# 6.5 Physical applications  (Page 6/11)

 Page 6 / 11

This is a Riemann sum, so taking the limit gives us the exact force. We obtain

$F=\underset{n\to \infty }{\text{lim}}\sum _{i=1}^{n}\rho \left[w\left({x}_{i}^{*}\right)\text{Δ}x\right]s\left({x}_{i}^{*}\right)={\int }_{a}^{b}\rho w\left(x\right)s\left(x\right)dx.$

Evaluating this integral gives us the force on the plate. We summarize this in the following problem-solving strategy.

## Problem-solving strategy: finding hydrostatic force

1. Sketch a picture and select an appropriate frame of reference. (Note that if we select a frame of reference other than the one used earlier, we may have to adjust [link] accordingly.)
2. Determine the depth and width functions, $s\left(x\right)$ and $w\left(x\right).$
3. Determine the weight-density of whatever liquid with which you are working. The weight-density of water is $62.4$ lb/ft 3 , or 9800 N/m 3 .
4. Use the equation to calculate the total force.

## Finding hydrostatic force

A water trough 15 ft long has ends shaped like inverted isosceles triangles, with base 8 ft and height 3 ft. Find the force on one end of the trough if the trough is full of water.

[link] shows the trough and a more detailed view of one end.

Select a frame of reference with the $x\text{-axis}$ oriented vertically and the downward direction being positive. Select the top of the trough as the point corresponding to $x=0$ (step 1). The depth function, then, is $s\left(x\right)=x.$ Using similar triangles, we see that $w\left(x\right)=8-\left(8\text{/}3\right)x$ (step 2). Now, the weight density of water is $62.4$ lb/ft 3 (step 3), so applying [link] , we obtain

$\begin{array}{cc}\hfill F& ={\int }_{a}^{b}\rho w\left(x\right)s\left(x\right)dx\hfill \\ & ={\int }_{0}^{3}62.4\left(8-\frac{8}{3}x\right)x\phantom{\rule{0.2em}{0ex}}dx=62.4{\int }_{0}^{3}\left(8x-\frac{8}{3}{x}^{2}\right)dx\hfill \\ & =62.4{\left[4{x}^{2}-\frac{8}{9}{x}^{3}\right]\phantom{\rule{0.2em}{0ex}}|}_{0}^{3}=748.8.\hfill \end{array}$

The water exerts a force of 748.8 lb on the end of the trough (step 4).

A water trough 12 m long has ends shaped like inverted isosceles triangles, with base 6 m and height 4 m. Find the force on one end of the trough if the trough is full of water.

$156,800$ N

## Chapter opener: finding hydrostatic force

We now return our attention to the Hoover Dam , mentioned at the beginning of this chapter. The actual dam is arched, rather than flat, but we are going to make some simplifying assumptions to help us with the calculations. Assume the face of the Hoover Dam is shaped like an isosceles trapezoid with lower base $750$ ft, upper base $1250$ ft, and height $750$ ft (see the following figure).

When the reservoir is full, Lake Mead’s maximum depth is about 530 ft, and the surface of the lake is about 10 ft below the top of the dam (see the following figure).

1. Find the force on the face of the dam when the reservoir is full.
2. The southwest United States has been experiencing a drought, and the surface of Lake Mead is about 125 ft below where it would be if the reservoir were full. What is the force on the face of the dam under these circumstances?
1. We begin by establishing a frame of reference. As usual, we choose to orient the $x\text{-axis}$ vertically, with the downward direction being positive. This time, however, we are going to let $x=0$ represent the top of the dam, rather than the surface of the water. When the reservoir is full, the surface of the water is $10$ ft below the top of the dam, so $s\left(x\right)=x-10$ (see the following figure).

To find the width function, we again turn to similar triangles as shown in the figure below.

