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  • 310354

    I don't understand the explanation on this one:

    I dont' get why to use 24000 Kg instead of 27000 if the formula states NEW total MASS (that would be 24000+3000).


  • #2

    If you think about it, 24000 kg is the new mass if we know the original mass is 27000 kg. It's a little bit reverse psychology how it's worded but we know the mass is 27000 kg (so this is the old mass, even though you calculated it by adding the 3000kg!). So the new mass would be the old mass of 27000 kg - 3000 kg = 24000. If we use the new mass we then use the distance between the mass and old CG which we know is 20m. So this gives us a sum of -3000/24000 (new mass) = CG change/20m (mass to old CG). This results in a shift of 2.5m and because we are removing 3000kg the CG will reduce from 30m to 27.5m.

    Incidentally, it's also worth noting that if the CG arm is 30m after we add 3000kg to the rear hold at 50m, it must follow that if we remove mass the CG will will move in the opposite direction, so the 30m has to reduce. None of the other answers reflect this, they all increase, inferring that as you remove the mass the CG moves towards the cargo hold, clearly impossible. The CG will always move towards a mass that is added and away from a mass that is removed. So there is only one answer it could possibly be.

    Hope this helps



    • #3
      FYI I have reworded the explanation and once approved will appear in the QB. Hopefully it will make a little more sense then as I can see how it was confusing before. :-)