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. (See the figure.) The density of brass is 8599 kg/m3. If you put this washer on a laboratory balance, what will it "weigh" in grams?

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. (See the figure.) The density of brass is 8599 kg/m3. If you put this washer on a laboratory balance, what will it "weigh" in grams?

anonymous · Answer

Just work out the Volume of the big cylinder, then subtract the volume of the 'air' cylinder. Need all measurements in metres...
large radius = R = 2.5 x10^-2 m
small radius = r = 0.625 x10^-2 m
depth = d = 1.5mm = 1.5 x10^-3 m
pi x R^2 x d = vol of large one (m^3)
pi x r^2 x d = vol of air (m^3)
vol of large - vol of air = volume of brass. (m^3)
Now..
density = mass / volume
density brass = mass of washer/ volume of brass.
mass of washer = density brass * volume of brass
Done!