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!

Potential energy in stretched spring

  1. May 6, 2016 #1
    1. The problem statement, all variables and given/known data
    Five identical masses of mass M are suspended by a spring stretched a distance of L. If three of the masses are removed, what is the potential energy stored in the spring?

    1) (4 / 25) * M * g * L
    2) (2 / 5) M * g * L^2
    3) (5 / 2) * M g * L
    4) (4 / 25 ) * M * g L^2
    5) 5 * M * g * L

    2. Relevant equations
    Fspring = -k * x

    3. The attempt at a solution

    Hi everyone. I get 2 to be the answer while my solutions manual says that 4 is correct. I suspect the solutions manual is wrong, but I wanted to get someone's opinion on this first.

    So firstly, I got the value of the "K" for the spring:
    Fspring = Fg of the masses

    k * x = 5 * M * g

    k = (5 * M * g) / L


    Then, I got the amount of stretch of the spring with only two masses.

    Fspring = Fg of the masses

    k * d = 2 * M * g

    [ (5 * M * g) / L ] * d = 2 * M * g

    solved for d = (2 /5 ) * L.

    Then, I took equation Uspring = (1 / 2) * k * x^2.

    I plugged the expressions for K and D for the stretch and I got the expression for answer 2.

    Have I done something wrong?

    Thanks in advance for the help!
     
  2. jcsd
  3. May 6, 2016 #2

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You did the Physics correctly, But if you did your math correctly, you would have got 2MgL/5. None of the choices match that correct answer.
     
  4. May 6, 2016 #3
    ohh yes, I got a single L, now that I remember, instead of L squared. Right. thanks
     
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: Potential energy in stretched spring
  1. Stretched Spring (Replies: 4)

  2. Vertical Spring (Replies: 1)

  3. Stretch of a Spring (Replies: 6)

Loading...