From the figure, we see that $w\left(x\right)=750+2r.$ Using properties of similar triangles, we get $r=250-\left(1\text{/}3\right)x.$ Thus,
$w\left(x\right)=1250-\frac{2}{3}x\phantom{\rule{0.2em}{0ex}}\text{(step 2).}$

Using a weight-density of $62.4$ lb/ft 3 (step 3) and applying [link] , we get
$\begin{array}{cc}\hfill F& ={\int }_{a}^{b}\rho w\left(x\right)s\left(x\right)dx\hfill \\ & ={\int }_{10}^{540}62.4\left(1250-\frac{2}{3}x\right)\left(x-10\right)dx=62.4{\int }_{10}^{540}-\frac{2}{3}\left[{x}^{2}-1885x+18750\right]dx\hfill \\ & =-62.4\left(\frac{2}{3}\right){\left[\frac{{x}^{3}}{3}-\frac{1885{x}^{2}}{2}+18750x\right]\phantom{\rule{0.2em}{0ex}}|}_{10}^{540}\approx 8,832,245,000\phantom{\rule{0.2em}{0ex}}\text{lb}=4,416,122.5\phantom{\rule{0.2em}{0ex}}\text{t}\text{.}\hfill \end{array}$

Note the change from pounds to tons $\left(2000$ lb = $1$ ton) (step 4). This changes our depth function, $s\left(x\right),$ and our limits of integration. We have $s\left(x\right)=x-135.$ The lower limit of integration is $135.$ The upper limit remains $540.$ Evaluating the integral, we get

$\begin{array}{cc}\hfill F& ={\int }_{a}^{b}\rho w\left(x\right)s\left(x\right)dx\hfill \\ & ={\int }_{135}^{540}62.4\left(1250-\frac{2}{3}x\right)\left(x-135\right)dx\hfill \\ & =-62.4\left(\frac{2}{3}\right){\int }_{135}^{540}\left(x-1875\right)\left(x-135\right)dx=-62.4\left(\frac{2}{3}\right){\int }_{135}^{540}\left({x}^{2}-2010x+253125\right)dx\hfill \\ & =-62.4\left(\frac{2}{3}\right){\left[\frac{{x}^{3}}{3}-1005{x}^{2}+253125x\right]\phantom{\rule{0.2em}{0ex}}|}_{135}^{540}\approx 5,015,230,000\phantom{\rule{0.2em}{0ex}}\text{lb}=\phantom{\rule{0.2em}{0ex}}2,507,615\phantom{\rule{0.2em}{0ex}}\text{t}\text{.}\hfill \end{array}$

Find the arc length of the graph of f(x) = In (sinx) on the interval [Π/4, Π/2].
Sand falling freely from a lorry form a conical shape whose height is always equal to one-third the radius of the base. a. How fast is the volume increasing when the radius of the base is (1m) and increasing at the rate of 1/4cm/sec Pls help me solve
show that lim f(x) + lim g(x)=m+l
list the basic elementary differentials
Differentiation and integration
yes
Damien
proper definition of derivative
the maximum rate of change of one variable with respect to another variable
terms of an AP is 1/v and the vth term is 1/u show that the sum of uv terms is 1/2(uv+1)
what is calculus?
calculus is math that studies the change in math, such as the rate and distance,
Tamarcus
what are the topics in calculus
Augustine
what is limit of a function?
what is x and how x=9.1 take?
what is f(x)
the function at x
Marc
also known as the y value so I could say y=2x or f(x)= 2x same thing just using functional notation your next question is what is dependent and independent variables. I am Dyslexic but know math and which is which confuses me. but one can vary the x value while y depends on which x you use. also
Marc
up domain and range
Marc
enjoy your work and good luck
Marc
I actually wanted to ask another questions on sets if u dont mind please?
Inembo
I have so many questions on set and I really love dis app I never believed u would reply
Inembo
Hmm go ahead and ask you got me curious too much conversation here
am sorry for disturbing I really want to know math that's why *I want to know the meaning of those symbols in sets* e.g n,U,A', etc pls I want to know it and how to solve its problems
Inembo
and how can i solve a question like dis *in a group of 40 students, 32 offer maths and 24 offer physics and 4 offer neither maths nor physics , how many offer both maths and physics*
Inembo
next questions what do dy mean by (A' n B^c)^c'
Inembo
The sets help you to define the function. The function is like a magic box where you put inside stuff(numbers or sets) and you get out the stuff but in different shapes (forms).
I dont understand what you wanna say by (A' n B^c)^c'
(A' n B (rise to the power of c)) all rise to the power of c
Inembo
Aaaahh
Ok so the set is formed by vectors and not numbers
A vector of length n
But you can make a set out of matrixes as well
I I don't even understand sets I wat to know d meaning of all d symbolsnon sets
Inembo
High-school?
yes
Inembo
am having big problem understanding sets more than other math topics
Inembo
So f:R->R means that the function takes real numbers and provides real numer. For ex. If f(x) =2x this means if you give to your function a real number like 2,it gives you also a real number 2times2=4
pls answer this question *in a group of 40 students, 32 offer maths and 24 offer physics and 4 offer neither maths nor physics , how many offer both maths and physics*
Inembo
If you have f:R^n->R^n you give to your function a vector of length n like (a1,a2,...an) where all a1,.. an are reals and gives you also a vector of length n... I don't know if i answering your question. Otherwise on YouTube you havr many videos where they explain it in a simple way
I would say 24
Offer both
Sorry 20
Actually you have 40 - 4 =36 who offer maths or physics or both.
I know its 20 but how to prove it
Inembo
You have 32+24=56who offer courses
56-36=20 who give both courses... I would say that
solution: In a question involving sets and Venn diagram, the sum of the members of set A + set B - the joint members of both set A and B + the members that are not in sets A or B = the total members of the set. In symbolic form n(A U B) = n(A) + n (B) - n (A and B) + n (A U B)'.
Mckenzie
In the case of sets A and B use the letters m and p to represent the sets and we have: n (M U P) = 40; n (M) = 24; n (P) = 32; n (M and P) = unknown; n (M U P)' = 4
Mckenzie
Now substitute the numerical values for the symbolic representation 40 = 24 + 32 - n(M and P) + 4 Now solve for the unknown using algebra: 40 = 24 + 32+ 4 - n(M and P) 40 = 60 - n(M and P) Add n(M and P), as well, subtract 40 from both sides of the equation to find the answer.
Mckenzie
40 - 40 + n(M and P) = 60 - 40 - n(M and P) + n(M and P) Solution: n(M and P) = 20
Mckenzie
thanks
Inembo
Simpler form: Add the sums of set M, set P and the complement of the union of sets M and P then subtract the number of students from the total.
Mckenzie
n(M and P) = (32 + 24 + 4) - 40 = 60 - 40 = 20
Mckenzie
how do i evaluate integral of x^1/2 In x
first you simplify the given expression, which gives (x^2/2). Then you now integrate the above simplified expression which finally gives( lnx^2).
by using integration product formula
Roha
find derivative f(x)=1/x
-1/x^2, use the chain rule
Andrew
f(x)=x^3-2x
Mul
what is domin in this question
noman
all real numbers . except zero
Roha
please try to guide me how?
Meher
what do u want to ask
Roha
?
Roha
the domain of the function is all real number excluding zero, because the rational function 1/x is a representation of a fractional equation (precisely inverse function). As in elementary mathematics the concept of dividing by zero is nonexistence, so zero will not make the fractional statement
Mckenzie
a function's answer/range should not be in the form of 1/0 and there should be no imaginary no. say square root of any negative no. (-1)^1/2
Roha
domain means everywhere along the x axis. since this function is not discontinuous anywhere along the x axis, then the domain is said to be all values of x.
Andrew
Derivative of a function
Waqar
right andrew ... this function is only discontinuous at 0
Roha
of sorry, I didn't realize he was taking about the function 1/x ...I thought he was referring to the function x^3-2x.
Andrew
yep...it's 1/x...!!!
Roha
true and cannot be apart of the domain that makes up the relation of the graph y = 1/x. The value of the denominator of the rational function can never be zero, because the result of the output value (range value of the graph when x =0) is undefined.
Mckenzie
👍
Roha
Therefore, when x = 0 the image of the rational function does not exist at this domain value, but exist at all other x values (domain) that makes the equation functional, and the graph drawable.
Mckenzie
👍
Roha
Roha are u A Student
Lutf
yes
Roha
What is the first fundermental theory of Calculus?
do u mean fundamental theorem ?
Roha
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