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Homework Help: Calculating force

  1. Oct 4, 2012 #1
    I'm having a little trouble conceptualizing calculations of force. The problem statement was:
    An object in space with a mass of 68 kg is propelled forward at a constant force ([itex]\vec{F}[/itex]) for 3.0 seconds. After 3.0 s, the object has moved 2.25 m. find [itex]\vec{F}[/itex].

    I can regurgitate the proper answer (34 N) by finding [itex]a_{x}=2 \Delta x/t^{2} = 0.50 m/s^{2}[/itex] and [itex]\vec{F}=ma_{x} = (68kg)(0.50 m/s^{2}) = 34 N[/itex].

    the problem is that my intuition tells me that it should be:
    t = 3.0s
    m = 68 kg
    Δx = 2.25 m
    So since [itex]N = kg \cdot m / s^{2}[/itex], [itex]N = 68 kg \cdot 2.25 m / 3.0 s^{2} = 17 N[/itex], which is obviously wrong. But, does that actually mean something else? Is that some sort of instantaneous value or something, or is it completely meaningless?
  2. jcsd
  3. Oct 4, 2012 #2
    What you have done is basically dimensional analysis. When you do this you need to keep in mind that the expression may contain a dimensionless constant, say 'k', which you need to include. Two quantities dimensionally equal aren't necessarily the same.

    So, in your case it would be,

    [tex]F \alpha [M][L][T]^{-2}[/tex]


    [tex]F = k * MLT^{-2}[/tex]

    Where the value of k is.....? :wink:

    ......And, I feel good to be able to post again. :biggrin:
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