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Pullign Blocks Question

  1. Mar 24, 2010 #1
    1. The problem statement, all variables and given/known data
    Three identical blocks connected by ideal strings are being pulled along a horizontal frictionless surface by a horizontal force F_vec. The magnitude of the tension in the string between blocks B and C is [tex]T = 3.00 {\rm N}[/tex]. Assume that each block has mass [tex]m = 0.400 {\rm kg}[/tex].

    a)What is the magnitude F of the force?

    b) What is the tension T_AB in the string between block A and block B?

    http://img340.imageshack.us/img340/3789/9374t.jpg [Broken]


    2. Relevant equations
    W=mg
    F=ma


    3. The attempt at a solution
    a) i made A and B as one object and calculated the acceleration of the object

    two forces are acting on this object.
    1st force
    w=mg
    w=0.4*9.8
    W=3.92N

    2nd force is tension = 3N

    Found Fnet to be 3+3.92+4.94
    Fnet = 11.86

    used F=ma formula

    a=11.86/0.4
    a=29.65m/s^2

    then to find the force i went: a=Fnet/3m

    so a=11.86/3*0.4

    a=9.88m/s

    To find force i use 9.88 * 0.4 = 4N ------> answer's says it 4.50N (tell me where i have gone wrong)

    b) Used f=ma

    f= 29.65 * 0.4
    =11.86 i know this is incorrect, but someone tell me the correct way of doing this.

    P.S
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Mar 24, 2010 #2

    Doc Al

    User Avatar

    Staff: Mentor

    There's also the upward normal force acting on the blocks, which exactly cancels the weight. (Since there's no acceleration in the vertical direction.)

    I thought you said there were two forces acting on the blocks? And since these forces act in different directions, they can't just be added like numbers.

    Hint: The vertical forces add to zero, so they can be ignored.
     
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