Closed or open voids, your choice. At a density of 7.5g/cc, my mild steel boat shaped thingy will float even though my mild steel has a mass density >1 g/cc.
I will now melt down the boat shaped thingy into a 65,000 ton cube and launch it with tears in my eyes as it sinks. It still has a mass density of 7.5g/cc.
The reason I can do this is because buoyancy is a force acting over an area.
By 'voids' I thought it was clear I was talking about sealed volumes within the denser material (otherwise you would not achieve a density reduction).
The density of your material does not change you are spreading it over a wider area.
If I put my sinking brick and floating boat shaped thingy in a huge wind tunnel I will find that with its higherCd and larger surface area facing the flow that my sinking brick has more drag than the floaty boat.
regards
Vince
Can we postpone the beer until I feel better, I had football stress overload last night.
Vince,
Oh come on - it's been pretty clear we've both been talking about composite density, but as I predicted you moved the goal posts.
Tell me - what volume of water does your ship displace? When you take the mass of the ship and divide it by this volume what answer do you think you'll get? That's its true density.
VGB wrote:Probably one for another thread but if I make my floater out of silicone and sinker out of solid PTFE, which one would have the greater diameter given that silicon has twice the material density of PTFE.
This was one of you original questions Vince - one I gave you an answer for. I then asked for your answer but you didn't respond.
You now are talking about essentially forming high diameter fly-lines from dense material that sink only because they flood with water (just like your warship would sink if flooded). And to think I accused you of being unrealistic :p .
I have never been talking about composite density, otherwise I would never have said that changing mass density does not directly effect drag. If you remember when we first started this discussion, I said that the only density used in the drag equation was air density. In fact I do not think that there is any such beast as composite density unless you are referring to carbon fibre or similar.
The problem is that you have assumed that density is mass/volume which is what we are taught in school. However, that assumes that the material is a solid, it goes out of the window if it is hollow. I did tell you that.
No matter how much air you blow in it the density of lead will always remain at 11340 kg/m^3. If you try to determine its density with voids in, you will get a misleading figure.
I also gave you the link that gave you BSI true density and true volume defintions:
True density (also called True particle density); The mass of a particle divided by its volume, excluding open pores and closed pores.
True volume: Volume excluding open and closed pores.
If you go back to where we started from I said that changing mateiral density does not affect drag. Putting voids in a material does not change its mass density it reduces its mass. If you took a line with a given set of physical properties, for the same velocity, it would have the same drag force whether it was solid or voided. However, the line with a lower mass has less momentum which is why your sinker will cast further than your floater even with the same diameter and same drag.
This is why I posed the 2 material question. If we assume that silicon has twice the mass density of PTFE. I can constuct 2 lines with the same drag from 2 materials with different mass densities by voiding. I think that I can get the silicone one to float by putting lenghtwise grooves in it because I am increasing the surface area but I am not sure that the drag will not go up. There is likely to be a compromise solution as a manufacturer has managed it.
You now are talking about essentially forming high diameter fly-lines from dense material that sink only because they flood with water (just like your warship would sink if flooded).
No I did not. I said:
Closed or open voids, your choice. At a density of 7.5g/cc, my mild steel boat shaped thingy will float even though my mild steel has a mass density >1 g/cc.
Ships have open voids where you put the engines, cabins and gubbins. I could have made a 65000 ton mild steel bowl and got it to float. I sank the 65000 tons of steel it by making it solid and increased the drag by changing its shape. Both designs have the same true volume and density.
VGB wrote:The problem is that you have assumed that density is mass/volume which is what we are taught in school. However, that assumes that the material is a solid, it goes out of the window if it is hollow. I did tell you that.
Vince,
We are talking about the density of an item that sits in water. My knowledge of density measurement goes a bit beyond school level, in fact I have one of the Micromeretics machines, metioned in your link, in my lab (mine does an immersion in helium measurement).
Composite density is indeed a made up term - I was trying to get across that your object contains more than one material i.e. steel and air (or plastic and air).
It's like me handing you a football and asking what the density is because I'm interested why it floats. You then cut my football up and measure the density of the material that makes up its skin. Although a correct measurement it is totally out of context as to why my ball floats.
You still haven't answered the question you set by the way...
Thank you for your explanation. I am comfortable with the momentum/drag effects
Vince,
That is progress.
If the positive acceleration comes from a change in the momentum of the moving mass in the fly leg and that momentum change is given by delta_p=rho_l*(v_fly/2).^2 do you now see that for a given fly velocity the linear mass density (rho_l) of the line is key to determining the positive acceleration force on the line rather than thinking:
What I have been trying to get across is that mass density is not very important
Gordy
"Flyfishing: 200 years of tradition unencumbered by progress." Ralph Cutter
Composite density is indeed a made up term - I was trying to get across that your object contains more than one material i.e. steel and air (or plastic and air).
At which point did you explain that you were using a made up term. I have heard you talk about your lab before and assumed that when I used the term "mass density" that we were both talking about the same thing and that you knew what we were talking about. In fact you never mentioned composite density until a couple of posts ago.
You are also aware that when you talk about an open or closed void that you discard it for density purposes. Why did you not point out your own interpretation of density when I talked about voids. Unless you tell me about your made up term, how am I supposed to guess.
It's like me handing you a football and asking what the density is because I'm interested why it floats. You then cut my football up and measure the density of the material that makes up its skin.
You are also well aware that quoting mass density implies that you are using a particular material and that implies mechanical properties including tensile and flexural strengths and moduli, all of which are very important if you are going to design a fly line. If I gave you a spec of density of 1g/cc without telling it was a made up composite density, you would think that I was making a line out of water.
If you are still intent on going on with your project of designing a fly line, I wish you luck because I believe it is more difficult than you realise. For instance, when making your floater you will have to decide how high it is floating before you calculate your displacement. As I alluded to earlier there are other properties of the material to consider other than whether we can make it buoyant, such as considering if it will survive the casting experience.
The reason I did not answer my own question despite your demands is becasue there is no right answer. I would like to put together a less terse answer but sitting in a hotel room with a flaky connection is already trying my patience
If the positive acceleration comes from a change in the momentum of the moving mass in the fly leg and that momentum change is given by delta_p=rho_l*(v_fly/2).^2 do you now see that for a given fly velocity the linear mass density (rho_l) of the line is key to determining the positive acceleration force on the line rather than thinking:
I answered this on Post 47 when I said that mass was important. Mass density of a material is a fixed value:
VGB wrote:You are also aware that when you talk about an open or closed void that you discard it for density purposes. Why did you not point out your own interpretation of density when I talked about voids. Unless you tell me about your made up term, how am I supposed to guess.
Hey Vince,
This is the bit I disagree with.
There are a number of density measurements, ranging from a theoretical figure derived from crystallographic measurements and atomic masses (usually by x-ray diffraction), through to a simple geometric measurement of of mass and (apparent) volume. Porosity is treated differently depending on the method used.
When you talk of 'mass density' which technique are you specifically referring to?
In terms of buoyancy, the figure that is most relevant is an Archimedes measurement - this discards open porosity and leaves in the closed porosity.
I should of explained what I meant by 'composite density' - I was trying to get to the point that fly-lines (or whatever) are made up of different materials (plastic doped with micro-balloons over a nylon core etc.) and it's the density of the 'composite' item that matters. This is the figure that a Archimedes measurement will return.
I am aware that there are differnet methods of measurement but to my limited knowledged they settle on true mass of material/true volume of material to give true density. The reason I posted the voids link was to get us on the same page.
I agree that it is the density of the solid that counts and was trying to point out that by varying the total mass value you would change momentum and hence casting distance. However, it is the volume side of the construction that will contribute to drag. this was the poo trap in my materials question, notice I did not mention mass
Mass density is why the net effect of drag on a sinking line is less than it is for a floating line. Same thing for ping pong balls and golf balls.
AND
Have you put your balls in the wind tunnel yet?
Vince,
You know that spherical objects with the same smooth surface and same diameter would have the same drag force for a given wind velocity.
However, the thing that is important (and the concept you seem to be missing) is that the net effect of that drag force on the deceleration rate that would impact the decrease in their propagation velocity from that identical drag force would be much different.
a=f/m, so for the same drag force an object with 10 times the mass will have deceleration rate that is one tenth the deceleration rate of the object with less mass.
As you found out from Dr. Perkins article on the falling rate of fly lines that is why a 12 wt floating line will have a higher terminal velocity than a 2 wt line having the same volume mass density of 0.85 g / cm^3 even though the 12 wt line will have more drag force since its diameter is larger.
Gordy
"Flyfishing: 200 years of tradition unencumbered by progress." Ralph Cutter
VGB wrote:I am aware that there are differnet methods of measurement but to my limited knowledged they settle on true mass of material/true volume of material to give true density. The reason I posted the voids link was to get us on the same page.
Vince,
You're missing the key point in my response; the various density measurements don't end up at the same value. They are all correct in the right context though.
As I stated before density measurements treat porosity differently. In fact if you want to truly understand the material you should perform density measurements via different techniques, this will give you information regarding the inherent porosity.
I looked up 'true density' as it's not a term I've heard used (where I am we use 'immersion' or 'pycnometer' density), anyway, I found this quote "However, a variety of definitions of particle density are available, which differ in terms of whether pores are included in the particle volume, and whether voids are included."
I looked up 'true density' as it's not a term I've heard used (where I am we use 'immersion' or 'pycnometer' density), anyway, I found this quote "However, a variety of definitions of particle density are available, which differ in terms of whether pores are included in the particle volume, and whether voids are included."
This is why I said that there is no right answer.
Unless terms are defined accurately to a standard they are a point of view, which is why I used a BSI reference. Different disciplines use different standards depending on the end use. In aircraft structures and performance calculations, we use the standard I referenced to give a true mass and area of a structure.
We are a bit averse to using immersion on aircraft structures
However, the thing that is important (and the concept you seem to be missing) is that the net effect of that drag force on the deceleration rate that would impact the decrease in their propagation velocity from that identical drag force would be much different.
a=f/m, so for the same drag force an object with 10 times the mass will have deceleration rate that is one tenth the deceleration rate of the object with less mass.
I take it from that response that you are comfortable that mass and not not mass density is the driver in momentum changes.
Additionally, I am not missing the point, you are assuming that all fly lines are a solid.
By putting different size voids in a line, you can have a line of the same diameter and the same mass using 2 different mass density materials.
The lower mass density line will have smaller voids and use more material. But both lines will have the same momentum due to having the same mass. They will also have the same drag due to presenting the same surface area to the airflow.
Additionally, I am not missing the point, you are assuming that all fly lines are a solid.
Vince,
So now you are saying that fly lines have voids in them so you cannot model a 1 meter length of a constant diameter section of fly line as being a cylinder?
That must make things very complicated for the way you would calculate the linear mass density (rho_l) of a flat section of fly line.
That being the case, how would you go about calculating the change in the momentum per unit time of a flat fly line due to the shortening of the moving mass in the fly leg? If you want to put voids in the line lets assume those voids have a distribution that has a constant volume per unit length.
Let's assume it is a floating line with a density of 850 kg/m^3 with a diameter of 1.3 mm and the fly is moving at 40 m/s.
It will be interesting to see how your number compares to one given by Hendry's formula that assume the line can be modeled as a long cylinder.
Gordy
"Flyfishing: 200 years of tradition unencumbered by progress." Ralph Cutter
So now you are saying that fly lines have voids in them so you cannot model a 1 meter length of a constant diameter section of fly line as being a cylinder?
You are being deliberately obtuse, I have not said that.
delta_p=rho_l*(v_fly/2).^2
That is true, so will a line with a higher linear mass density have more mass change or less mass mass change vs time than a lower density line assuming they have the same loop velocity? Which line has a higher linear mass density throughout, a level line or a tapered line?
You are confusing the model with the real world. Linear mass density is not the same as mass unless the fly line is solid as I have just shown you.
