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Buoyancy dependent variable?

  1. Nov 24, 2008 #1
    1. The problem statement, all variables and given/known data
    well, this isnt really a homework question, but its a lab we're doing. the thing is that we were trying to "verify" archimede's principle but what we ended up doing is just using it. Let me explain. We took a ball and measured its mass and volume.. and found its specific gravity. Okay.. then we took the formula Density of Object/Density of Water and extended it to get that Weight of Substance/Weight of Displaced Water, which the denominator should equal to the buoyant force. We also measured this a different way using the elongation of a spring on a jolly balance with the formula

    Elongation of Spring in air/(Elongation of Spring in air - Elongation of Spring when object is submerged)

    So, this is suppose to be a lab with an independent variable and a dependent variable.. however, really what was did was just use the same formula 3 times, right? because all three ways of measuring the specific gravity were derived from the same formula.. and i think the third one even applies archimedes's principle..

    you really cant VERIFY something by using it to prove itself.. can you?


    2. Relevant equations
    Anyways, all that leads up to my question: how can we salvage this lab?

    more specifically, what would be a good way to make an independent variable and a dependent variable without all of those calculations going to waste..?



    3. The attempt at a solution
    Well, what i was thinking was that we could just measure the buoyant force with something that measures force.. like whats that thing called? the one that hooks up to the computer or TI calculator.. or whatever, i dont remember, BUT we dont have access to one anyways.. however, i was thinking that maybe we could use the jolly balance to measure the force.. i mean.. doesnt it measure force?

    so the independent variable would be the specific gravity using weight of the displaced water.. and the dependent variable would be the specific gravity using the force measured by using the jolly balance.. (the 2 second ways we measured specific gravity).. and our reasoning for using specific gravity is because the with hooke's law.. if we divide those the spring constant cancels out and we just have to worry about the elongation and stuff..

    however, im still not sure if that would be okay.. since the 2nd calculated value is kinda derived from the first.. unless it would be okay to say.. BECAUSE they were derived from eachother we found them to be equal.. which supports archimedes's principle.. but i dont know if thats good enough?

    This post was probably kinda long.. but thanks a lot to those who try to help! lol. :biggrin:


    ** if you want me to show you why i keep saying theyre derived from eachother, i can show you the equations i used..
     
  2. jcsd
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