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Homework Help: Energy principle example problem in classical mechanics book

  1. Sep 11, 2016 #1
    1. The problem statement, all variables and given/known data
    A man of mass 100 kg can pull on a rope with a maximum force equal to two fifths of his own weight. [Take g = 10 ms^2] In a competition, he must pull a block of mass 1600 kg across a smooth horizontal floor, the block being initially at rest. He is able to apply his maximum force horizontally for 12 seconds before falling exhausted. Find the total work done by the man and confirm that the energy principle is true in this case.

    2. Relevant equations
    F*v*dt=W , Work-Energy principle

    3. The attempt at a solution

    0http://physics.stackexchange.com/questions/279513/energy-principle-example-problem-in-mechanics-book# [Broken]

    The problem as stated in the book Classical Mechanics by R Douglas Gregoryhttps://books.google.co.uk/books?id...equal to two fifths of his own weight"&f=false .I only have an issue with a portion of the problem.Part of the solution to that example problem states that the force he applies is a constant 200 N. But shouldn't it be 400 N since the problem states that he pulls with a maximum force of two-fifths of his own weight and his weight is 1000 N and two-fifths *1000 is 400 N? Maybe I am missing something here.
    Last edited by a moderator: May 8, 2017
  2. jcsd
  3. Sep 11, 2016 #2


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    Staff: Mentor

    Hi certainice, Welcome to Physics Forums!

    It's not unheard of for there to be typos in text books, particularly if example problems are "updated" for new editions. 2/5 of 1000 is indeed 400, not 200.

    Do the derivation yourself symbolically rather than plugging in numbers, or just show that your own numerical results demonstrate the principle being examined.
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