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!

Energy Conservation.

  1. Jun 18, 2008 #1
    1. The problem statement, all variables and given/known data

    A 10 kg vox slides 4.0 m down the frictionless ramp shown in the following link. It then collides with the spring whose spring constant is 250 N/m.
    a. What is the maximum compression of the spring?
    b. At what compression of the spring does the box have its maximum velocity?

    http://i196.photobucket.com/albums/aa59/aliatehreem/chapter_10.jpg


    2. Relevant equations

    Usf= Ug [ spring potential energy final = gravitational potential energy initial.
    0.5 k x^2 = mgy

    3. The attempt at a solution
    Find height corresponding to 4 m.
    h= 4 sin30

    a. Then use conservation of energy to find compression.
    Usf= Ug [ spring potential energy final = gravitational potential energy initial.
    0.5 k x^2 = mgy
    0.5 (250) (x ^2) = (10) (9.81) (4sin30)
    x= 1.25 m

    Apparently, the correct answer is 1.46 m for a. and 19.6 cm for b. Can someone please help me figure out what I am doing wrong? It would be greatly appreciated! I wish that they would give the length of the spring, then I could calculate the gravitational potential energy more accurately.
     
  2. jcsd
  3. Jun 18, 2008 #2

    rl.bhat

    User Avatar
    Homework Helper

    The force acting on the block is mgsin(theta) and distance moved by the block is (4 + x). In solving this problem the length of the spring is not required.
     
  4. Jun 18, 2008 #3

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    Have you taken into account that the pe of the box is minimum when the spring stops not at first contact, so the box slides (4+x) down the slope

    For (b) write the energy as KE+PE+spring=0 ( get the sign's correct! ) to find an equation only in V and x.

    (Oops posts clashed)
     
  5. Jun 18, 2008 #4
    by writinf 4 + x, i get 2.2 m, which is still not the correct answer.

    Also, for Ke +PE + spring=0, why is the sum 0?
     
  6. Jun 18, 2008 #5

    rl.bhat

    User Avatar
    Homework Helper

    0.5 (250) (x ^2) = (10) (9.81) (sin30)(4+x) .
    Solve for x.
     
  7. Jun 18, 2008 #6


    Nice!! Thank you so much!!!!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?