If you allow me to rephrase the question:
    The boy and tricycle accelerate to a velocity of 9 km/hr. (the amount of time it takes isn't relevant for the problem, it could be 1 second or 1 hour) After reaching that velocity he maintains it for some time (again, amount of time is not relevant). Finally he stops pedaling and coasts to a stop going 7.5 m from the instant he stopped pedaling. Assume that the ground is flat and friction is constant. He and the bike together weigh 21 kg. While he was pedaling at 9 km/hr, what (average) power was he expending? Figuring out the amount of energy he had the instant he stopped pedaling is a great first step. Why did he stop? Where did that energy go? You can use the answers to those questions for the next step.
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