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Homework Help: Force Applied to a Box on a Flat Surface, Find the Force

  1. Oct 13, 2014 #1
    1. The problem statement, all variables and given/known data
    A box of mass 72 kg is at rest on a horizontal frictionless surface. A constant horizontal force of magnitude F then acts on the box, accelerating it to the right. You observe that it takes the box 3.4 seconds to travel 13 meters. What is the magnitude of the force F?

    m = 72kg
    t = 3.4s
    s = 13m

    2. Relevant equations
    v = s/t
    a = v/t
    F = ma

    3. The attempt at a solution
    My first attempt:
    v = s/t = (13m) / (3.4s) = 3.8235m/s
    a = v/t = (3.8235m/s) / (3.4s) = 1.1245m/s^2
    F = ma = (72kg)(1.1245m/s^2) = 80.968N == 81N

    My second attempt:
    v_i = 0m/s
    a = F/m
    s = (v_i)t + 0.5 (F/m)t^2
    => F = 2sm / t^2 = (2(13m)(72kg)) / (3.4s)^2 = 161.93N == 162N

    Which answer is correct and why is one method correct and the other not?

    Thank-you
     
    Last edited: Oct 13, 2014
  2. jcsd
  3. Oct 13, 2014 #2
    Hi logan.
    Your second attempt looks good to me.I am not sure about calculations.
    We use ## v=s/t## when there is uniform motion.But in this case there is uniform accelerated motion.
     
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