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Homework Help: Three blocks connected by two strings: tensions?

  1. Sep 12, 2011 #1
    1. The problem statement, all variables and given/known data

    Three blocks on a frictionless surface are connected by strings like so (from left to right):
    Block 1 (m=10kg) to string with Tension 1 to Block 2 (m=20kg) to string with Tension 2 to Block 3 (m=30kg) to string with Tension 3.
    The blocks are pulled to the right with force T3=60N.
    Find T1 and T2.

    2. Relevant equations

    I'm not even sure... maybe

    3. The attempt at a solution

    I would guess T1 is 20N and T2 is 40N, because it's proportional to T3. But this doesn't seem correct. Could I please get some feedback?

    Thank you!
  2. jcsd
  3. Sep 13, 2011 #2


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    Staff: Mentor

    T3 accelerates 3 blocks, T2 accelerates 2 blocks, etc.
  4. Sep 13, 2011 #3
    if i am not wrong,

    consider the entire system of 3 blocks

    force to the right 60N = mass of 3 blocks (60kg) x acceleration

    'a' is the same for the entire system. so , 'a' is the same for each block

    so your acceleration is = 60/60 = 1

    so for tension 1, which pulls only block 1, to have a = 1, it must have a force acting on it by f=ma = (10kg)(1) = 10N, so tension is = 10N

    for tension 2, which pulls block 1 and block 2, a = 1, f=ma = (10+20)(1) = 30N

    for tension 3, which pulls block 1,2,3, a = 1, f= ma = (10+20+30)(1) = 60N
  5. Sep 13, 2011 #4
    Thanks a ton! That makes much more sense.
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