Stupid math question (volume-mass-weight)

[ARGV]

For some reason (bad math teachers) I can't get this figured out, even though I suspect its very simple.

 

I have the volume of a 3Dsolid, obviously it's weight depends on it's density, which depends on it's mass.

 

Suppose I know one material is basic metal (steel) which is about 480 lbs/ft^3 and one is mdf which is 42 lbs/ft^3

 

Using the volume property of my solid, how do I determine the weight of the object given it is one of these materials?

 

I know weight = volume*density. But how do I accurately figure out it's density, since density = weight/volume?

 

 

Thanks!

 

-ArgV

[dbroada]

I haven't been taught in imperial so I may have missed the gravity affect but I think in this case you can ignore it. Keep an eye on your units. You have you solid of z ft^3 so multiply that by the density of 480 lbs/ft^3 and you get z * 480 lbs as the ft^3 cancel each other out.

 

or have I missed your point?

[nukecad]

For a wealth of engineering data, including calculations such as the one you want, check out the following website-

 

http://www.roymech.co.uk/

 

I refer to it often when I cant remember some equation or other; its nearly as good as the Machinereys Handbook.

[ARGV]

I haven't been taught in imperial so I may have missed the gravity affect but I think in this case you can ignore it. Keep an eye on your units. You have you solid of z ft^3 so multiply that by the density of 480 lbs/ft^3 and you get z * 480 lbs as the ft^3 cancel each other out.

 

or have I missed your point?

 

You might have missed it, I dunno. My goal is to find the density of these materials really. The volume doesn't do me any good unless I know the density of material, which would help determine it's weight.

 

my questions:

 

what is the density of a material if it is 480lbs ft^3?

what is the density of a material if it is 42lbs ft^3?

 

So far, I've come up with .283 and .028 respectively. I just don't know if thats accurate.

 

thank you,

 

-ArgV

[eldon]

my questions:

 

what is the density of a material if it is 480lbs ft^3?

what is the density of a material if it is 42lbs ft^3?

 

What is it in the above? it is density, so the above statements are akin to tautolology. You are answering your own question before you ask it.

 

Definition of Density. The degree of consistency of a body or substance measured by the quantity of mass in unit volume.

 

So the density of the material is 480 pounds per cubic foot, or 42 pounds per cubic foot.

 

So far, I've come up with .283 and .028 respectively. I just don't know if thats accurate.

 

thank you,

 

 

How have you got these figures because they do not make a lot of sense in view of the fact that the figures are 480 and 42

[ARGV]

What is it in the above? it is density, so the above statements are akin to tautolology. You are answering your own question before you ask it.

 

Definition of Density. The degree of consistency of a body or substance measured by the quantity of mass in unit volume.

 

So the density of the material is 480 pounds per cubic foot, or 42 pounds per cubic foot.

 

 

 

How have you got these figures because they do not make a lot of sense in view of the fact that the figures are 480 and 42

 

I'm sorry, you're right. yes, that is the density per cubic foot, and I decided to go with cubic inch, hence the much smaller figures.

 

I guess that is all I wanted, but I got lost somewhere between converting from kg / m^3 and lbs / ft^3.

 

Well, I guess back to math class.

 

So, if I know the density of material in ft^3, and I know the volume of an object, and I want the weight, I would ______________________.

 

I get screwed up because volume is such an undefined measurement. I.e. Volume of what? Air? wood? metal? glass? water? And it's weight depends on the mass or density of the material.. then in what units do you use? Cubic inch? cubic ft?

 

Then, if the "volume" of the object is, say 1.25. If I multiply the volume by that density (480 lbs / ft^3), that isn't going to work, because my part is only about 1 pound, and 1.25 * 480 = 600lbs!

 

So, I converted it to cubic inches, and get a much more accurate result, but what is the failproof method?

[eldon]

When you work out the volume of material, separate out the air space from the actual material.

 

Then if you are working in cubic inches, the density of steel is 480 divided by 12 times 12 times 12 = 0.278 pounds per cubic inch.

 

When I went to school, the gravitational effect was not bothered with. Then this fellow Newton came along, and confusion has reigned ever since

[ARGV]

When you work out the volume of material, separate out the air space from the actual material.

 

Then if you are working in cubic inches, the density of steel is 480 divided by 12 times 12 times 12 = 0.278 pounds per cubic inch.

 

When I went to school, the gravitational effect was not bothered with. Then this fellow Newton came along, and confusion has reigned ever since

 

haha. Well, I definitely get confused when an objects "weight" is referred to as it's "gravitational pull" if that is what you're talking about, however in the grand scheme of things, thats the only thing it could be I guess. But really.. An object doesn't have weight just because it's got lots of mass, or lots of volume, but it's mass and volume compared to the mass and volume of the elements it is sitting in.. water, air, dirt, etc.

 

From what i've read, water is used as the basis for volume. Maybe because air is so much less dense that the numbers get too big. ? So, if it floats, it's volume is water.

 

or something like that.

 

thanks!

[eldon]

Gravitational pull does have an effect, so the same object would have a different weight at different parts of the world.

 

Also different steels will have different densities, so the figure you are using is probably an average one. For better accuracy, you have to measure the actual density of the material that you are using.

[ARGV]

Gravitational pull does have an effect, so the same object would have a different weight at different parts of the world.

 

Also different steels will have different densities, so the figure you are using is probably an average one. For better accuracy, you have to measure the actual density of the material that you are using.

 

Oh, I didn't know that weight changed by location. Hmm.. that works I guess.

 

yeah, I don't need to be THAT accurate.. within 5 pounds for a 200 pound unit is acceptable. This is mostly for shipping weight. I made a routine to go through all the blocks in our models and calculate the weight of the solids depending on the layer.

 

Before they were using ft^2 which meant having to know the exact thickness.. thats great for surface area.. not so much for volume to weight.

 

thanks again.

 

-ArgV