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: Test Question (Potential Energy): Need help finding answer

  1. Apr 18, 2005 #1
    I can't figure out what I did wrong on this test question.

    A 4kg block is dropped from a height of 0.8m above a spring, k=120N/m. Find the maximum compression of the spring.

    I have the PE at the top is equal to KE at the bottom.

    The potential energy in the spring when compressed should equal the KE at the bottom. Based upon that I should be able to find the maximum distance the spring will compress.

    Rearranging x=sqrt(2*31.4/120) and I get x=-0.723m

    What am I doing wrong here? Can someone point it out.

    Thanks, peace
  2. jcsd
  3. Apr 18, 2005 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Remember that the spring is compressing in the vertical direction, so there is more gravitational potential energy to take into account:

    Initial gravitational potential energy = Final spring potential energy

    Last edited: Apr 18, 2005
  4. Apr 18, 2005 #3
    Your equation will give me the exact same answer I got.
    mg(0.8 + 0) =.5kx^2
  5. Apr 18, 2005 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    No, in my response, x_spring is the same as x in your response. On the left side, you set the second term to 0, when it should be mgx. It's a quadratic equation.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook