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Consider a square-based prism water tank with dimensions [tex]$1 \text{ m} \times 1 \text{ m} \times 2 \text{ m}$[/tex].
Assume the density of water is [tex]$\rho=1000 \text{ kg} / \text{m}^3$[/tex] and the gravitational acceleration is [tex]$g=9.8 \text{ m} / \text{s}^2$[/tex].

(a) (8 points) Calculate the work done to fill the tank with water by pumping water into the base, if it is positioned so that the square face is on the ground. In other words, the height is [tex]$2 \text{ m}$[/tex], and the other two dimensions are [tex]$1 \text{ m}$[/tex]. Assume that the water is pumped from the bottom to the top of the tank and that the tank starts out empty.

(b) (8 points) What would be the work to fill the tank if it is placed with the long side on the ground, the length being [tex]$2 \text{ m}$[/tex] and the other two dimensions being [tex]$1 \text{ m}$[/tex]?

(c) (4 points) What is the work done to fill it by pouring it into the top for each example? (Hint: It is the same work required to lift the weight of all the water from the ground to the top of the tank). Compare the difference between filling from the bottom up and filling from the top, and explain why they are different.


Sagot :

Let's solve the problem step by step.

### Given Data:
- Dimensions for the first configuration: [tex]\(1\, \text{m} \times 1\, \text{m} \times 2\, \text{m}\)[/tex]
- Dimensions for the second configuration: [tex]\(2\, \text{m} \times 1\, \text{m} \times 1\, \text{m}\)[/tex]
- Density of water, [tex]\(\rho = 1000 \, \text{kg/m}^3\)[/tex]
- Gravitational acceleration, [tex]\(g = 9.8 \, \text{m/s}^2\)[/tex]

### (a) Work Done to Fill the Tank in the First Configuration (Height = 2m)

1. Volume Calculation:
The volume of the tank is:
[tex]\[ V = 1 \, \text{m} \times 1 \, \text{m} \times 2 \, \text{m} = 2 \, \text{m}^3 \][/tex]

2. Mass Calculation:
The mass of the water filling the tank is:
[tex]\[ m = \rho \times V = 1000 \, \text{kg/m}^3 \times 2 \, \text{m}^3 = 2000 \, \text{kg} \][/tex]

3. Work Calculation:
To pump the water to various heights from 0 to 2 meters, we need to integrate the work done:
[tex]\[ W = \int_0^2 \rho g A h \, dh \][/tex]
Here [tex]\(A = 1 \, \text{m} \times 1 \, \text{m} = 1 \, \text{m}^2\)[/tex] and [tex]\(A\)[/tex] is constant.

Thus,
[tex]\[ W = \rho g A \int_0^2 h \, dh = \rho g A \left[ \frac{h^2}{2} \right]_0^2 = \rho g A \times \frac{2^2}{2} = \rho g \times 2 = 2000 \times 9.8 = 19600 \, \text{J} \][/tex]

### (b) Work Done to Fill the Tank in the Second Configuration (Height = 1m)

1. Volume Calculation:
The volume of the tank remains the same:
[tex]\[ V = 2 \, \text{m} \times 1 \, \text{m} \times 1 \, \text{m} = 2 \, \text{m}^3 \][/tex]

2. Mass Calculation:
The mass of the water filling the tank is:
[tex]\[ m = \rho \times V = 1000 \, \text{kg/m}^3 \times 2 \, \text{m}^3 = 2000 \, \text{kg} \][/tex]

3. Work Calculation:
To pump the water to various heights from 0 to 1 meter, we need to integrate the work done:
[tex]\[ W = \int_0^1 \rho g A h \, dh \][/tex]
Here [tex]\(A = 2 \, \text{m} \times 1 \, \text{m} = 2 \, \text{m}^2\)[/tex] and [tex]\(A\)[/tex] is constant.

Thus,
[tex]\[ W = \rho g A \int_0^1 h \, dh = \rho g A \left[ \frac{h^2}{2} \right]_0^1 = \rho g A \times \frac{1^2}{2} = \rho g \times 1 = 2000 \times 9.8 = 19600 \, \text{J} \][/tex]

### (c) Work Done to Fill by Pouring Water into the Top

For both configurations:
1. Mass is the same: [tex]\(2000 \, \text{kg}\)[/tex]
2. Height for the work done:
- First configuration: [tex]\(2 \, \text{m}\)[/tex]
- Second configuration: [tex]\(1 \, \text{m}\)[/tex]

3. Work Calculation when Pouring into the Top:
- For the first configuration:
[tex]\[ W_{\text{top, first}} = m g h = 2000 \, \text{kg} \times 9.8 \, \text{m/s}^2 \times 2 \, \text{m} = 39200 \, \text{J} \][/tex]
- For the second configuration:
[tex]\[ W_{\text{top, second}} = m g h = 2000 \, \text{kg} \times 9.8 \, \text{m/s}^2 \times 2 \, \text{m} = 39200 \, \text{J} \][/tex]

### Comparisons:

1. Difference in Works:
- For the first configuration:
[tex]\[ \Delta W_{\text{first}} = |W_{\text{fill}} - W_{\text{top}}| = |19600 - 39200| = 19600 \, \text{J} \][/tex]
- For the second configuration:
[tex]\[ \Delta W_{\text{second}} = |W_{\text{fill}} - W_{\text{top}}| = |19600 - 39200| = 19600 \, \text{J} \][/tex]

### Explanation:
The work done to fill the tank by pouring directly into the top is larger than the work done by filling it from the bottom because, in the latter method, the water needs to be pumped incrementally from the bottom to the top, meaning each incremental volume of water is lifted to progressively higher heights as the tank fills. In contrast, pouring into the top involves lifting the entire mass of water to the maximum height of 2 meters in an idealized scenario.

Thus, the work required to pour water into the top directly is always more by exactly [tex]\(19600 \, \text{J}\)[/tex] than filling from the bottom for the given dimensions.