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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???

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

    HallsofIvy

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    Staff Emeritus
    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.
     
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