# Energy principle example problem in classical mechanics book

#### certainice

1. Homework Statement
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. Homework 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=uAfUQmQbzOkC&pg=PA132&lpg=PA132&dq="A man of mass 100 kg can pull on a rope with a maximum force equal to two ﬁfths of his own weight"&source=bl&ots=4vYfjiGFRQ&sig=nXFm9dihLDUiUbd8UA4myb_cWdo&hl=en&sa=X&ved=0ahUKEwjZ0tD8_4bPAhUGORoKHUdNDAwQ6AEIHjAA#v=onepage&q="A man of mass 100 kg can pull on a rope with a maximum force 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:
Related Introductory Physics Homework Help News on Phys.org

#### gneill

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.

"Energy principle example problem in classical mechanics book"

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving