Horizontal force due to tension from vertical wieght

  1. 1. The problem statement, all variables and given/known data

    Block C (m= 4 kg) sits on a frictionless horizontal surface. Block B of (m
    = 2kg) sits on top of block C, and is attached to a rope that runs over the massless
    pulley as shown in the figure. Block A (m=1 kg) hangs vertically from the rope. With
    what horizontal force F must you push Block C so that block A rises with an upward
    acceleration of a = 3m/s2? All surfaces are frictionless.

    Diagram:
    question7pic.png


    2. Relevant equations

    F=ma


    3. The attempt at a solution

    Using free body diagrams came up with:

    F=mac
    T=2ac
    T/2=ac

    F=maa
    T-mg=(3)(1)
    T=3+(9.8)(1)
    T=12.8 N

    (12.8)/2=ac
    ac=6.4 m/s2

    F=(mc+ma)ac
    F=(6)(6.4)
    F=38.4 N

    Actual answer = 59.8N
     
    Last edited: Oct 18, 2011
  2. jcsd
  3. PeterO

    PeterO 2,319
    Homework Helper

    It looks like you are mixing up the mass of A and B. A is 1 ; B is 2 not the other way.
     
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