1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Work and kinetic energy

  1. Oct 31, 2012 #1
    1. The problem statement, all variables and given/known data

    Block A is moving downward at 5 ft/s at a certain time when the spring is compressed 6 inches. The coefficient of friction between block B and the plane is 0.2, the pulley is light, and the weights of A and B are 161 and 193 lb, respectively.

    a) Find the distance that A falls from its initial position before coming to zero speed
    b) Determine whether or not body A will start to move back upward
    2. Relevant equations

    work by spring = (-1/2)(x)

    3. The attempt at a solution
    The answers are a) 4.78 ft b) A will start back upward

    No idea how to start. I suppose I apply F=ma to each A and B separately? Then somehow invoke the work energy equation to figure it out? How do I handle the spring? And the spring compressed 6 inches confuses me. Thanks for the help.

    Attached Files:

    • pic.png
      File size:
      2.2 KB
  2. jcsd
  3. Nov 1, 2012 #2


    User Avatar

    Staff: Mentor

    The energy stored in a spring = ½kx²

    As mass A loses potential energy, that energy reappears in the system in other forms. Write the equation that accounts for this. I presume that, since the spring has compressed 6", then the masses have moved the same 6". This should allow you to find the spring constant, k.

    (Your sketch shows the spring stretching, whereas I expect it should be drawn as a compression spring.)

    Beyond this, I'm afraid I can't help further. I think you'd be looking at a 2nd order DE?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook