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Two blocks with different mass are attached to either end of a light rope that passes

  1. Feb 18, 2008 #1
    1. The problem statement, all variables and given/known data
    Two blocks with different mass are attached to either end of a light rope that passes over a light, frictionless pulley that is suspended from the ceiling. The masses are released from rest, and the more massive one starts to descend. After this block has descended a distance 1.30 m, its speed is 3.50m/s .

    If the total mass of the two blocks is 14.0 kg, what is the mass of the more massive block?



    2. Relevant equations

    (1/2)mv1^2+mgy1=(1/2)mv2^2+mgy2

    3. The attempt at a solution
    Can someone help me get started with this problem? I'm not sure I am using the right equation or approaching it correctly
     
  2. jcsd
  3. Feb 18, 2008 #2
    Since you know that a 14 Kg mass has been accelerated from rest to 3.50 m/s over a distance of 1.30 m, you can detemine the acceleration from the equation:
    [tex]v^{2} = u^{2} + 2as[/tex]

    From there you can calculate the force required to accelerate the mass at that rate with:
    [tex]F = ma[/tex]

    That will give you the difference in the weights of the two masses.
     
  4. Feb 18, 2008 #3
    What does u^2 stand for?
     
  5. Feb 18, 2008 #4
    v is the final velocity, u is the initial velocity. Perhaps you use a different type of notation. The same formula is listed last here in a different notation.
     
  6. Feb 18, 2008 #5
    I found that the F=ma is 65.94N, but where do I go from there?
     
  7. Feb 18, 2008 #6
    Like I said, that's the difference in their weights (though I actually get 65.96 N). You can divide that by g to find the difference in their mass since weight = mass * gravitational field strength.

    If you call the two weights a and b, that will give you:
    a - b = 65.96/g

    You also have
    a + b = 14

    Now you have to solve the simultaneous equations.
     
  8. Feb 18, 2008 #7
    Thanks, I got it. You guys are amazing
     
  9. Feb 18, 2008 #8
    Nah, not amazing just ... yeah OK, amazing. :wink:

    Happy to help. :smile:
     
  10. Apr 2, 2010 #9
    Re: Two blocks with different mass are attached to either end of a light rope that pa

    This appears to be a common question. I got the same thing with different values, but the formulae here worked!
     
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