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: Conservation of Energy Problem-I think

  1. Dec 6, 2004 #1
    Conservation of Energy Problem--I think

    In a switchyard, freight cars start from rest and roll down a 2.8 m incline and come to rest against a spring bumper at the end of the track (Fig. 8-27). If the spring constant is 4.9 x 106 N/m, how much is the spring compressed when hit by a 57,000 kg freight car?

    I thought that this was a conservation of energy problem, so I set up K_i + U_si + U_gi = K_f + U_sf + U_gf, and after cancelling out all the things that equal zero, I got PE_gi = PE_sf. Then I did mgy_i = .5(k)(x)^2. When I solved for change in x, I got 0.255 m, but that was wrong, so I don't know where to go from there. Any help would be greatly appreciated. Thanks!! Beverly
  2. jcsd
  3. Dec 6, 2004 #2
    PE_final + KE_final = PE_initial + KE_initial
    0 + KE_final = (57000)*g*2.8 + 0

    KE_final = (1/2)mv^2 = 0.5kx^2 = 57000*g*2.8
    x = root (57000*g*2.8 / (0.5*4.9*10^6)) = 0.8 meters?
  4. Dec 7, 2004 #3
    Thanks! I had forgotten to multiply by g somehow!! I can't believe I didn't notice that before!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook