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An ornament weighing 36 gm in air weighs only 34 gm in water. Assuming that some copper is mixed with gold to prepare the ornament. Find out the amount of copper in it.

Specific gravity of gold is 19.3 and that of copper is 8.9.

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Data: m$_{air}$=36 gm

m$_{water}$=34 gm

specific gravity of gold=S$_{gold}$=19.3

specific gravity of copper=S$_{copper}$=8.9

∴ Weight of ornament in air is=W$_{air}$=$36 \times 10^{-3} \times 9.81$=0.35316 N

∴ Weight of ornament in water is=W$_{water}$=$34 \times 10^{-3} \times 9.81$=0.33354 N

Buoyant force=$F_{B}=W_{air}-W_{water}$=0.01962 N

Weight of the fluid displaced $(F_{B})=W_{water} \times $volume of the ornament

0.01962=9810$\times $volume of the ornament

∴ Volume of the ornament=$2 \times 10^{-6} m^{3}$

But, the ornament contains gold and copper

∴ Volume of the ornament (V) = $V_{gold}+V_{copper}$ ---------------(1)

$V_{gold}=\cfrac{\text{Weight of gold}}{W_{gold}}$

$V_{gold}=\cfrac{\text{Weight of gold}}{19.3 \times 9810}$

Similarly,

$V_{copper}=\cfrac{\text{Weight of copper}}{W_{copper}}$

$V_{copper}=\cfrac{\text{Weight of copper}}{8.9 \times 9810}$

Equation (1) becomes,

$2 \times 10^{-6}$= $V_{gold}+V_{copper}$

$2 \times 10^{-6}$=$\cfrac{\text{Weight of gold}}{19.3 \times 9810} + \cfrac{\text{Weight of copper}}{8.9 \times 9810}$ ---------------(2)

Also, weight of the ornament (V) = $W_{gold}+W_{copper}$=0.35316 ---------------(3)

Solving equation (2) and (3) we get,

$W_{gold}$=0.33133 N and $W_{copper}$=0.0218 N

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