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Center of Mass!

  1. Oct 31, 2006 #1
    Question: Three cubes of sides l, 2l, and 3l are placed next to one another (in contact) with their centers along a straight line and the l=2l cube in the center. What is the position, along the line, of the CM of this system? Assume the cubes are made of the same uniform material.

    My reasoning: So the equation for CM= M1XI +M2X2 + M3X3/ TM ... where M= mass, and X= distance ....So M(l +2l +3l)/3m = CM
    M(6L)/3m = 2L
    So, I got the center of Mass is 2L

    I dont think my answer is correct, and even if my approach is correct. It is labeled a pretty easy problem, but I get really confused with problems without numbers. Also, I am not sure if you can conclude the total mass is 3M considering that means all the cubes would have to be the same mass .... and I also do not know what the question maker meant when he wrote the l=2l cube in the center .....

    Please help!
     
  2. jcsd
  3. Oct 31, 2006 #2

    radou

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    The coordinates X1, X2, and X3 are the coordinates of the center of mass of each cube. So, all you have to do is place the origin wherever you want and start to calculate. If you place the origin at the beginning of the first cube, then X1 = 0.5 L, and so on..
     
  4. Oct 31, 2006 #3
    radou ... thanks for the help but i am still confused.... how would you find the total mass of the cubes??
     
  5. Oct 31, 2006 #4

    radou

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    Well, since the cubes are made of the same uniform material, you may assume the density of the cubes is equal. You know the volume, so, you can calculate the mass.
     
  6. Oct 31, 2006 #5
    finding the volume is easy , but how would you go about finding the density? is it simply a known value?
     
  7. Oct 31, 2006 #6

    radou

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    Yes, call it [tex]\rho[/tex] or something. It will cancel out in the further calculation.
     
  8. Oct 31, 2006 #7
    A diagram might help.

    center-of-mass.png
     
  9. Oct 31, 2006 #8
    is the answer 3.08Lo (thats what i got) ???
     
    Last edited: Oct 31, 2006
  10. Oct 31, 2006 #9

    radou

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    According to my calculation, it's 3.83L. But I may be wrong. Nevertheless, it's important you understand the principle. :smile:
     
  11. Oct 31, 2006 #10
    yeaaa... i just did it again and got 3.83L ..... but radou, how does that make sense??
     
  12. Oct 31, 2006 #11

    radou

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    What exactly do you mean?
     
  13. Oct 31, 2006 #12
    well........if the center of mass is 3.83L .... then wouldnt the center of mass be out of the object given (which is impossible)?
     
  14. Oct 31, 2006 #13

    radou

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    No it wouldn't, because the total length of the object is L + 2L + 3L = 6L.
     
  15. Oct 31, 2006 #14
    thanks radou!!!!!!!!!!!!
     
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