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: (21-23) thermodynamics problem. Finding efficiency for the human body

  1. Dec 9, 2011 #1
    1. The problem statement, all variables and given/known data

    While running up stairs at a (vertical) rate of 0.30 m/s, a man of 70 kg generates waste heat at a rate of 1300 J/s. What efficiency for the human body can you deduce from this?

    2. Relevant equations

    efficiency of heat engine=e=[itex]\frac{W}{Q1}[/itex]=[itex]\frac{Q1-Q2}{Q2}[/itex]=1-[itex]\frac{Q2}{Q1}[/itex]

    3. The attempt at a solution

    Kinetic energy= [itex]\frac{1}{2}[/itex](70)(.30)2= 3.15

    e[itex]\frac{3.15}{1300}[/itex]=0.0024 X 100=0.024 %

    According to the book the answer is 14%

    How do I get to this answer?
  2. jcsd
  3. Dec 9, 2011 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    Your formula for efficiency is correct: η = W/Qh = Qh-Qc/Qh

    Use W = Qh - Qc to find Qh from W and Qc.

    Note: The kinetic energy of the man is irrelevant since it is not changing. The only work being done is in raising the mass of the runner through a vertical height. That is W. You are given Qc so find Qh. Then apply the formula for η.

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