Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

3 blocks no friction forces question

  1. Oct 5, 2010 #1
    1. The problem statement, all variables and given/known data
    [PLAIN]http://img834.imageshack.us/img834/6920/captureqt.jpg [Broken]


    2. Relevant equations

    f=ma

    3. The attempt at a solution

    Fsum=Fl + Fr=(m1+m2+m3)a
    26+-8=(2+8+4)a
    a=18/14=1.28571

    f=ma
    f=4(1.28571)=5.14268N

    I know this is incorrect. The force of 4kg block pushes on the 8kg the same as the 8kg pushes on the 4kg, but the way I calculated it doesn't work with that but I know of no other way.
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 5, 2010 #2

    ehild

    User Avatar
    Homework Helper

    You are right that the resultant force acting on the 4 kg block is F=4a=5.143 N. See the figure: What forces act on this block?

    ehild
     
  4. Oct 6, 2010 #3
    The homework systems seems to disagree.

    Call the blocks from left to right A,B,C

    The way the teacher explained it in class, the force of C acting on B is the same as the force of B acting on C. But the way I calculated it Fc=4a but Fb=8a
     
  5. Oct 6, 2010 #4

    ehild

    User Avatar
    Homework Helper

    Look at the 4 kg mass. There are two forces acting on it: the force from the 8kg mass (F8) ponting to the right, and Fr=8 N, pointing to the left. The resultant force points to the right, and it is F8-Fr=4*a. So F8=4a+Fr.

    ehild
     
  6. Oct 6, 2010 #5
    Is the correct answer then 13.14268N?
     
  7. Oct 6, 2010 #6

    ehild

    User Avatar
    Homework Helper

    Yes, the 8 kg mass exerts 13.14 N on the 4 kg one, and a force of the same magnitude and opposite direction is exerted by the 4 kg mass on the 8 kg one.

    ehild
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook