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!

Given some body with a constant force F acting on it upwards

  1. Jun 5, 2013 #1
    Given some body with a constant force F acting on it upwards
    (The body is subject to the gravitational force due to the earth)
    What's the correct statement?

    gif.gif

    Or:

    gif.gif

    K - Kinetic Energy
     
    Last edited: Jun 5, 2013
  2. jcsd
  3. Jun 5, 2013 #2

    Nugatory

    User Avatar

    Staff: Mentor

    Think about how the potential energy changes with [itex]\Delta{x}[/itex]. The change in total energy, which includes both kinetic and potential energy, will equal the total amount of work done on the body.

    You might also find it helpful to consider the cases: F is only very slightly larger than mg; and F is so large relative to mg that we can ignore the effects of gravity.
     
  4. Jun 5, 2013 #3
    what if F is small?
    how come gravity has no affect on the change in kinetic energy?
     
  5. Jun 5, 2013 #4

    Doc Al

    User Avatar

    Staff: Mentor

    If you want to apply the "Work - KE" theorem, you must use the net force on the body. That net force must include gravity.
     
  6. Jun 5, 2013 #5
    So gif.gif is the right statement?
     
  7. Jun 5, 2013 #6

    Doc Al

    User Avatar

    Staff: Mentor

    Yes.
     
  8. Jun 5, 2013 #7
    Thank you very much!! :)
    i knew it was the right answer but i had an argument with my high school teacher(she claims to have Ph.D in physics) about that.
     
  9. Jun 9, 2013 #8

    Philip Wood

    User Avatar
    Gold Member

    Multiply out the rhs, then
    [tex]\Delta K = F \Delta x - mg \Delta x.[/tex]
    Identifying the terms on the right as work or energy:
    [tex]\Delta K = work\;done\;by\;F - \Delta (grav\;potential\;energy).[/tex]
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Given some body with a constant force F acting on it upwards
  1. Body acting on itself (Replies: 1)

Loading...