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: Work of a heat engine

  1. Dec 6, 2006 #1
    1. The problem statement, all variables and given/known data

    An engine works at 26% efficiency. The engine raises a 6-kg crate from rest to a vertical height of 11 m, at which point the crate has a speed of 5 m/s. How much heat input is required for this engine?

    2. Relevant equations

    e=work output / heat input = W/Qin

    3. The attempt at a solution

    I assume the velocity is insignificant since W=Fd; W=(6*.981)*(11)= 647.46J

    Qin=.0004016 J

    the value for calculated heat is wayy too low; where did I screw up? Thanks
  2. jcsd
  3. Dec 7, 2006 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    For starters, you have calculated 1/Qin not Qin.

    You are also ignoring the work that is required to give the load the kinetic energy. Add that to your calculation for W and then do the algebra properly and you will be fine.

  4. Dec 7, 2006 #3
    for KE do I just say that delta E = 1/2mv^2 + mgh = W = eQ ?

    .5(6kg)(5^2)= 75J

    mgh = 647.46J
    What do i do with these two values?
  5. Dec 7, 2006 #4

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    Looks good to me.

    Why not put them into your equation and find Q?

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