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Thread: Middle School Buoyancy Question

  1. #1 Middle School Buoyancy Question 
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    I am so confused right now about buoyancy. If an object displaces the same amount of water as itself does it float? Theoretically it should because it's buoyed up by its own weight, but then if we place a clay ball in water and it sinks, but it still displaces the Same amount of water as itself. How does that work??
    Also, if a boat displaces less weight of water as itself doesn't it sink? Though, my science teacher said it floats because she said if it displaces more or equal amount, it would sink??
    I really need a reply because my unit test is on Wednesday, thanks!


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  3. #2  
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    Archimedes principle states that the buoyant force on an object is equal to the weight of the displaced FLUID.

    Any object that is completely submerged will obviously displace a volume equal to it's own, that's just common sense. But the weight that the water pushes up with is equal to the weight of an amount of water with the same volume of the object.
    So to quickly examine a problem, for example your clay ball problem, just ask yourself what is heavier, this clay ball? Or a water ball that is shaped exactly like that clay ball to the exact dimensions?
    If the answer is that the clay ball weighs more than the water ball, then the clay ball would sink in water. If it's lighter then it will float.

    So, when doing calculations simply find the volume of the object, then find how much mass a similar volume of water has, then find the weight of that mass. That will give you your buoyant force.
    If anything is still unclear please let us know.


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    Hey, ok well you have to think about densities, its not just weight but density. An object that has more "stuff" then water will sink since gravity will effect it more. The boat example you have to think about water. The water molecules dont want to seperate ( belly flop=pain) and since the boat has mostly a flat hull the molecules attraction (surface tension) causes it too float. Also the volume is the amount of "stuff", all stuff has voume. So if we say A is water and B is your clay ball A+B=A+B, still same amount of stuff.

    Bouyancy is just the different densities and size of the object
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    For the simple question of flotation, the shape of the object isn't important. Boats usually have either V-shaped or flat hulls depending on what you want them to do in the water, but they float either way. Also, I'm not actually sure how much of an effect surface tension has on larger objects. I know that a small piece of aluminum foil will float due to surface tension, when it would normally sink, but I don't think that applies to something like a boat.
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    Surface tension has nothing to do with the problem at hand. Just reread what Street_Physicist wrote. That's really all there is to say about it.
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    So if the boat displaced less weight of water than itself, it would obviously sink right? This is the original question.
    Tamara made a toy boat that weighed 340N. The amount of water it displaced when it was put in water was 270N, did her toy boat float?
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    Quote Originally Posted by Jamesun95
    So if the boat displaced less weight of water than itself, it would obviously sink right? This is the original question.
    Tamara made a toy boat that weighed 340N. The amount of water it displaced when it was put in water was 270N, did her toy boat float?
    Correct, it will sink.

    An object that floats will displace its own weight. An object that sinks will displace its own volume, which will be less than its weight.
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  9. #8  
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    Thank you very much Harold! Now if this question reappears on my test, I know how to do it! I sincerely thank you from my heart!
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