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!

Two blocks and a spring system

  1. Jun 16, 2013 #1
    1. The problem statement, all variables and given/known data
    Two bars of masses ##m_1## and ##m_2## connected by a weightless spring of stiffness ##k## rest on a smooth horizontal plane. Bar 2 is shifted a small distance ##x## to the left and then released. Find the velocity of the centre of inertia of the system after bar 1 breaks off the wall.
    attachment.php?attachmentid=59577&stc=1&d=1371360642.png

    2. Relevant equations



    3. The attempt at a solution
    The bar 1 breaks off from the wall when the bar 2 just passes its original position but I don't have any idea about how to begin making the equations here. :confused:
     

    Attached Files:

  2. jcsd
  3. Jun 16, 2013 #2
    What will be the velocity of 1 when it breaks off the wall?
    What will be the velocity of 2 when 1 breaks off the wall?(conserve energy)
     
  4. Jun 16, 2013 #3

    verty

    User Avatar
    Homework Helper

    What can you tell us about the momentum of the system at various times?
     
  5. Jun 16, 2013 #4
    Conserving energy,
    [tex]m_1v_1^2+m_2v_2^2=kx^2[/tex]
    where ##v_1## and ##v_2## are the velocities of 1 and 2 when 1 breaks off the wall.

    Do you ask me to conserve linear momentum? :confused:
     
  6. Jun 16, 2013 #5
    What will be the velocity of block 1 when it breaks off the wall?

    No need to conserve momentum.Linear momentum will not be conserved till block 1 remains in contact with the wall.
     
  7. Jun 16, 2013 #6
    If I assume ##v_1=0##, I get the right answer. Thanks! :smile:
     
  8. Jun 16, 2013 #7
    Great ...:approve:

    But do you understand why should you do so ?
     
  9. Jun 16, 2013 #8
    As the question asks ##v_{CM}## at the instant 1 breaks off the wall, so we can assume ##v_1=0## and I don't see how will ##v_1## reach a finite value in an instant, right?
     
  10. Jun 16, 2013 #9
    When block 2 is released,it will move towards right,but block 1 will remain pushed against the wall.Block 2 will reach a point A such that the spring is in its natural uncompressed length.The force on block 1 at this point will be zero .

    Now due to the momentum gained,Block 2 will continue moving towards right.Just as block 2 crosses point A,there will be a stretching in the spring which will pull the block 1 off the wall with an initial speed zero.
     
  11. Jun 16, 2013 #10
    Thanks!

    I had somewhat similar scenario in my mind when I began with this question. I figured out that 1 will separate from wall when 2 just passes A. I did not think of ##v_1=0##. Should have been more careful. :rolleyes:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted