1. Not finding help here? Sign up for a free 30min 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!

What is the work that he does on his body?

  1. Apr 29, 2016 #1
    1. The problem statement, all variables and given/known data

    A 75-kg man pushes a crate 4.5 m up along a ramp that makes an angle of 17.5° with the horizontal. He exerts a force of 510 N on the crate parallel to the ramp and moves it at a constant speed.

    Calculate the work done by man to move the crate, in joules. Be sure to include the work he does on the crate and on his body to get up the ramp.

    2. Relevant equations

    W = F cos theta d

    3. The attempt at a solution
    I calculated the work that he does on the box using that above equation but could not figure out what the secind part of the question is getting at
     
  2. jcsd
  3. Apr 29, 2016 #2

    cnh1995

    User Avatar
    Homework Helper

    Work done by the man(against gravity) would be equal to the change(increase) in his potential energy.
     
  4. Apr 29, 2016 #3

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    I'm sure that is the intent of the question, but strictly speaking he does no work on the crate or on his own body. Work done on a rigid object equates to the gain in its KE, which is zero here. The work done by the man consists of work done on the objects (zero) plus the work done on the gravitational system consisting of those objects and the Earth.
     
  5. Apr 29, 2016 #4

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    True, the net work done on the crate is zero. However, that consists of the man doing (positive) work on the crate and gravity doing (negative) work on the crate.
     
  6. Apr 29, 2016 #5

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Doesn't seem any more reasonable as a way to view it, but if you wish to go that path you'd have to say the man does some work on the crate and some on the Earth, with gravity doing negative work on both.
     
  7. Apr 29, 2016 #6

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    The work that the man does on the Earth is essentially zero.

    Added in Edit:

    I do agree that the issue of what work the man does on himself is problematical.
     
    Last edited: Apr 29, 2016
  8. Apr 30, 2016 #7
    If the man's potential energy is added to the work he does on the crate, it does not yield the right answer which in this case is 3.14x10^3 J.
     
  9. Apr 30, 2016 #8

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    In that equation, what is the necessary geometric relationship between F, d and theta in the physical arrangement?
     
  10. Apr 30, 2016 #9
    F is the magnitude of the force acting on the system.
    d is the magnitude of the displacement of the system.
    and theta is the angle between the force vector and the displacement vector?
     
  11. Apr 30, 2016 #10

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Ok, now please post your working.
    (I get nearly 3300J.)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted