1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I Reduction of the Centre of Gravity

  1. Jun 5, 2017 #1
    Hi, I'm trying to figure out the equivalent change of the Centre of Gravity when an amount of liquid is reduced. This could be a fuel oil tank on a ship.

    In other words, if I e.g. reduce the content what would be the equivalent vertical change of Centre of Gravity on a ship with a total displacement (weight) of x tonne?

    Example: On a ship with a deplacement of 6000 tonne the fuel oil (947 kg/m3) tank is reduced 50%. Tank measurements: 18 m long, 8 m wide and 2.2 m deep (see attached illustration). Screen Shot 2017-06-05 at 22.50.58.png

    Hope I've made myself understandable and that some of you can help me

    Thanks in advance!
     
  2. jcsd
  3. Jun 5, 2017 #2
    I've thought of using the formula:

    GG2 = q*d/disp

    GG2 = change of centre of gravity
    q = mass moved
    d = distance moved
    disp = displacement of the ship

    But cannot figure out what would be the right q and d since it is not quit a relocation of mass?
     
    Last edited: Jun 5, 2017
  4. Jun 5, 2017 #3
    Isn't the center of gravity always half of the height of the filling level in this setup? Doesn't seem you would need a formula here.
     
  5. Jun 5, 2017 #4
    Yes, if it was of the tank. But I want the new centre of gravity for the ship...
     
  6. Jun 5, 2017 #5
    You are lacking sufficient information for calculating that. Without the knowledge of the weight of the ship itself you can't know what percentage of the total mass is the fuel.
     
  7. Jun 5, 2017 #6

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    It's in three stages. You just find the CM and mass of the ship plus empty tank, and the CM and mass of the remaining fuel and do a third calculation to find the overall CM position by combining the two.
     
  8. Jun 5, 2017 #7
    The weight of the ship is 6000 tonne as mentioned in the text.
     
  9. Jun 5, 2017 #8

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    You still need to know where the tank is, relative to the CM of the ship.
    Ahh- from the diagram are you assuming that the oil is filling the bilge of the ship?
     
  10. Jun 5, 2017 #9
    Yes, the tank is placed at the very bottom of the hull
     
  11. Jun 5, 2017 #10
    I'm not sure what CM stands for... I'm used to danish notaions :) Can you maybe exlpain, please?
     
  12. Jun 5, 2017 #11
    In the attached pdf it is assignment 4.4 where I have some trouble finding the reduction of the GMt (centre of gravity to meta center hight)
     

    Attached Files:

  13. Jun 5, 2017 #12

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Sorry. Centre of Mass. CM is a Physicists pedantic name for Centre of Gravity.
    You would still need to know the depth of the hull and the shape so you can work out where the CM of the empty ship is. (Open top? and very thin roof to tank?)
     
  14. Jun 5, 2017 #13
    Thanks. I have just uploaded the assignment as an pdf. The only information I have is the hydrostatic data... Don't know if you can see it?
     
  15. Jun 5, 2017 #14

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    OMG
    Not trivial then. But the part you are asking about is straightforward, I think. Point is that you can work out the various masses and CM positions and then work out overall CM position.

    Relative to any origin
    Total mass times distance of overall CM = mass of one times CM distance + mass of the other times CM distance + + +
    It's basically the principle of moments.
     
  16. Jun 5, 2017 #15
    Yeah, that was what I was afraid of. I have uploaded my solution here if it could have anyones interest. Thank you very much sophiecentaur!
     

    Attached Files:

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Reduction of the Centre of Gravity
  1. Reduction of gravity (Replies: 4)

  2. Centre Of Gravity. (Replies: 23)

  3. Centre of Gravity/Mass (Replies: 8)

  4. Centre of gravity (Replies: 9)

Loading...