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: Energy Problem with Spring, Gravity, and Friction

  1. Dec 4, 2017 #1
    1. The problem statement, all variables and given/known data
    An elevator is traveling at 30 m/s just as it touches a cushioning spring. The safety clamps engage at that moment and provide 20,000N of frictional force. The mass of the elevator is 3000kg and the spring constant is 15000 N/m. How fast will the elevator be traveling after the spring compresses 0.8m ?

    2. Relevant equations
    K1+Wf = Ug2+Us2+K2
    Allow the point that the elevator touches the spring to be y=0
    3. The attempt at a solution
    1/2*m(v1)2+Ffd=mgy2+1/2*kx2+1/2*mv22
    Manipulating this around and plugging in numbers gives an answer of 30.03 m/s for me. Am I missing something here?
    Edit: d is just the distance along which friction occurs, -.8 m?
    x is the compression of the spring, also .8m
    y2 is the distance traveled by the elevator, so -.8m
     
    Last edited: Dec 4, 2017
  2. jcsd
  3. Dec 4, 2017 #2

    gneill

    User Avatar

    Staff: Mentor

    Hi FettuccineAlfredo,

    Welcome to Physics Forums!

    Can you define your variables used in the attempt at solution? For example, what distinguishes ##d##, ##x##, and ##y_2##?
     
  4. Dec 4, 2017 #3

    Doc Al

    User Avatar

    Staff: Mentor

    I didn't check your calculation, but realize that the work done by friction is negative. (As will be y2.)
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted