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Newton's Laws/ Tension Forces Problem

  1. Apr 14, 2012 #1
    1. The problem statement, all variables and given/known data

    Two Objects are hung from strings. The top object (m1) has a mass of 18 kg and the bottopm object (m2) has a mass of 12 kg.

    Looks like this:
    http://www.google.ca/imgres?q=physi...w=158&start=0&ndsp=15&ved=1t:429,r:6,s:0,i:77

    (a). Calculate tension in each string.

    (b) Calculate the tension in each string if you pull down on m1 with a force of 45 N [down]
    2. Relevant equations

    FNet = ma
    Fg = mg
    g = 9.8

    3. The attempt at a solution
    I knew how to do (a).
    I creted a free body diagram for each weight first.
    Then for m1:
    FNet = T1 - Fg
    0 = T1 - Fg ----> FNet = 0 because it is at rest
    T1 = Fg
    T1 = (18)(9.8)
    T1= 176.4 N
    And then i did the same for the second one, and came up with T2 = 117.6

    Then I got to (b). But im lost.
    I attemted to find acceleration by doing:
    FNet = Fa ----> Fa = the 45 N
    FNet = 45 N
    ma = 45 N
    (m1 + m2)a= 45 ---- a= 45/(18 + 12)
    a = 1.5
    So:
    FNet = T1 - Fg1
    (18)(1.5) + (9.8)(18) = T1
    T1 = 203.4 N
    According to my answer key, this is not right. Please help.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Apr 14, 2012 #2

    PeterO

    User Avatar
    Homework Helper

    Red line above.

    T1 is supporting BOTH masses so the mass used to calculate T1 should be 30kg.

    T2 is supporting only M2 so your answer is probably correct.

    With a free body diagram:-

    for M2:

    M2g down, T2 up so your answer should be correct

    for M1:

    M1g down + T2 down, T1 up

    For part b, there is no acceleration; merely an EXTRA 45N down in the free body diagram for M1
     
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