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2 mass spring system

  1. Nov 23, 2007 #1
    Two blocks of masses m1 and m2 are connected by a spring of spring constant k.now forces F1 and F2 start acting on the two blocks in a manner to stretch the spring.Find the max.elongation of the spring.



    What I could figure out was to conserve the energy. Energy stored in spring when elongation is x would be 1/2kx^2. Work done by F1 would be F1x and by F2 would be F2x. So,equating them, we would get x=2(F1 + F2)/k.

    But I think this is not right. Can you please help ?

    Thanks
     
  2. jcsd
  3. Nov 24, 2007 #2
    Anyone ?
    The system will have some acceleration too. So, there must be some other catch in the problem.
     
  4. Nov 24, 2007 #3
    Hmmmm....

    Because the forces are not equal,the whole system starts to move.

    [tex]F_1-kx=m_1a[/tex]

    [tex]kx-F_2=m_2a[/tex]
     
  5. Nov 24, 2007 #4
    Yes..but that isnt really helping in find the max elongation. The elongation will increase and after reaching the max value, will start to decrease again.
     
  6. Nov 24, 2007 #5
    [tex]F_1-F_2=(m_1+m_2)a[/tex]

    [tex]F_1-T_1=m_1a[/tex]

    [tex]T_2-F_2=m_2a[/tex]

    energy stored in the spring equals to

    [tex]E=T_1x_1+T_2x_2[/tex]
     
  7. Nov 24, 2007 #6
    Isnt T1=T2=kx ?
     
  8. Nov 24, 2007 #7
    No,I just confused they are not egual.Spring is oscillating,this makes them to be inequal.
     
  9. Nov 24, 2007 #8
    Hey thanks

    Got the answer as 2(f1M1 +f2M2)/k(M1+M2)

    Thanks again
     
  10. Nov 25, 2007 #9
    Must be

    [tex]\frac{1}{2}k(x_1+x_2)^2=\frac{(F_1m_2+F_2m_1)}{m_1+m_2}(x_1+x_2)[/tex]

    so [tex](x_1+x_2)=0[/tex] minimum elongation and

    [tex](x_1+x_2)=2\frac{F_1m_2+F_2m_2}{k(m_1+m_2)}[/tex] maximum.
     
  11. Nov 25, 2007 #10
    Hey wait....but wouldnt energy stored in spring be [tex]F_1x_1 + F_2x_2[/tex] since [tex]F_1 , F_2[/tex] are the external forces on the spring system ?
     
  12. Nov 25, 2007 #11
    [tex]T_1x_1+T_2x_2=(F_1-m_1a)x_1+(F_2+m_2a)x_2[/tex]

    [tex](F_1-F_2)d=\frac{1}{2}kx^2+\frac{1}{2}(m_1+m_2)v^2[/tex]

    if energy stored in the spring were [tex]F_1x_1 + F_2x_2[/tex],it means that spring is not moving.
     
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