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Constant Acceleration and Resolution of Forces

  1. Feb 20, 2013 #1
    1. The problem statement, all variables and given/known data
    A block of concrete 5kg is initially at rest on a smooth (frictionless) horizontal surface. It is then pulled along by a constant force A, inclined at 45° from the horizontal. In the first 5 seconds of motion, the block moves a distance of 10m.

    A: Find the acceleration of the body
    B: Find the magnitude of the force A
    C: Find the normal reaction force


    2. Relevant equations
    [itex]
    v=u+at\\
    s=\frac{1}{2}(u+v)t\\
    s=ut+\frac{1}{2}at^2\\
    v^2=u^2+2as\\
    F=ma\\
    R=mg\\
    [/itex]

    3. The attempt at a solution
    I wasn't quite sure how to start but with the known variables I started with equation 2 from above and then the rest is dependent of whether I started it correctly.
    [itex]
    Part A:\\
    s=\frac{1}{2}(u+v)t\\
    10=0.5(0+v)5\\
    10=2.5v\\
    v=4\\
    .\\
    v=u+at\\
    4=0+a5\\
    a=\frac{4}{5}=0.8\\
    .\\
    Part B:\\
    Acos45=ma\\
    Acos45=5 \times 0.8\\
    0.71A= 4\\
    A=\frac{4}{0.71}=5.63N\\
    .\\
    Part C:\\
    R+Asin45=mg\\
    R+5.63sin45=5 \times 9.8\\
    R+3.98=49\\
    R=49-3.98=45.02N
    [/itex]

    The figures I got just seem low to me, any help or advice/pointers is greatly appreciated.
     
  2. jcsd
  3. Feb 20, 2013 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Looks good to me.
     
  4. Feb 20, 2013 #3
    OK, thank you :) .
     
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