Question

A brass washer has an outside diameter of 4.50 cm with a hole of diameter 1.25 cm and is 1.50 mm thick. The density of brass is 8599 kg/m3. If you put this washer on a laboratory balance, what will it "weigh" in grams?

Answer 1

We will find the mass from 
mass = density x volume 
We are told the density and must find the volume from the dimensions given 
the volume of the washer will be the area x thickness (remembering to convert all measurements to meters) 
if the washer had no hole, its area would be pi (0.0225m)^2 (remember to convert to meters and to use radius) 
the area of the hole is pi(0.00625m)^2 
so the area of the washer is pi[(0.0225m)^2 - (0.00625m)^2] = 1.5x10^-3 m 
the volume of the washer is 1.5x10^-3 m x 1.5x10^-3 m = 2.25x10^-6 m^3 (the thickness of the washer is 1.5 mm = 1.5x10^-3m) 
thus, the mass of the washer = 8598kg/m^3 x 2.25x10^-6m^3 = 0.0189kg = 18.9 grams