Spearguns and buoyancy

    I think that was the initial confusion neither one of us new what the other meant.


    Are you saying the volume is the same or different between pipes A and B?

    The volume of metal in the tube is the same in both cases. But the total volume of the system (and therefore the volume of water displaced) is different. When submerged, the tube without the hole will displace more water than the tube with a hole. In the case of the tube with the hole, some of the water that would have been displaced flows through the hole goes inside the tube.

    Quote

    But the total volume of the system (and therefore the volume of water displaced) is different


    No they are the same volumes displaced same shape same material. I will show you give me a second, it won't be a pipe but the fundamental concept is the same.

    Help me Seaweed! I don't know how else to say it.


    If I take a sealed barrel, and stick it in a full bucket of water, a certain amount of water will overflow the bucket. That is the displacement of the barrel, wich includes the volume of the metal in the pipe, plus the volume of the air contained therein.


    If I take a second barrel, with a hole in the side, and submerge it in a full bucket of water. I promise you that LESS water will overflow the bucket. Because some of the water which would have overflowed, has leaked through the hole and into the interior of the barrel, displacing the air (which we assume leaks out as the water leaks in). The volume of water that overflows the bucket is equal only to the volume of the metal used to construct the pipe.


    The displacement of the sealed barrel is greater. Consequently, its buoyancy is greate (ignoring the weight of the air and plugs).


    I'm going to bed.

    If you want me to measure it out I will but not now my girlfriend is already asking me what the hell is going on.


    But the volume displaced by each is the same so the Buoyant Force on each is the same.


    To determine if it floats or not is another story but Buoyant Force is not the MAIN determining factor at this point, that is what I was trying to say.

    Jeff's example he used a closed system (sealed pipe) and an open system (water coming into pipe). Kyle is talking about 2 closed systems.


    BF=V*rho(water)- m*g


    Buoyancy force= volume of body times density of water minus mass of body times accceleration due to gravity


    Bouyancy force is not the same for the bottles. Only the first term is the same in both cases, second term differs since the bottle with water has more mass.



    Things float due solely to buoyancy so it is the only determining factor.

    Davie Peguero

    Wait Davie if I am wrong I will admit it maybe there is a miscommunication here.


    Archimedes 1 st principle states " A body immersed in a fluid experiences a vertical buoyant force equal to the weight of the fluid it displaces."


    If that is wrong tell me, if not then, the weight of that fluid is equal to the Volume of the fluid * Density of the fluid


    So why is the Buoyancy in both cases not the same.


    When you minus the mass of the body times gravity you are calculating the immersed weight of the body.

    Quote from Kyle c

    Archimedes 1 st principle states " A body immersed in a fluid experiences a vertical buoyant force equal to the weight of the fluid it displaces.".


    You are absolutely right in this respect. I think that we are hung up on semantics. Maybe the definitions that I learned and use are slightly different. You are talking about a buoyant force. Just one force due to the volume of water displaced.


    But Buoyancy is the total sum of ALL vertical static forces acting on the object. A concrete block sitting on the seabed experiences a "buoyant force" due to the water that it displaces, but the "buoyancy" is still negative because of the weight.


    Quote from Kyle c

    So why is the Buoyancy in both cases not the same. .


    The buoyant force due to displaced water is the same in both bottles in your example. But he buoyancy is different because the weight of the bottle filled with water is greater.


    Quote from Kyle c

    When you minus the mass of the body times gravity you are calculating the immersed weight of the body.

    This IS the buoyancy of the object.

    I was done and I didn't want to offend anyone and I am still not trying to but you pop your head in and copy and paste something from Google that you obviously do not understand. Everyone is a genius because of Google these days. Along with quick quips and funny emot-icons internet forums have become a dangerous place for knowledge.





    And what does what you posted have to do with the discussion we were having?

    Edited once, last by Kyle c ().

    I really don't see why anyone can't see what I am saying is right?


    The Buoyant Force of the similar objects is the same. Buoyant Force is based solely on VOLUME DISPLACED.
    The ability of an object to sink or float is based on the objects weight and if it is greater than the Buoyant Force.


    So in one post seaweed says,


    Quote

    Bouyancy force is not the same for the bottles.


    Then Tin man says,

    Quote

    The buoyant force due to displaced water is the same in both bottles in your example


    And I am the one that is wrong.....WTF?




    I like you... you're "special" :)

    The knuckle dragger,short bus kinda special or the damn he is right kinda special.


    Why did you delete your post Fuzz?

    Quote from Kyle c

    The Buoyant Force of the similar objects is the same. Buoyant Force is based solely on VOLUME DISPLACED.
    The ability of an object to sink or float is based on the objects weight and if it is greater than the Buoyant Force.


    I agree 100% with this part, so let's see if we can figure out where we got off track. This is the point where you lost me a little:


    Quote from Kyle c

    Yes but the factor is more deciding on whether it sinks or floats is based on the density of the material used.


    My point was that the density has nothing to do with the buoyant force. Buoyant force is based only on displacement (as you said above), and not on the properties of the material. The net Buoyancy, however, is the sum of the upward acting buoyant force, and the downward acting force due to gravity (weight).


    I thought that you meant the density of the tube. Maybe you were refering to the "total density" of the entire barrel? This being a composite of the aluminum barrel and the air inside? If that is the case, then your definition of density does come into play because it (along with volume) determines the weight, which is one of the two forces which determine the net buoyancy.


    Help me understand which part we do not agree on.

    Edited 3 times, last by Guest ().

    I don't even know anymore I forgot why I posted initially, now I got people telling me I don't understand principle laws of physics then deleting there post I don't know what the hell is going on now. I was trying to help....really.


    I think I started out with the idea that people wanted to know how to get a neutral buoyant point on there guns, I think somewhere along the line someone suggested changing the buoyant force of the gun. I wanted to state you cannot do this without changing the actual dimensions of the gun.


    The thing you can change in the gun is it's weight you CANNOT change the buoyant force of the gun without changing the shape and size of the gun.



    That is why I was trying to say the Buoyant Force of the gun is not the determining factor if you want the gun to sink, float or stay neutral.

    Quote from Kyle c

    The knuckle dragger,short bus kinda special or the damn he is right kinda special.


    Why did you delete your post Fuzz?



    My mommy says I'm special... I'm not sure which type she's referring to either... :crazy:



    If you read my post, I was agreeing with Tin Man's assertion that much of the confusion is semantics-based. Net effects aside, I also agreed with you that the buoyancy FORCE for both bottles in your example is the same. Forgive my gratuitous use of googled physics images... I thought the images along with a description would help alleviate some of the misunderstandings. I deleted the post because you seem more inclined to continue bickering. This is a very basic physics principle, with only two relevant vertical forces. Why you choose to make it more difficult than it needs to be... is beyond my comprehension.








    Quote from Kyle c

    you pop your head in and copy and paste something from Google that you obviously do not understand. Everyone is a genius because of Google these days. Along with quick quips and funny emot-icons internet forums have become a dangerous place for knowledge.


    This quote is why I deleted my post. I do fully understand the material. I also fully understand what Tin Man has been saying. Just don't want to get caught up in silly bullshit... I can go to Spearboard for that. :thumbsup2:

    Quote from Kyle c


    The thing you can change in the gun is it's weight you CANNOT change the buoyant force of the gun without changing the shape and size of the gun.


    That is why I was trying to say the Buoyant Force of the gun is not the determining factor if you want the gun to sink, float or stay neutral.



    I understand what you're trying to say, but this thread has been a very convoluted way of getting your point across. Semantics.






    How's this for a wrap-up summary?
    Very simple - only 2 relevant vertical forces. If one's a constant, the other is the variable determining force.

    • If buoyancy force (displacement volume) is constant, then changing weight (and average density) of the object will affect its NET buoyancy. (Your point)
    • If weight is constant, then changing shape/size (volume) of gun will change the buoyancy force and therefore affect its NET buoyancy.

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