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Forces between two blocks on a frictionless surface

  1. Oct 31, 2012 #1
    1. The problem statement, all variables and given/known data

    A block of mass 7.18 kg lies on a frictionless
    horizontal surface. The block is connected
    by a cord passing over a pulley to another
    block of mass 5.6 kg which hangs in the air, as
    shown. Assume the cord to be light (massless
    and weightless) and unstretchable and the
    pulley to have no friction and no rotational
    inertia.

    Calculate the acceleration of the first block.
    The acceleration of gravity is 9.8 m/s^2
    Answer in units of m/s


    2. Relevant equations

    Fnet=ma

    3. The attempt at a solution
    I used the equation Fnet=ma to find my horizontal equation T1-T2+M2G=MA
    I then reduced that to M2G=MA since the two tensions are the same
    After that I plugged in the numbers to get an acceleration of 9.8m/s^2
    But i know this answer isn't right because when i checked it it was wrong
     
  2. jcsd
  3. Oct 31, 2012 #2

    ehild

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    Homework Helper
    Gold Member

    Hi Carmey, welcome to PF!

    You are right, the tension is the same along the string, so T1=T2.

    What did you denote by M2 and what is M? Show your work in detail, please.

    ehild
     
  4. Oct 31, 2012 #3
    Sorry M2 is the mass of the second block (5.6 kg block) and M is the mass. I just figured out my mistake though, when i had the equation M2G=MA i forgot to use both masses for M. So when i did that I got the right answer of 4.294209703
     
  5. Oct 31, 2012 #4

    ehild

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    Homework Helper
    Gold Member

    Well done !:smile: But do not forget to add rthe unit (m/s2 ), and round off the result to so many significant digits as in the data.


    ehild
     
  6. Oct 31, 2012 #5
    Ya thanks i got that. I do have another question though if you could help me with that. How would you find the tension in the same problem?
     
  7. Oct 31, 2012 #6
    Never mind i got it.
     
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