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Two blocks connected by string with an angled force applied

  1. Oct 29, 2008 #1
    A 3.5 kg block (m1) and a 1 kg block (m2) are connected by a string and are pushed across a horizontal surface by a force applied to the 1 kg block as shown below. The coefficient of kinetic friction between the blocks and the horizontal surface is 0.3. If the magnitude of vector F is 27 N, what is the tension in the string that connects the blocks? (θ = 20)

    http://www.webassign.net/hrw8/6-p-096.gif

    ----
    Ok so my FBD say for block m1:
    x: t-fk1= m1a
    y: N1 - m1g = m1a(0)

    which gives me N1 = 34.3

    FBD for block m2:
    x:Fcos[the]-t-fk2=m2a
    y:n2-m2g-fsin[the]=m2a(0)

    which gives me N2 = 13.904 ( = m2g+fsin20)

    and fk1=10.29 and fk2 = 4.171 i know because I have the N's and the kinetic friction coefficients.

    so with all that!! i did

    T= fk1+m1a (from first eq for x of m1)

    so i sub that into the m2's x equation

    Fcos20-fk1-m1a-fk2=m2a
    12cos20-10.29-3.5a-4.171 = 1a
    a = -.7077

    which is weird cause i didnt think there would be a negative acceleration.

    Then i plugged the acceleration into the first eq for x on block 1
    T-fk1=m1a which gave me 7.81305 for T but thats apparently the wrong answer, so if anyone can help find what i did wrong it would be greatly appreciated.

    I have another problem like this i'm stuck on and i think if i figure out this one i should be able to get the next one too. Thanks for lookin
     
  2. jcsd
  3. Oct 29, 2008 #2

    PhanthomJay

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    Gold Member

    Your method and equations looks OK after a cursory look, but I note that the value of F is given as 27 and you used 12. 12 isn't enough to make the system move! Correct the typo and you should be good.
     
  4. Oct 29, 2008 #3
    oh man haha, why would i do that! thanks for looking it over i got it right now!!!
     
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