Buoyancy dependent variable?

[HelgaaMan]

Homework Statement

well, this isn't 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 can't VERIFY something by using it to prove itself.. can you?

Homework 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..?

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 what's that thing called? the one that hooks up to the computer or TI calculator.. or whatever, i don't remember, BUT we don't have access to one anyways.. however, i was thinking that maybe we could use the jolly balance to measure the force.. i mean.. doesn't 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, I am 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 each other we found them to be equal.. which supports archimedes's principle.. but i don't know if that's good enough?

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


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

 

[lippyka]

but it may be easier to just look up archimede's principle and buoyant force equations **So, my suggestion is that you use the jolly balance to actually measure the buoyant force of the object. You can then use this as your dependent variable, and the weight of the object in air as your independent variable. You can then plot a graph to show how the buoyant force increases with an increase in the weight of the object. This will still allow you to verify Archimedes' Principle, as it will show that the buoyant force is directly proportional to the weight of the object.

 

[thomate1]

I can understand your concern about the validity of your lab results and the need for an independent and dependent variable. It is important to have these variables in an experiment to establish a cause-and-effect relationship.

In this case, the independent variable could be the mass of the object, as this can be varied and controlled by you. The dependent variable could be the buoyant force, which is dependent on the mass of the object and the density of the fluid it is submerged in.

To measure the buoyant force, you can use a force sensor or a spring scale as you suggested. This will provide a direct measurement of the force acting on the object due to its displacement of the fluid. You can then compare this value to the calculated value of the buoyant force using the formula you mentioned in your post.

It is true that all three ways of measuring specific gravity are derived from the same formula. However, by using the force measurement as your dependent variable, you are introducing a new, direct way of measuring the buoyant force and verifying Archimedes' principle. This will provide more evidence to support your results and conclusions.

In summary, to salvage your lab, you can use the mass of the object as the independent variable and the buoyant force measured by a force sensor or spring scale as the dependent variable. This will provide a more direct way of measuring the buoyant force and verifying Archimedes' principle.