# 7.6 Modeling with trigonometric equations  (Page 6/14)

 Page 6 / 14

## Bounding curves in harmonic motion

Harmonic motion graphs may be enclosed by bounding curves. When a function has a varying amplitude    , such that the amplitude rises and falls multiple times within a period, we can determine the bounding curves from part of the function.

## Graphing an oscillating cosine curve

Graph the function $\text{\hspace{0.17em}}f\left(x\right)=\mathrm{cos}\left(2\pi x\right)\mathrm{cos}\left(16\pi x\right).$

The graph produced by this function will be shown in two parts. The first graph will be the exact function $\text{\hspace{0.17em}}f\left(x\right)\text{\hspace{0.17em}}$ (see [link] ), and the second graph is the exact function $\text{\hspace{0.17em}}f\left(x\right)\text{\hspace{0.17em}}$ plus a bounding function (see [link] . The graphs look quite different.

Access these online resources for additional instruction and practice with trigonometric applications.

Visit this website for additional practice questions from Learningpod.

## Key equations

 Standard form of sinusoidal equation $y=A\text{\hspace{0.17em}}\mathrm{sin}\left(Bt-C\right)+D\text{\hspace{0.17em}}\text{or}\text{\hspace{0.17em}}y=A\text{\hspace{0.17em}}\mathrm{cos}\left(Bt-C\right)+D$ Simple harmonic motion Damped harmonic motion $f\left(t\right)=a{e}^{-c}{}^{t}\mathrm{sin}\left(\omega t\right)\text{\hspace{0.17em}}\text{or}\text{\hspace{0.17em}}f\left(t\right)=a{e}^{-ct}\mathrm{cos}\left(\omega t\right)$

## Key concepts

• Sinusoidal functions are represented by the sine and cosine graphs. In standard form, we can find the amplitude, period, and horizontal and vertical shifts. See [link] and [link] .
• Use key points to graph a sinusoidal function. The five key points include the minimum and maximum values and the midline values. See [link] .
• Periodic functions can model events that reoccur in set cycles, like the phases of the moon, the hands on a clock, and the seasons in a year. See [link] , [link] , [link] and [link] .
• Harmonic motion functions are modeled from given data. Similar to periodic motion applications, harmonic motion requires a restoring force. Examples include gravitational force and spring motion activated by weight. See [link] .
• Damped harmonic motion is a form of periodic behavior affected by a damping factor. Energy dissipating factors, like friction, cause the displacement of the object to shrink. See [link] , [link] , [link] , [link] , and [link] .
• Bounding curves delineate the graph of harmonic motion with variable maximum and minimum values. See [link] .

## Verbal

Explain what types of physical phenomena are best modeled by sinusoidal functions. What are the characteristics necessary?

Physical behavior should be periodic, or cyclical.

What information is necessary to construct a trigonometric model of daily temperature? Give examples of two different sets of information that would enable modeling with an equation.

If we want to model cumulative rainfall over the course of a year, would a sinusoidal function be a good model? Why or why not?

Since cumulative rainfall is always increasing, a sinusoidal function would not be ideal here.

Explain the effect of a damping factor on the graphs of harmonic motion functions.

## Algebraic

For the following exercises, find a possible formula for the trigonometric function represented by the given table of values.

 $x$ $y$ $0$ $-4$ $3$ $-1$ $6$ $2$ $9$ $-1$ $12$ $-4$ $15$ $-1$ $18$ $2$

$y=-3\mathrm{cos}\left(\frac{\pi }{6}x\right)-1$

 $x$ $y$ $0$ $5$ $2$ $1$ $4$ $-3$ $6$ $1$ $8$ $5$ $10$ $1$ $12$ $-3$
 $x$ $y$ $0$ $2$ $\frac{\pi }{4}$ $7$ $\frac{\pi }{2}$ $2$ $\frac{3\pi }{4}$ $-3$ $\pi$ $2$ $\frac{5\pi }{4}$ $7$ $\frac{3\pi }{2}$ $2$

$5\mathrm{sin}\left(2x\right)+2$

 $x$ $y$ $0$ $2$ $\frac{\pi }{4}$ $7$ $\frac{\pi }{2}$ $2$ $\frac{3\pi }{4}$ $-3$ $\pi$ $2$ $\frac{5\pi }{4}$ $7$ $\frac{3\pi }{2}$ $2$
 $x$ $y$ $0$ $1$ $1$ $-3$ $2$ $-7$ $3$ $-3$ $4$ $1$ $5$ $-3$ $6$ $-7$

$4\mathrm{cos}\left(\frac{x\pi }{2}\right)-3$

 $x$ $y$ $0$ $-2$ $1$ $4$ $2$ $10$ $3$ $4$ $4$ $-2$ $5$ $4$ $6$ $10$

#### Questions & Answers

a colony of bacteria is growing exponentially doubling in size every 100 minutes. how much minutes will it take for the colony of bacteria to triple in size
Divya Reply
I got 300 minutes. is it right?
Patience
no. should be about 150 minutes.
Jason
It should be 158.5 minutes.
Mr
ok, thanks
Patience
what is the importance knowing the graph of circular functions?
Arabella Reply
can get some help basic precalculus
ismail Reply
What do you need help with?
Andrew
how to convert general to standard form with not perfect trinomial
Camalia Reply
can get some help inverse function
ismail
Rectangle coordinate
Asma Reply
how to find for x
Jhon Reply
it depends on the equation
Robert
whats a domain
mike Reply
The domain of a function is the set of all input on which the function is defined. For example all real numbers are the Domain of any Polynomial function.
Spiro
foci (–7,–17) and (–7,17), the absolute value of the differenceof the distances of any point from the foci is 24.
Churlene Reply
difference between calculus and pre calculus?
Asma Reply
give me an example of a problem so that I can practice answering
Jenefa Reply
x³+y³+z³=42
Robert
dont forget the cube in each variable ;)
Robert
of she solves that, well ... then she has a lot of computational force under her command ....
Walter
what is a function?
CJ Reply
I want to learn about the law of exponent
Quera Reply
explain this
Hinderson Reply
what is functions?
Angel Reply
A mathematical relation such that every input has only one out.
Spiro
yes..it is a relationo of orders pairs of sets one or more input that leads to a exactly one output.
Mubita
Is a rule that assigns to each element X in a set A exactly one element, called F(x), in a set B.
RichieRich
If the plane intersects the cone (either above or below) horizontally, what figure will be created?
Feemark Reply

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Source:  OpenStax, Precalculus. OpenStax CNX. Jan 19, 2016 Download for free at https://legacy.cnx.org/content/col11667/1.6
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