# 4.1 Related rates  (Page 4/7)

 Page 4 / 7

## Water draining from a funnel

Water is draining from the bottom of a cone-shaped funnel at the rate of $0.0{3\phantom{\rule{0.2em}{0ex}}\text{ft}}^{3}\text{/sec}.$ The height of the funnel is 2 ft and the radius at the top of the funnel is $1\phantom{\rule{0.2em}{0ex}}\text{ft}.$ At what rate is the height of the water in the funnel changing when the height of the water is $\frac{1}{2}\phantom{\rule{0.2em}{0ex}}\text{ft}?$

Step 1: Draw a picture introducing the variables. Water is draining from a funnel of height 2 ft and radius 1 ft. The height of the water and the radius of water are changing over time. We denote these quantities with the variables h and r , respectively.

Let $h$ denote the height of the water in the funnel, $r$ denote the radius of the water at its surface, and $V$ denote the volume of the water.

Step 2: We need to determine $\frac{dh}{dt}$ when $h=\frac{1}{2}\phantom{\rule{0.2em}{0ex}}\text{ft}.$ We know that $\frac{dV}{dt}=-0.03\phantom{\rule{0.2em}{0ex}}\text{ft/sec}.$

Step 3: The volume of water in the cone is

$V=\frac{1}{3}\pi {r}^{2}h.$

From the figure, we see that we have similar triangles. Therefore, the ratio of the sides in the two triangles is the same. Therefore, $\frac{r}{h}=\frac{1}{2}$ or $r=\frac{h}{2}.$ Using this fact, the equation for volume can be simplified to

$V=\frac{1}{3}\pi {\left(\frac{h}{2}\right)}^{2}h=\frac{\pi }{12}\phantom{\rule{0.1em}{0ex}}{h}^{3}.$

Step 4: Applying the chain rule while differentiating both sides of this equation with respect to time $t,$ we obtain

$\frac{dV}{dt}=\frac{\pi }{4}\phantom{\rule{0.1em}{0ex}}{h}^{2}\frac{dh}{dt}.$

Step 5: We want to find $\frac{dh}{dt}$ when $h=\frac{1}{2}\phantom{\rule{0.2em}{0ex}}\text{ft}.$ Since water is leaving at the rate of $0.0{3\phantom{\rule{0.2em}{0ex}}\text{ft}}^{3}\text{/sec},$ we know that $\frac{dV}{dt}=-0.03{\phantom{\rule{0.2em}{0ex}}\text{ft}}^{3}\text{/sec}.$ Therefore,

$-0.03=\frac{\pi }{4}{\left(\frac{1}{2}\right)}^{2}\frac{dh}{dt}\text{},$

which implies

$-0.03=\frac{\pi }{16}\phantom{\rule{0.2em}{0ex}}\frac{dh}{dt}.$

It follows that

$\frac{dh}{dt}=-\frac{0.48}{\pi }=-0.153\phantom{\rule{0.2em}{0ex}}\text{ft/sec}.$

At what rate is the height of the water changing when the height of the water is $\frac{1}{4}\phantom{\rule{0.2em}{0ex}}\text{ft}?$

$-0.61\phantom{\rule{0.2em}{0ex}}\text{ft/sec}$

## Key concepts

• To solve a related rates problem, first draw a picture that illustrates the relationship between the two or more related quantities that are changing with respect to time.
• In terms of the quantities, state the information given and the rate to be found.
• Find an equation relating the quantities.
• Use differentiation, applying the chain rule as necessary, to find an equation that relates the rates.
• Be sure not to substitute a variable quantity for one of the variables until after finding an equation relating the rates.

For the following exercises, find the quantities for the given equation.

Find $\frac{dy}{dt}$ at $x=1$ and $y={x}^{2}+3$ if $\frac{dx}{dt}=4.$

$8$

Find $\frac{dx}{dt}$ at $x=-2$ and $y=2{x}^{2}+1$ if $\frac{dy}{dt}=-1.$

Find $\frac{dz}{dt}$ at $\left(x,y\right)=\left(1,3\right)$ and ${z}^{2}={x}^{2}+{y}^{2}$ if $\frac{dx}{dt}=4$ and $\frac{dy}{dt}=3.$

$\frac{13}{\sqrt{10}}$

For the following exercises, sketch the situation if necessary and used related rates to solve for the quantities.

[T] If two electrical resistors are connected in parallel, the total resistance (measured in ohms, denoted by the Greek capital letter omega, $\text{Ω}\right)$ is given by the equation $\frac{1}{R}=\frac{1}{{R}_{1}}+\frac{1}{{R}_{2}}.$ If ${R}_{1}$ is increasing at a rate of $0.5\phantom{\rule{0.2em}{0ex}}\text{Ω}\text{/}\text{min}$ and ${R}_{2}$ decreases at a rate of $1.1\text{Ω/min},$ at what rate does the total resistance change when ${R}_{1}=20\text{Ω}$ and ${R}_{2}=50\text{Ω}\text{/}\text{min}?$

A 10-ft ladder is leaning against a wall. If the top of the ladder slides down the wall at a rate of 2 ft/sec, how fast is the bottom moving along the ground when the bottom of the ladder is 5 ft from the wall? $2\sqrt{3}$ ft/sec

A 25-ft ladder is leaning against a wall. If we push the ladder toward the wall at a rate of 1 ft/sec, and the bottom of the ladder is initially $20\phantom{\rule{0.2em}{0ex}}\text{ft}$ away from the wall, how fast does the ladder move up the wall $5\phantom{\rule{0.2em}{0ex}}\text{sec}$ after we start pushing?

#### Questions & Answers

find the domain and range of f(x)= 4x-7/x²-6x+8
Nick Reply
find the range of f(x)=(x+1)(x+4)
Jane Reply
-1, -4
Marcia
That's domain. The range is [-9/4,+infinity)
Jacob
If you're using calculus to find the range, you have to find the extrema through the first derivative test and then substitute the x-value for the extrema back into the original equation.
Jacob
Good morning,,, how are you
Harrieta Reply
d/dx{1/y - lny + X^3.Y^5}
mogomotsi Reply
How to identify domain and range
Umar Reply
hello
Akpevwe
He,,
Harrieta
hi
Dr
hello
velocity
I only talk to girls
Dr
women are smart then guys
Dr
Smarter
Adri
sorry
Dr
hi adri ana
Dr
:(
Shun
was up
Dr
hello
Adarsh
is it chatting app?.. I do not see any calculus here. lol
Adarsh
Find the arc length of the graph of f(x) = In (sinx) on the interval [Π/4, Π/2].
mukul Reply
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
ade
show that lim f(x) + lim g(x)=m+l
BARNABAS Reply
list the basic elementary differentials
Chio Reply
Differentiation and integration
Okikiola Reply
yes
Damien
proper definition of derivative
Syed Reply
the maximum rate of change of one variable with respect to another variable
Amdad
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)
Inembo Reply
what is calculus?
BISWAJIT Reply
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?
Geoffrey Reply
what is x and how x=9.1 take?
Pravin Reply
what is f(x)
Inembo Reply
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
Adri
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).
Adri
I dont understand what you wanna say by (A' n B^c)^c'
Adri
(A' n B (rise to the power of c)) all rise to the power of c
Inembo
Aaaahh
Adri
Ok so the set is formed by vectors and not numbers
Adri
A vector of length n
Adri
But you can make a set out of matrixes as well
Adri
I I don't even understand sets I wat to know d meaning of all d symbolsnon sets
Inembo
Wait what's your math level?
Adri
High-school?
Adri
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
Adri
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
Adri
I would say 24
Adri
Offer both
Adri
Sorry 20
Adri
Actually you have 40 - 4 =36 who offer maths or physics or both.
Adri
I know its 20 but how to prove it
Inembo
You have 32+24=56who offer courses
Adri
56-36=20 who give both courses... I would say that
Adri
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

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