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A 9.0 killogram mass and a 3.0 kg mass are at rest on a frictionless

  1. Oct 21, 2011 #1
    1. a 9.0 killogram mass and a 3.0 kg mass are at rest on a frictionless table. A massless spring is compressed between the masses, which are held together by a thread. The thread breaks and the 9.0 kg mass moves to the left with a velocity of 2.0 m/s. Find the velocity of the 3.0 kg mass.



    2, m1v1+m2v2=(m1+m2)v3



    3. 9(2)+3
    18+0=12v3
    v3=1.5m/s
    this was my original attempt, but i realize it is not solving for the velocity of the 3.0kg mass but i don't know what else to do
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Oct 22, 2011 #2

    ehild

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    Re: velocity

    What does v3 mean?


    ehild
     
  4. Oct 24, 2011 #3
    Re: velocity

    It stands for the final combined velocity but i am not sure if this equation is right for this problem.
     
  5. Oct 24, 2011 #4

    ehild

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    Re: velocity

    No, it is not the final combined velocity. Read the problem: the final velocity of one mass is 2 m/s and you need to find the velocity of the other mass. So there is no combined final velocity. But what do you know about of the initial velocities? Do the masses move?

    ehild
     
  6. Oct 25, 2011 #5
    Re: velocity

    The masses are initially at rest and then they move in opposite directions
     
  7. Oct 26, 2011 #6

    ehild

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    Re: velocity

    What is the momentum of the system then?

    ehild
     
  8. Oct 26, 2011 #7
    Re: velocity

    The force that moves the blocks after the thread breaks?
     
  9. Oct 26, 2011 #8

    PeterO

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    Re: velocity



    The spring will apply the same sized force on each mass - but opposite in direction.

    because the masses are 9kg and 3 kg, the effect of the force on the 3 kg mass will give 3 times the acceleration it causes to the 9kg mass.

    That means the 3kg mass will be travelling at 3 times the speed of the 9kg mass at all times.

    It sounds like after a while the masses have moved away from the spring. By then the 9 kg mass has accelerated to 2.0 m/s

    What do you think that means for the 3 kg mass?
     
  10. Oct 26, 2011 #9
    Re: velocity

    That its speed is 6m/s?
     
  11. Oct 26, 2011 #10

    PeterO

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    Re: velocity

    That sounds entirely reasonable!!
     
  12. Oct 26, 2011 #11
    Re: velocity

    Thank you so much!!:)
     
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