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!

Sliding block- thermal energy - please

  1. Nov 4, 2007 #1
    1. The problem statement, all variables and given/known data
    You push a block of lead across the floor, applying a horizontal force of 9 N. The block moves at a constant speed. The mass of the block is 1.5 kg and the heat capacity of lead is 130 J/(kg K). After pushing the block a distance of 20 m, you measure its temperature and find that it has increased by 0.3 K.
    In answering the following problems, state explicitly your choice of system!
    (a) What is the change in thermal energy of the block?
    (b) How much work is done by the frictional force of the floor acting on the block?
    (c) What is the change in the thermal energy of the floor?


    2. Relevant equations

    Etherm = mC deltaT

    Etherm = Force * delta r = W

    3. The attempt at a solution

    PART A : i was thinkin mCdeltaT

    Part B: either -Fd or could be zero since the force is perpendicular to motion

    Part C: work minus etherm maybe



    ****need help asap*****THANKS
     
  2. jcsd
  3. Nov 4, 2007 #2

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    yes
    explain why you think the force is perpendicular (or parallel?) to the motion
    explain your logic here.



    ****need help asap*****THANKS[/QUOTE]
     
  4. Nov 4, 2007 #3
    well the block is moving to the right
    and the work by floor is moving upward
    making it perpendicular to motion

    (honestly, part b and c are the ones im very unsure about)

    thank you
     
  5. Nov 4, 2007 #4

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Oh, OK, there are two forces exerted by the floor on the block; the upward normal force of the floor on the block, and the friction (leftward) force of the floor on the block (acting parallel to the floor). The motion of the block is to the right. Using the definition of work, frcostheta, note that the normal force does no work,(since costheta =0), and only the friction force does work. How much work does it do?
     
  6. Nov 4, 2007 #5
    -Fd ?
     
  7. Nov 4, 2007 #6

    learningphysics

    User Avatar
    Homework Helper

    The total change in energy is mc*deltaT.

    So the work done by friction + the work done by the 9N force = mc*deltaT
     
  8. Nov 4, 2007 #7
    oh ok that makes sense

    so Wfric = mct - mgh

    then would the etherm for floor be zero?
     
  9. Nov 4, 2007 #8

    learningphysics

    User Avatar
    Homework Helper

    not mgh... the displacement is not vertical... what is the work done by the 9N force... what is the definition of work?
     
  10. Nov 4, 2007 #9
    well i meant change in distance

    work = mgdetlar
     
  11. Nov 4, 2007 #10
    or wait no ... Fd
     
  12. Nov 4, 2007 #11

    learningphysics

    User Avatar
    Homework Helper

    yes, exactly.

    so:

    Wfric = mct - Fd
     
  13. Nov 4, 2007 #12
    alright, so how would i get to the etherm of the floor?
     
  14. Nov 4, 2007 #13

    learningphysics

    User Avatar
    Homework Helper

    What did you get as the work done by friction?

    Note that friction is the work done by the floor on the block.

    The only object doing any work on the floor is the block...

    if x is the work done by the floor on the block... what is the work done by the block on the floor?
     
  15. Nov 4, 2007 #14
    negative x
     
  16. Nov 4, 2007 #15

    learningphysics

    User Avatar
    Homework Helper

    exactly. what numbers do you get?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?