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Think the Mass is correct, unsure on Reaction Force

  1. Apr 28, 2015 #1
    1. The problem statement, all variables and given/known data
    physics 1.PNG



    2. Relevant equations

    kg/m3 = mass / volume

    Downward forces on the column = Weight + Additional force
    (F=ma)


    3. The attempt at a solution

    kg/m3 = mass / volume
    Therefore, 2400 = mass / (0.3 x 0.4)
    Mass = 576 kg
    Downward forces on the column = Weight + Additional force
    (F=ma) = (576 x 9.81) + 10 kN
    = 15.7 kN
    Every force has an equal and opposite reaction (Newtons 3rd Law). Hence the reaction force at the base of the column = 15.7 kN

     
  2. jcsd
  3. Apr 28, 2015 #2

    SteamKing

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    The volume of the concrete is 2 m * 0.3 m * 0.4 m = 0.24 m3.
     
  4. Apr 28, 2015 #3
    oh ok so does that make the volume 1000?
     
  5. Apr 28, 2015 #4
    I meant 0.24 sorry
     
  6. Apr 28, 2015 #5

    SteamKing

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    1000 what? Always use units.

    What I wrote was very plain: the volume of the concrete ... = 0.24 m3. No more. No less.
     
  7. Apr 28, 2015 #6
    sorry yes i did not realise you had worked it out . Thanks.

    So would this now make the total mass 1000kg?

    Therefore (1000 x 9.81) + 10 KN = overall 19.8 KN ???
     
  8. Apr 28, 2015 #7
    Where did the 1000 kg come from?

    Chet
     
  9. Apr 28, 2015 #8

    PhanthomJay

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    Math aside for the moment, the so called reaction force at the base has nothing to do with Newtons 3rd law. Rather, it is a consequence of his first law, from equilibrium considerations. This is a very important distinction.
     
  10. Apr 28, 2015 #9

    SteamKing

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    You seem to be confused by the meaning of density.

    Since concrete has a density of 2400 kg/m3, this means that a 1 m3 block of concrete has a mass of 2400 kg.

    If you want to calculate the mass of concrete contained in a solid rectangular block, you must calculate the volume of the block, which is what I showed in Post #2.

    The formula for density ρ is ρ = M / V, where M is the mass and V is the volume.

    For concrete, ρ = 2400 kg/m3 and I showed you how to calculate V. Solve for M. :wink:
     
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