how do I calculate the weight of a plastic part, knowing the volume in cm3 and its specific gravity?

Answer 1

Weight of Plastic = W
Volume = V (convert into m3 by dividing 1,000,000)
Specfic G = Sg
Dp = plastic density
Dw = water density = 1000 kg/m3

W = Dp * V ...............(1)
Sg = Dp / Dw ................(2)

Substitute 2 into 1 hence,

W = (Sg * Dw) * V
W = Sg * 1000 x V

Answer 2

Bingo, you got it! Remember that your units should work out in your question. The formula is Vd = w, where V is volume, d is density (or specific gravity), and w is weight. Let's say you got mixed up and thought it was V = dw. Then you would work the problem like this: V = d * w V = something in (lb/ft^3) * something in lb V = lb^2 / ft^3 You'd notice that the units make no sense! When you do it the right way, you get: w = Vd w = (something in ft^3) * (something in lb/ft^3) w = something in lb This is called "dimensional analysis". It helps you keep track of units, and can provide a quick check that your equation is correct. It won't help you to find constants - for example, the volume of a sphere is: V = (4/3) * pi * r^3 Dimensional analysis will help you get the "r^3" part, since volume is in inches cubed and the radius is in inches. If you said r^2, you'll get a volume in in^2, or square inches, which makes no sense. BUT...it fails to help you get the (4/3)*pi part of the equation, since that part has no units.

Answer 3

Maybe it will help if I tell you the definition of specific gravity is the density of an object in terms of the density of water, for example water has a specific gravity of 1 and gold has a specific gravity about 18 or so.

Answer 4

Multiply the two together, keeping a good eye on your orders of magnitude and units...
I.E .. were the decimal places are and the units,, KG, Grammes, Feet, Yards,.... etc.

Answer 5

Umm - multiply?