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: Springs: Inclined ramp, object moving down towards spring

  1. Mar 17, 2009 #1
    1. The problem statement, all variables and given/known data

    You are designing a delivery ramp for crates containing exercise equipment. The crates weighing 1470 N will move at a speed of 1.80 m/s at the top of a ramp that slopes downward at an angle 25.0. The ramp exerts a kinetic friction force of 540 N on each crate, and the maximum static friction force also has this value. Each crate will compress a spring at the bottom of the ramp and will come to rest after traveling a total distance of 7.70 m along the ramp. Once stopped, a crate must not rebound back up the ramp.

    2. Relevant equations

    K1 + Ugrav1 + Uelastic1 - Wfriction = K2 + Ugrav2 + Uelastic2

    3. The attempt at a solution

    K1 = 0.5*150*1.8^2
    Ugrav1 = 0
    Uelastic1 = 0
    Wfriction = (540)(7.7) = 4158

    K2 = 0
    Ugrav2 = 1470*(-7.7)(sin25) = -4783
    Uelastic2 = 0.5k(7.7^2)

    Using the above equation, I got k = 29.27. Unfortunately, this is not the correct answer, so I am a little stuck as to what I need to do next.
  2. jcsd
  3. Mar 17, 2009 #2


    User Avatar
    Homework Helper

    I am guessing that the question here is to determine the spring constant?

    What is also not determined here is the distance over which the spring will compress. But since they don't want it to rebound from the bottom position then we know at least it must supply a force that satisfies is Hookes F = -kx. And that force would be 1470N*Sin25 + the frictional force from the rollers of 540N preventing it from rolling back up seems to me.

    So now you can solve your energy equation

    PE loss = 1470*7.7*sin25 ; basically your usual m*g*h
    Then there is the work of friction over the distance as you found as well as the initial kinetic energy also found.

    And all that you can now resolve into one equation that relates 1/2kx2. That and your other equation should allow you to determine x and k right?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook