Help me please. (finding density)

[SoySauce]

I'm new here but i was searching the web for help forums and found this site where everyone seems nice and friendly. After 3 weeks of doing nothing, I finally thought up of an experiment that relates to buoyancy and today my teacher tells me that my write-up is extremely shoddy. My project experiment is really simple (elementary level probably). I have to mold clay into different 4 shapes (each weighs 1.13kg) then put it in water and time how long it takes to sink. The time will be my quantative data although I'm not even sure if the clay will sink. Blah, I sure beated around the bush. Today, while criticizing my experiment (didn't really understand her b/c of her strong Chinise accent) she told me that i needed to find the density of my shapes. The problem is I don't know how and I dont think I have the proper materials to do so. Yeah, so what do I need to find the density of a cube/or a pyramid, rectangular prism, sphere, and cylinder and what materials do I need? For the density I think I got the formulas covered but I need someone to verify just to be sure (teacher won't tell me). ...Beated around the bush again. I really hate myself right now .

[deleted-71588]

Don't berate yourself for anything but starting a late! What grade are you in? Knowing your grade level will help the volunteers here to tune our answers and provide you more grade appropriately worded answers and help.

Density of an object is just the ratio of mass (or weight) to the volume of the object.

For the moment, if we ignore any ability for your particular "clay" to absorb water or dissolve in water and all of your clay is identical (has the same density to start with), then...

If you start with the same weight (or mass) of identical clay, then the density of the clay portion of each of your objects should be identical no matter how you form it. If it were just about the density of the material you use to build a boat, then we wouldn't use steel in building boats because the density of steel is greater than the density of water, so they would always sink.

Your experiment as I understand it deals more with buoyancy of objects which is NOT the same as density. If you teach doesn’t understand that, then you and I aren’t communicating effectively, or you and your teacher are not communicating effectively, or your teacher hasn’t thought about what buoyancy really is scientifically in a while.

Buoyancy of an object (formed clay in your case) in a liquid (water in your case) is simply a question of whether the liquid the object displaces weighs more or less than the object.

Unless your clay absorbs water or dissolves in water, the time to sink might not be all that interesting of a dependent variable. Note: These are assumptions that you need to validate (experimentally or reading up on the properties of the material provided by others’ experiments).

Apologies if you are in a high school physics class and have taken integral calculus as one can actually predict buoyancy in terms of individual densities and volumes of all of the materials involved (including air), but it isn’t easy even with simple shapes. Professional engineers involved in boat building will use scale models to validate a design because that don’t dare rely solely on mathematical models.

[deleted-71447]

Can you tell us a little more about what you mean by "how long it takes to sink"? Do you want to make objects that float, and then eventually sink, or are you talking about measuring the downward velocity of the objects as they sink? If you are interested in the downward velocity, then you might consider the effects of friction, or "drag" on your various shapes of clay as they sink:
http://en.wikipedia.org/wiki/Drag_%28physics%29
There are a lot of possibilities here. I'm looking forward to hearing more about your project.

[deleted-71555]

Cheer up! Things are not that hard

You experiment seems very simple from doing but complicated in theory. I would rather do it in opposite way.

IN your proposed experiment, three factors will play the role on the time (speed) to reach the bottom: gravity(down), liquid friction (up), and buoyancy force (up). Among these three the friction is more variant and depends on the shape. And maybe you are right it wont sink if you make it like a boat . In short, it's hard to quantify the friction in your level. Besides, the speed of sinking maynot be constant and exact formular may need college-level knowledge ( Integration).

If you like, you can do a simpler experiment project adapting what your teacher suggested and make her happy:)

An experiment to obtain the density of different shapes of clays.

This project involves a few experiments and will execise some physics formulars. First you measure weight by scale, and then measure the volume by the water it expels using a measuring cup. Then devide the weight by volume to get the density. Then compare the density value you got from different shapes.

In fact the density is nothing to do with the shape for a uniform material; it is one the intrinsic properties of a solid; it is constant. Your experiment should verify this: if you mixed the clay well the density value shall be the same for different shapes. But if you mix it unevenly, the density value can vary. You can play with two variables: the way you mix them and different ingredients.

Enjoy!

[SoySauce]

Wow, that's a lot of words (everyone seems so friendly ). Mr. Bridge, I'm a highschool freshman (that belongs in the 6th grade) taking basic biology (no honors anymore due to last year's freshmen) and geometry :[ (my cousin tells me the education on the mainland is more advanced than Hawaii's). So far what I'm getting from you is if i start with 1.13kg of identical clay for all of my objects, the density of each object should be indentical no matter what shape? Sorry if i misinterpreted but I'm not sure what clay portion is, the entire object? Also, is there a way to make sure my clay doesn't absorb water or dissolve in water? Such as wrapping around that plastic wrap thingy for food (pitching crappy ideas)?

Mr. Chris G, I'm planned on the objects to sink and then if it had more bouyancy it would be pushed back making it sink slower. The time would then be the quantative data. If it floated I'd record it as 0. However, now that I know this will involve downward velocity/effects of friction (misinterpretted?) I think I may not be able to do what I originally planned to do.

Mr. Duan, you also mention friction as a factor. That really makes me think if I should think about continuing this particular experiment (the drag formulas that Mr. Chris G provided was mumbo-jumbo to me). I might try to ask my teacher to switch to your experiment, but I'm pretty sure she won't be too happy about me changing so close to the deadline. Like Mr. Bridge, you also mentioned that the density value shall be the same for different shapes. I'm not sure but what is mixing clay? Different ingredients (ingredients for making clay?)? I didn't mix the clay that I currently have, I bought the Crayola air-dry kind. In my brain of junk, I always thought that a shape's volume can be different from another shape's volume even if they had the same mass, so I thought mass/diff volume = diff density for both.

Part of the reason for my lack of knowledge (besides my intellect) is probably because I've never had physics. It's just that my teacher, who is a biology teacher, told us that our science fair project could be about anything related to science. So all the information I've gotten so far about how buoyancy works I found out by myself (which isn't a lot nor good). I think I'm the only kid that did something in physics. I just google'd buoyancy, found a few things about Archimedes and etc. What confuses me right now is my experiment deals more with the buoyancy of objects which is not the same as density, according to Mr. Bridge. From what i aquired through the internet I'd thought that buoyancy was related to density (An object with a higher average density than the fluid has less buoyancy than weight and it will sink). Is that related to the center buoyancy and the metacenter (spelling?) because I recall finding that online but I wasn't sure how it was related to density. Ughh...I was gonna ask something...got it. Would you think it would matter much if left out the friction factor completely? As well as the factor that the clay can absorb/dissolve in water (unless there is a way to prevent that)? Thanks for all of your help so far Mr. Duan, Mr. Chris G, and Mr. Bridge.

PS-Sorry if I'm too comma crazy. I'm trying to write with at least acceptable grammar here (used to IM/chat typing). Sorry if you think I'm too polite (I'm not usually like this, in fact, I get annoyed when I witness this in real life, but I don't want to disrespect anyone here) Also I'm sorry about any sentences that don't make sense. Right now is 1:05 AM (i started reading this thread, trying to understand the replies, and constructing a response at 12:XX AM) so I'm really out of it. Please, if anyone has any more useful suggestions please tell me (something that a 7th~8th grader can do, maybe 9th grade). I know that I had a lot more questions than I typed out here as I was reading the replies but I just don't remember. Thank you to whoever helps and I'm sorry for causing inconvenience.

PSS(spelling?)-It takes me a while to reply, very sorry.

[deleted-71447]

SoySauce,
Thomas Edison said, "Genius is one percent inspiration and ninety-nine percent perspiration." That is certainly true in my work, and many other jobs and activities, so I suggest letting go of ideas about whether you are "smart". Even if it were all about brains, you seem plenty bright to me.

I think you can still do your experiment. You will just need to make some small adaptations. If you want to exclude the effects of friction from your experiment, you will need all your objects to be the same size and shape. One approach would be to make several spheres of the same size, and to change the density by inserting different objects into the clay. You could use steel bolts to make the spheres more dense and pieces of wood to make them less dense.

I expect that your spheres would sink fast enough that it would not matter whether the clay could eventually absorb a little water or fall apart. If you use clay that dries out, make sure to measure the mass of the sphere just before you run the experiment, because the mass will change as the clay dries.

Looking forward to hearing more about your experiment,
Chris

[ghariman]

Hey, that's the quote in my signature .... hehehe.

Another quote of Edison that I like is "Everything should be made as simple as possible, but not simpler" !!!

SoySauce,
Thomas Edison said, "Genius is one percent inspiration and ninety-nine percent perspiration." That is certainly true in my work, and many other jobs and activities, so I suggest letting go of ideas about whether you are "smart". Even if it were all about brains, you seem plenty bright to me.

I think you can still do your experiment. You will just need to make some small adaptations. If you want to exclude the effects of friction from your experiment, you will need all your objects to be the same size and shape. One approach would be to make several spheres of the same size, and to change the density by inserting different objects into the clay. You could use steel bolts to make the spheres more dense and pieces of wood to make them less dense.

I expect that your spheres would sink fast enough that it would not matter whether the clay could eventually absorb a little water or fall apart. If you use clay that dries out, make sure to measure the mass of the sphere just before you run the experiment, because the mass will change as the clay dries.

Looking forward to hearing more about your experiment,
Chris

[SoySauce]

Thank you Mr. Chris G for your comment.
On Friday I went to my teacher to talk about Mr. Duan's proposed experiment and she told me if I wanted to do it, it's all up to me (students are supposed to be independent for this science fair). But, she told me if I used the same type of clay, no matter how I shaped my clay, the density will remain the same. Due to his experiment and mines both being based on density, I was reminded of Mr. Bridge's fact that density wasn't the same as buoyancy. So I asked her to confirm that fact and she said it was true. So I asked her if the proposed project was density or buoyancy and she said it was density. So I just decided to do my shape experiment and asked her how I would do it. She told me to I had to find the buoyant force of the shapes. I then went home and found out that buoyant force is = to the weight of the displaced fluid (I actually learned this while conducting my research but forgot about it). So my question is if the density is the same but the shape of the clay is different, will it displace a different mass of water/have different buoyant force? Also what is the unit for buoyant force? For example, if the shape displaces 50g of water will the buoyant force be 50g?
My teacher told me that I can ignore the friction, water absorption, and the water dissolving the clay factors.

PS. Chris G, would you be kind enough to tell me how the results of your experiment will "benifit mankind?" The reason I chose to do my current experiment is because with my results I may be able to design a better shaped watercraft. My first proposed experiment to my teacher was kind of rejected. It was basically the same as this one, but this time, the clay was a constant and remained the same shape and mass while I had solutions as the variable. I would mix different solutions into the solvent, water, then sink the clay and use the time as the data. But, my teacher told me that how would a boat be able to float on body of water mixed with sugar/dishwashing soap/ice tea mix? So I just trashed that idea.

Thank you to everyone who replies.

PSS. I had a better reply but on my ibm laptop there's a key by the directional keys that makes the web page go back or forward. I accidentally pressed it when constructing my first (much, much better than this one; addressed nearly everyone and everything) so when i went forward again, whatever I had was deleted. I made another post too which was also probably better than this one but I made the same mistake twice :[. So sorry if this post doesn't make much sense.

[deleted-71447]

Hi SoySauce,'
You're welcome for the reply, and thanks to Ghariman for putting the quote in his signature, which is probably where I saw it in the first place.

. . . So my question is if the density is the same but the shape of the clay is different, will it displace a different mass of water/have different buoyant force? Also what is the unit for buoyant force? For example, if the shape displaces 50g of water will the buoyant force be 50g?

The shape of the submerged clay will have no effect on the buoyant force, because the shape of the clay does not substantially change the volume (unless you incorporate some air pockets or other impurities). As you have discovered, if the volume of the submerged object and the displaced water does not change, then the buoyant force does not change either. The typical scientific units for force are Newtons, and the buoyant force is equal to the weight (not mass) of the displaced water.

Can you please tell us more about the specific scientific question that you want to answer and the exact procedure that you plan to use? I am a little confused about what variable you want to test and also about what you hope to achieve.

If you would like to know more about how density, buoyancy, and friction of a solid object immersed in a liquid are important to humanity, you should explore the topic of "fluid dynamics." Unfortunately, it can get pretty complicated. The topic is extremely important for designing watercraft and aircraft, as well as any stationary structure that withstands flowing water or air.

Sorry that you lost your previous post. That's a bummer.

[SoySauce]

Mr. Chris G., thanks for sticking with me this far (even when I make no sense and dont understand anything).

My scientific question is "what shape floats the best?"
My procedure is pretty much the same as before. Shape clay into 4 different shapes, each with the same mass. Obtain a clear container to fill with water. Fill it up with x amount of water. Place shape over the surface of the water. Let go. If object sinks record the amount of time it takes to reach the bottom. If the object floats record as 0.
The variable I want to test is shape. Maybe with my results I can design a better floatation device?

Now that I know that density and buoyant force will remain the same if I use the same clay, what is it that makes some shapes float better than others? The first time I discussed this with my teacher (before winter break), I think I heard say a triangle shape has a better chance of sinking than some shape I didn't hear (not sure if she said that, too scared to ask again, plus she'll probably tell me to go find out on my own). Why is that? What is that factor? Not density, not buoyant force, does it have something to with metacentric height? I hope not because that would mean that I researched the totally wrong thing.

PS-After these questions are answered I really think I have to stop checking up on this site because I've become too dependent on it. If I don't answer, I'll try to update on what happened on the deadline of the project.

[SoySauce]

Whoops. Forgot to mention:
I found out that weight = mass times acceleration due to gravity (9.8 m/s squared?). So if the mass is 1.13 grams I multiply it by 9.8, would the weight be 11.074 newtons?
Not sure if I'll need that information anymore though, but just in case.

[deleted-71447]

My scientific question is "what shape floats the best?"
My procedure is pretty much the same as before. Shape clay into 4 different shapes, each with the same mass. Obtain a clear container to fill with water. Fill it up with x amount of water. Place shape over the surface of the water. Let go. If object sinks record the amount of time it takes to reach the bottom. If the object floats record as 0.
The variable I want to test is shape. Maybe with my results I can design a better floatation device?

Now that I know that density and buoyant force will remain the same if I use the same clay, what is it that makes some shapes float better than others? The first time I discussed this with my teacher (before winter break), I think I heard say a triangle shape has a better chance of sinking than some shape I didn't hear (not sure if she said that, too scared to ask again, plus she'll probably tell me to go find out on my own). Why is that? What is that factor? Not density, not buoyant force, does it have something to with metacentric height? I hope not because that would mean that I researched the totally wrong thing.

Hi SoySauce,
I have a few questions.
What do you mean by "floats the best"? Would that be whether an object sinks or doesn't sink? If that is your question, then how will it help to measure how long it takes the objects to sink to the bottom? It seems like you might be interested in two different experiments here: (1) which shapes float versus which shapes sink or (2) if they sink, how does the shape affect the downward velocity. If that is true, I would recommend that you choose one topic or the other. If you choose #1, don't measure how long it takes them to sink. If you choose #2, don't include any data for objects that float.

Good work finding out about the difference between weight and mass. There is one more step that you need to include in your calculation.
1 Newton = 1 kg*m/s squared
so, you need to divide by 1000 to convert from g to kg.

[deleted-71447]

PS. If you are not able to visit the site, that is fine, but please feel free to take advantage of this resource as much as you possibly can. We volunteer as experts because we enjoy science and we enjoy working with young scientists, so it is not a problem for anyone.