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Spring and block problem

  1. Apr 19, 2005 #1
    Please help, I can not figure out if I would set up the equation as 3F=-kx

    The problem is A 0.70-kg block is hung from and stretches a spring that is attached from the ceiling. A 2nd block is attached to the first one and the amount that the srping stretches from its unstrained length triples. What is the mas of the second block?
     
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  3. Apr 19, 2005 #2

    OlderDan

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    If F1 is -kx, then F2 is -3kx. F1 is the weight of the first block. F2 is the weight of the two blocks together.
     
  4. Apr 20, 2005 #3
    ok am I using the right equation for this problem, I know I have to find the mass
     
  5. Apr 20, 2005 #4

    OlderDan

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    Yes, you have the right equation. You just need to relate the forces to one another, as outlined previously, and relate those forces (weights) to the masses of the objects involved.
     
  6. Apr 20, 2005 #5
    F1=F2 and use -kx=-3kx ??
     
  7. Apr 20, 2005 #6

    OlderDan

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    F1 does not equal F2. From F1 = -kx and F2 = -3kx we find that F2 = 3F1 by substitution
     
  8. Apr 20, 2005 #7
    Im confused sorry
     
  9. Apr 20, 2005 #8

    OlderDan

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    OK Let's step through it
    The first block causes the spring to stretch a distance x. The force the spring provides is proportional to the stretch. It is -kx. The minus sign indicates that the force is in the direction opposite the stretch. Adding the second block triples the distance of stretch, but k stays the same. k is a property of the spring (as long as you dont stretch it too far and ruin it).

    There are two cases

    F1 = -kx
    F2 = -3kx

    If you replace the -kx in the second equation with F1 (because they are equal) you get

    F2 = 3F1

    Now F1 is equal in magnitude to the weight of the first block

    F1 = M1g

    and F2 is equal to the weight of the two blocks combined

    F2 = M1g + M2g

    Replce F2 and M1g with their equals

    3F1 = F1 + M2g

    Subtract F1 from both sides

    2F1 = M2g

    Replace F1 with its equal M1g

    2M1g = M2g

    divide both sides by g

    2M1 = M2

    The mass of the second block is twice the mass of the first block. Use the known mass of the first block to compute the mass of the second block
     
  10. Apr 20, 2005 #9
    i will work through it tonight
     
  11. Apr 20, 2005 #10
    I still cant figure it out
     
  12. Apr 20, 2005 #11

    OlderDan

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    What do you not understand, the derivation I did to get the result 2M1 = M2, or where to go from there?
     
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