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: Spring Constant Problem

  1. Jan 29, 2009 #1
    1. The problem statement, all variables and given/known data
    A 75 kg student is standing atop a spring in an elevator that is accelerating upward at 3.0m m/s2. The spring constant is 2700 N/m
    By how much is the spring compressed?

    2. Relevant equations
    I know I have to use the spring constant formula in this

    So 75kg*9.8m/s2=735N
    And I thought that I just needed to just divide by 2700N/m and that equals .272222

    3. The attempt at a solution

    So I put in .2722m and it said I was wrong?
    Is there something missing?
  2. jcsd
  3. Jan 29, 2009 #2


    User Avatar
    Homework Helper

    I hate elevator problems because you can't get away with using a = 9.81.
    If the thing is accelerating upward with a = 3, you must take that into account.
    You've got the mg and the ma combined. Would you expect your total "weight" to be larger or smaller when elevator is accelerating whichever way it is?
  4. Jan 30, 2009 #3
    Newton's third law states that for every action there is an equal and opposite reaction. The elevator is exerting a force on the guy to get him accelerating upwards and the guy must exert an equal and opposite force down onto the floor of the elevator (or, in this case, the spring). Think about it. This is why you feel heavier when you are in an elevator which is going up.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook