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

Homework Help: 2008 AP Physics B Free Response MISTAKE?

  1. Jan 20, 2010 #1
    2008 AP Physics B Free Response MISTAKE???

    1. The problem statement, all variables and given/known data

    Question number two can be found here...
    http://apcentral.collegeboard.com/apc/public/repository/ap08_physics_b_frq.pdf

    2. Relevant equations



    3. The attempt at a solution

    Ok I was working out question number two and was doing (e). How come when they solved it in the rubric

    http://www.collegeboard.com/prod_downloads/ap/students/physics/ap08_physics_b_sgs_rev.pdf

    they solved it this way

    .5 (mass of block b) v^2 = .5 kx^2

    how come they only included the mass of block b and not the mass of the whole system???

    The wording of the question states specifically

    In a new situation, the blocks and spring are moving together...

    this would allow one to conclude that the whole system was moving which would mean block A and block B but yet they only include the mass of block b in their calculation...

    Can you please explain why they chose only the mass of B in this calculation and didn't include the other block thanks!
     
    Last edited by a moderator: Apr 24, 2017
  2. jcsd
  3. Jan 20, 2010 #2

    cepheid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Re: 2008 AP Physics B Free Response MISTAKE???

    The question says that block A hits the wall and sticks there. If it stops, all of its kinetic energy is gone. Because block B is still moving and block A has stopped, the spring will begin to compress. All of the kinetic energy of the still moving block B will be tranformed into elastic potential energy of the spring. This is the conservation of energy equation that has been set up in your "rubric."
     
  4. Jan 20, 2010 #3

    rl.bhat

    User Avatar
    Homework Helper

    Re: 2008 AP Physics B Free Response MISTAKE???

    After impact block A comes to rest. But due to dynamic inertia, block B continues to move with the velocity V. Before impact the spring is in un stretched position. Kinetic energy of the block B is converted into potential energy of the spring by compressing it.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook