Science:Math Exam Resources/Courses/MATH101/April 2011/Question 01 (d)

MATH101 April 2011

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Question 01 (d)

For this question, the exam asks to put your answer in the box provided but also to show your work. This question is worth 3 marks. Full marks is given for correct answers placed in the box and at most 1 mark is given for an incorrect answer. You are required to simplify your answers as much as possible. A tank in the shape of a cube with edges 2 $ft$ long is filled with a fluid that weighs 50 $lb/ft^{3}$. Find the hydrostatic force, in $lb$, against one of the vertical sides of the tank.

Make sure you understand the problem fully: What is the question asking you to do? Are there specific conditions or constraints that you should take note of? How will you know if your answer is correct from your work only? Can you rephrase the question in your own words in a way that makes sense to you?

If you are stuck, check the hint below. Consider it for a while. Does it give you a new idea on how to approach the problem? If so, try it!

Hint
Divide the tank wall into many small horizontal strips and consider the hydrostatic force (${\text{Force}}={\text{density}}\times {\text{pressure}}\times {\text{depth}}\times {\text{area}}$) on each. The total force will be approximately the sum of the forces on all the strips. Let the number of strips go to infinity to make the approximation exact.

Checking a solution serves two purposes: helping you if, after having used the hint, you still are stuck on the problem; or if you have solved the problem and would like to check your work. If you are stuck on a problem: Read the solution slowly and as soon as you feel you could finish the problem on your own, hide it and work on the problem. Come back later to the solution if you are stuck or if you want to check your work. If you want to check your work: Don't only focus on the answer, problems are mostly marked for the work you do, make sure you understand all the steps that were required to complete the problem and see if you made mistakes or forgot some aspects. Your goal is to check that your mental process was correct, not only the result.

Solution

Found a typo? Is this solution unclear? Let us know here. Please rate my easiness! It's quick and helps everyone guide their studies. The depth of the water is $2{\text{ }}ft$, so we divide the tank wall into $n$ horizontal strips of height $\Delta x=(2-0)/n.$ The width of every strip is $2{\text{ }}ft$, so the area of each strip is $A_{i}=2\Delta x.$ The hydrostatic force on a strip at depth $x_{i}^{\ast }$ is $F_{i}=\rho gx_{i}^{\ast }A_{i}=\rho gx_{i}^{\ast }(2\Delta x).$ We are given that the weight density of the fluid is $\rho g=50{\text{ }}lb/ft^{3}.$ To get the total force on the wall, we add up all the forces $F_{i}$ and take the limit as $n\rightarrow \infty :$ $F=\lim _{n\rightarrow \infty }\sum _{i=1}^{n}F_{i}=2\rho g\lim _{n\rightarrow \infty }\sum _{i=1}^{n}x_{i}^{\ast }\Delta x=2\rho g\int _{0}^{2}xdx=2\cdot 50\cdot 2=200{\text{ }}lb.$ We integrate from $0$ to $2$ because the water extends from depth $0{\text{ }}ft$ to $2{\text{ }}ft$. This is also why we set $\Delta x=(2-0)/n.$

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