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!

Work and Energy Problem

  1. Apr 26, 2006 #1
    A 60.0 kg crate, starting from rest, is pulled across the floor with a constant force of 100.0 N. For the first 10.0 m the floor is frictionless and for the next 10.0 m the coefficient of friction is 0.20. What is the final speed of the crate?

    Is it possible to use KE + PE = KE + PE
    1/2mv^2 + mgh = 1/2mv^2 + mgh
     
  2. jcsd
  3. Apr 26, 2006 #2

    Doc Al

    User Avatar

    Staff: Mentor

    No, since energy is not conserved. Instead, consider the work done by the applied forces. (The work-energy theorem.)
     
  4. Apr 26, 2006 #3
    How can you tell if energy is coserved or not?
     
  5. Apr 26, 2006 #4
    Work is done, both by the puller and the frictional force. The friction generates heat, which certainly tells us that energy is not conserved.
     
  6. Apr 26, 2006 #5

    Doc Al

    User Avatar

    Staff: Mentor

    Mechanical energy is conserved if the only forces acting are conservative forces, like gravity. As Captain Zapp0 points out, here we have additional forces acting: the applied force and the friction. These forces do work on the object, changing its kinetic energy.

    If you attempted to use the conservation equation you quoted, you'll quickly find that it gets you nowhere since the gravitational PE doesn't change. (We assume that the crate is moving across a horizontal floor.)
     
  7. Apr 26, 2006 #6
    m=60.0kg u=0.20
    F= 100.0 N
    d1==10.0m

    KE(final) - KE(initial)
    1/2mv^2 - 1/2mv^2

    umm... how do i isolate v? or is this even correct?
     
  8. Apr 26, 2006 #7

    Doc Al

    User Avatar

    Staff: Mentor

    Not clear what you've done.

    Find the work done by the forces during the first 10m and the second 10m. The total work done will equal the change in the crate's KE.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Work and Energy Problem
  1. Energy Work Problem (Replies: 3)

Loading...