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!

Why does Hooke's law not work here?

  1. Oct 20, 2015 #1
    1. The problem statement, all variables and given/known data
    1. A 1200-kg car moving on a horizontal surface has speed v = 85 kmh when it strikes a horizontal coiled spring and is brought to rest in a distance of 2.2 m. What is the spring stiffness constant of the spring?
    2. Relevant equations
    F=-kx
    KE=(1/2)mv^2
    PE(spring)=(1/2)kx^2

    3. The attempt at a solution
    I tried to find the average acceleration to slow the car from 85kmh to 0. I used the formula vfinal^2=vinitial^2 + 2ax, where velocity initial is 23.6m/s and x is 2.2m. This gave me an acceleration of -126m/s^2. Then I multiplied the acceleration by the mass to find the average force. This gave me -151200N. Then, I plugged that into Hooke's law with x being 2.2m. This gave the spring constant being 69000N/m.

    However, I am supposed to use the conservation of energy principle where KE=PE. This gives the correct answer. Why does my method not work?
     
  2. jcsd
  3. Oct 20, 2015 #2

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    I does not work because the acceleration is not constant. Hook's Law works, but not your expression ##v_f^2 = v_i^2 + 2ax##.
     
    Last edited: Oct 20, 2015
  4. Oct 20, 2015 #3
    By Hooke's law I assume you mean ##F=kx## where ##F## is the magnitude of the force and ##x## is the distance?

    In that formula ##F## is not the average force. It's the magnitude of the force when the spring is stretched (or compressed) a distance ##x##.

    If the force were constant, that would work, but the force is not constant. You could integrate the force, or use energy concepts.
     
  5. Oct 20, 2015 #4
    Oh, okay it makes sense now. Thank you both very much.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Why does Hooke's law not work here?
  1. Work and hookes law (Replies: 4)

  2. Work and Hooke's Law (Replies: 2)

Loading...