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: Concept of Force Confusion

  1. May 20, 2009 #1
    1. The problem statement, all variables and given/known data

    Object A and B are placed on a spring. Object A has twice as much mass as object B. If the spring is depressed and released, propelling the objects into air, object A will:

    answer: rise to the same height as object B

    2. Relevant equations

    3. The attempt at a solution

    Based on my reasoning and math, I am confused why both objects would rise to the same height:

    The force on both objects is the same, F. And 2*mass of B = mass of A

    F = (mass of A)*(accl. A) = (mass of B)*(accl. B)
    F = 2*(mass of B)*(accl. A) = (mass of B)*(accl. B)

    cancel out (mass of B):
    F = 2*(accl. A) = (accl. B)

    Since the upward acceleration of B is twice the that of A, why would the height be the same?
  2. jcsd
  3. May 20, 2009 #2
    I've only briefly thought about your attempt:

    You are only considering the upward forces due to the spring.

    What about the force on each object due to gravity..... will it be the same???

    Last edited: May 20, 2009
  4. May 20, 2009 #3


    User Avatar
    Science Advisor

    I think the simplest way to do this problem is to use "conservation of energy". The spring has a specific amount of energy stored in it when compressed and, on release gives the same energy to A and B. They will rise to heights that give the same potential energy to both.
  5. Jun 10, 2011 #4
    any one know how to approach this problem?
  6. Jun 10, 2011 #5
    As the spring moves up A and B are pushed up with at all times equal speeds and their speeds are also equal when they lose contact with the spring.Take it from there.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook