Hooke's Law spring stiffness (k) problem - help!

    • Thread Starter

    Hi there

    I was given the following problem in my coursework:

    "A spring is initially compressed by 50mm when a steel ball of mass 2kg is released from just being in contact with the uncompressed spring. Determine the spring stiffness (k) of the spring."

    I gave the answer below, but the teacher has marked it as wrong. Can anyone shed any light on where I may have made a mistake? I thought it was just a simple application of Hooke's Law.

    F = mg
    Restorative force F = -kx

    F = mg
    F = 2 x -9.81
    F = -19.62 N

    F = -kx
    19.62 = -k(-0.05)
    19.62 = 0.05k
    k = 19.62/0.05
    k = 392.4

    Is this correct? Thanks.

    I think you've answered the question as if the ball were being gently lowered onto the spring until it reached the equilibrium point... which is a type of question which gets asked.

    This question seems to be more dynamic - if the ball is suddenly released it'll accelerate downwards and gain momentum... and travel past the equilibrium position. In an ideal system with no losses it'll be in SHM and will be stationary again for an instant when it's on the opposite side of the equilibrium position to where it started, that position is I think 50mm below the release position.
Write a reply… Reply
Submit reply


Thanks for posting! You just need to create an account in order to submit the post
  1. this can't be left blank
    that username has been taken, please choose another Forgotten your password?
  2. this can't be left blank
    this email is already registered. Forgotten your password?
  3. this can't be left blank

    6 characters or longer with both numbers and letters is safer

  4. this can't be left empty
    your full birthday is required
  1. Oops, you need to agree to our Ts&Cs to register
  2. Slide to join now Processing…

Updated: September 18, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

What social media "stories" do you use?
Study resources

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Quick reply
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.