A cube of volume [tex]$5.50 \times 10^{-3} \, m^3$[/tex] and density [tex]$7.50 \times 10^3 \, kg/m^3$[/tex] hangs from a cable. When the cube has the lower half of its volume submerged in an unknown liquid, the tension in the cable is 375 N. What is the density of the liquid? (Ignore the small buoyant force exerted by the air on the upper half of the cube.)

Express your answer with the appropriate units.

[tex]\rho_{\text{liquid}} = 1100 \, \frac{kg}{m^3}[/tex]



Answer :

Certainly! Let's break down the process step-by-step to find the density of the liquid:

### Step 1: Determine the mass of the cube
Given:
- Volume of the cube [tex]\( V_{\text{cube}} = 5.50 \times 10^{-3} \, \text{m}^3 \)[/tex]
- Density of the cube [tex]\( \rho_{\text{cube}} = 7.50 \times 10^3 \, \text{kg/m}^3 \)[/tex]

The mass [tex]\( m \)[/tex] of the cube can be found using the formula:
[tex]\[ m = \rho_{\text{cube}} \times V_{\text{cube}} \][/tex]

[tex]\[ m = 7.50 \times 10^3 \, \text{kg/m}^3 \times 5.50 \times 10^{-3} \, \text{m}^3 \][/tex]

[tex]\[ m = 41.25 \, \text{kg} \][/tex]

### Step 2: Calculate the weight of the cube
The weight [tex]\( W \)[/tex] of the cube is given by:
[tex]\[ W = m \times g \][/tex]
where [tex]\( g \)[/tex] is the acceleration due to gravity [tex]\( \left( 9.8 \, \text{m/s}^2 \right) \)[/tex].

[tex]\[ W = 41.25 \, \text{kg} \times 9.8 \, \text{m/s}^2 \][/tex]

[tex]\[ W = 404.25 \, \text{N} \][/tex]

### Step 3: Determine the volume of the cube that is submerged
Since only the lower half of the cube is submerged, the submerged volume [tex]\( V_{\text{submerged}} \)[/tex] is half of the total volume of the cube:
[tex]\[ V_{\text{submerged}} = \frac{V_{\text{cube}}}{2} \][/tex]

[tex]\[ V_{\text{submerged}} = \frac{5.50 \times 10^{-3} \, \text{m}^3}{2} \][/tex]

[tex]\[ V_{\text{submerged}} = 2.75 \times 10^{-3} \, \text{m}^3 \][/tex]

### Step 4: Apply the principle of buoyancy
The buoyant force [tex]\( F_b \)[/tex] acting on the submerged part of the cube is given by:
[tex]\[ F_b = \rho_{\text{liquid}} \times V_{\text{submerged}} \times g \][/tex]

### Step 5: Establish the equilibrium condition for the forces
In equilibrium (when the cube is hanging), the tension [tex]\( T \)[/tex] in the cable plus the buoyant force balances the weight of the cube:
[tex]\[ T + F_b = W \][/tex]

Given that the tension [tex]\( T \)[/tex] in the cable is 375 N:
[tex]\[ 375 \, \text{N} + \rho_{\text{liquid}} \times V_{\text{submerged}} \times g = 404.25 \, \text{N} \][/tex]

### Step 6: Solve for the density of the liquid [tex]\( \rho_{\text{liquid}} \)[/tex]
Rearrange the equation to solve for [tex]\( \rho_{\text{liquid}} \)[/tex]:
[tex]\[ \rho_{\text{liquid}} = \frac{W - 375 \, \text{N}}{V_{\text{submerged}} \times g} \][/tex]

Substitute the known values into the equation:
[tex]\[ \rho_{\text{liquid}} = \frac{404.25 \, \text{N} - 375 \, \text{N}}{2.75 \times 10^{-3} \, \text{m}^3 \times 9.8 \, \text{m/s}^2} \][/tex]

[tex]\[ \rho_{\text{liquid}} = \frac{29.25 \, \text{N}}{2.75 \times 10^{-3} \, \text{m}^3 \times 9.8 \, \text{m/s}^2} \][/tex]

[tex]\[ \rho_{\text{liquid}} = \frac{29.25}{0.02695} \][/tex]

[tex]\[ \rho_{\text{liquid}} \approx 1085.34 \, \text{kg/m}^3 \][/tex]

Thus, the density of the liquid is approximately [tex]\( 1085.34 \, \text{kg/m}^3 \)[/tex].