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Homework Help: 2 blocks sliding down a incline connected by string (check please)

  1. Dec 20, 2005 #1
    Two blocks with masses 4.0kg and 8.0 kg are connected by a string and slide down a 30 degree inclined plane. The coefficient of kinetic friction between the 4.0 kg block and the plane is 0.25; that between the 8.0 kg block and the plane is 0.35
    a) calculate the acceleration of each block

    This part is easy i got it right, don't even need anyone to check it... 4 kg = 2.775 , 8 kg =1.4750

    used for both blocks : mgx-ff=ma

    b) calculate the tension in the string

    I said since the bottom block accelerates faster it will create a tension which means both blocks will be connected by a rigid string with a tension in it... so this means i could solve them as a system ... this is where i need someone to verify i did it right or if not correct me....

    i said that

    (mgx1+mgx2)-(ff1+ff2) = (m1+m2)a

    They are connected so they will have a new a in common for both which comes out to 1.9 .

    So now i can find the tension by using either box and the first equations trying to find their acceleration alone... instead now i have

    (for the bottom box)


    So i get T = 3.5N

    if i use this t in a newtons second law equation for the top one ill get 1.9 acceleration for it as well...

    Can anyone let me know if i did any mistakes please? Thank you :) I really appreciate it
  2. jcsd
  3. Dec 21, 2005 #2


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    Homework Helper

    the equations are
    m1 g sin(theeta) - mu1 m1 g cos(theeta) - T = m1 a
    m2 g sin(theeta) - mu2 m2 g cos(theeta) + T = m2 a
    check your calculations
    first for a2 and then for common a.
  4. Dec 21, 2005 #3
    i used this for the second part... if you look at my equation but i combined them together.... am i wrong?

    (mgx1+mgx2)-(ff1+ff2) = (m1+m2)a

    if you combine them the T's cancel but i can find the T by setting it in a separate equation? whats wrong with my method?

  5. Dec 21, 2005 #4


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    Homework Helper

    No the method is not wrong, I told to check the calculations because I am gatting a different answer for acceleration for the second block alone and the commom acceleration.
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