When a container of water is placed on a laboratory scale, the scale reads 120 g. Now a 20-g piece of copper $\text { (of density } 8.9 \mathrm {~g} / \mathrm { cm } ^ { 3 } \text { ) }$
is suspended from a thread and lowered into the water, not touching the bottom of the container. What will the scale now read? The density of water is 1.0 $\mathrm { g } / \mathrm { cm } ^ { 3 }$
A) 120 g
B) 138 g
C) 122 g
D) 140 g

Question

When a container of water is placed on a laboratory scale, the scale reads  120 g. Now a  20-g piece of copper $\text { (of density } 8.9 \mathrm {~g} / \mathrm { cm } ^ { 3 } \text { ) }$
is suspended from a thread and lowered into the water, not touching the bottom of the container. What will the scale now read? The density of water is  1.0  $\mathrm { g } / \mathrm { cm } ^ { 3 }$
A)  120 g
B)  138 g
C)  122 g
D)  140 g

Quizplus · Accepted Answer

The scale will read 122 g. The buoyant force on the copper is equal to the weight of the water displaced by the copper. The volume of water displaced is equal to the volume of the copper, which is  $ 20 \mathrm { g } / 8.9 \mathrm { g } / \mathrm { cm } ^ { 3 } = 2.25 \mathrm { cm } ^ { 3 }$. The weight of the water displaced is  $ 2.25 \mathrm { cm } ^ { 3 } \times 1.0 \mathrm { g } / \mathrm { cm } ^ { 3 } = 2.25 \mathrm { g }$. The scale will read the weight of the water plus the weight of the copper minus the buoyant force, which is  $ 120 \mathrm { g } + 20 \mathrm { g } - 2.25 \mathrm { g } = 122 \mathrm { g }$.

When a Container of Water Is Placed on a Laboratory (of density 8.9 g/cm3 ) \text { (of density } 8.9 \mathrm {~g} / \mathrm { cm } ^ { 3 } \text { ) } (of density 8.9 g/cm3 )

When a Container of Water Is Placed on a Laboratory $\text { (of density } 8.9 \mathrm {~g} / \mathrm { cm } ^ { 3 } \text { ) }$