# 7.6 Modeling with trigonometric equations  (Page 4/14)

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## Simple harmonic motion

A type of motion described as simple harmonic motion    involves a restoring force but assumes that the motion will continue forever. Imagine a weighted object hanging on a spring, When that object is not disturbed, we say that the object is at rest, or in equilibrium. If the object is pulled down and then released, the force of the spring pulls the object back toward equilibrium and harmonic motion begins. The restoring force is directly proportional to the displacement of the object from its equilibrium point. When $\text{\hspace{0.17em}}t=0,d=0.$

## Simple harmonic motion

We see that simple harmonic motion    equations are given in terms of displacement:

where $\text{\hspace{0.17em}}|a|\text{\hspace{0.17em}}$ is the amplitude, $\text{\hspace{0.17em}}\frac{2\pi }{\omega }\text{\hspace{0.17em}}$ is the period, and $\text{\hspace{0.17em}}\frac{\omega }{2\pi }\text{\hspace{0.17em}}$ is the frequency, or the number of cycles per unit of time.

## Finding the displacement, period, and frequency, and graphing a function

For the given functions,

1. Find the maximum displacement of an object.
2. Find the period or the time required for one vibration.
3. Find the frequency.
4. Sketch the graph.
1. $y=5\text{\hspace{0.17em}}\mathrm{sin}\left(3t\right)$
2. $y=6\text{\hspace{0.17em}}\mathrm{cos}\left(\pi t\right)$
3. $y=5\text{\hspace{0.17em}}\mathrm{cos}\left(\frac{\pi }{2}t\right)$
1. $y=5\text{\hspace{0.17em}}\mathrm{sin}\left(3t\right)$
1. The maximum displacement is equal to the amplitude, $\text{\hspace{0.17em}}|a|,$ which is 5.
2. The period is $\text{\hspace{0.17em}}\frac{2\pi }{\omega }=\frac{2\pi }{3}.$
3. The frequency is given as $\text{\hspace{0.17em}}\frac{\omega }{2\pi }=\frac{3}{2\pi }.$
4. See [link] . The graph indicates the five key points.
2. $y=6\text{\hspace{0.17em}}\mathrm{cos}\left(\pi t\right)$
1. The maximum displacement is $\text{\hspace{0.17em}}6.$
2. The period is $\text{\hspace{0.17em}}\frac{2\pi }{\omega }=\frac{2\pi }{\pi }=2.$
3. The frequency is $\text{\hspace{0.17em}}\frac{\omega }{2\pi }=\frac{\pi }{2\pi }=\frac{1}{2}.$
4. See [link] .
3. $y=5\text{\hspace{0.17em}}\mathrm{cos}\left(\frac{\pi }{2}\right)\text{\hspace{0.17em}}t$
1. The maximum displacement is $\text{\hspace{0.17em}}5.$
2. The period is $\text{\hspace{0.17em}}\frac{2\pi }{\omega }=\frac{2\pi }{\frac{\pi }{2}}=4.$
3. The frequency is $\text{\hspace{0.17em}}\frac{1}{4}.$
4. See [link] .

## Damped harmonic motion

In reality, a pendulum does not swing back and forth forever, nor does an object on a spring bounce up and down forever. Eventually, the pendulum stops swinging and the object stops bouncing and both return to equilibrium. Periodic motion in which an energy-dissipating force, or damping factor, acts is known as damped harmonic motion    . Friction is typically the damping factor.

In physics, various formulas are used to account for the damping factor on the moving object. Some of these are calculus-based formulas that involve derivatives. For our purposes, we will use formulas for basic damped harmonic motion models.

## Damped harmonic motion

In damped harmonic motion    , the displacement of an oscillating object from its rest position at time $\text{\hspace{0.17em}}t\text{\hspace{0.17em}}$ is given as

where $\text{\hspace{0.17em}}c\text{\hspace{0.17em}}$ is a damping factor, $\text{\hspace{0.17em}}|a|\text{\hspace{0.17em}}$ is the initial displacement and $\text{\hspace{0.17em}}\frac{2\pi }{\omega }\text{\hspace{0.17em}}$ is the period.

## Modeling damped harmonic motion

Model the equations that fit the two scenarios and use a graphing utility to graph the functions: Two mass-spring systems exhibit damped harmonic motion at a frequency of $\text{\hspace{0.17em}}0.5\text{\hspace{0.17em}}$ cycles per second. Both have an initial displacement of 10 cm. The first has a damping factor of $\text{\hspace{0.17em}}0.5\text{\hspace{0.17em}}$ and the second has a damping factor of $\text{\hspace{0.17em}}0.1.$

At time $\text{\hspace{0.17em}}t=0,$ the displacement is the maximum of 10 cm, which calls for the cosine function. The cosine function will apply to both models.

We are given the frequency $\text{\hspace{0.17em}}f=\frac{\omega }{2\pi }\text{\hspace{0.17em}}$ of 0.5 cycles per second. Thus,

The first spring system has a damping factor of $\text{\hspace{0.17em}}c=0.5.\text{\hspace{0.17em}}$ Following the general model for damped harmonic motion, we have

$f\left(t\right)=10{e}^{-0.5t}\mathrm{cos}\left(\pi t\right)$

[link] models the motion of the first spring system.

The second spring system has a damping factor of $\text{\hspace{0.17em}}c=0.1\text{\hspace{0.17em}}$ and can be modeled as

$f\left(t\right)=10{e}^{-0.1t}\mathrm{cos}\left(\pi t\right)$

[link] models the motion of the second spring system.

#### Questions & Answers

how fast can i understand functions without much difficulty
Joe Reply
what is set?
Kelvin Reply
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
100•3=300 300=50•2^x 6=2^x x=log_2(6) =2.5849625 so, 300=50•2^2.5849625 and, so, the # of bacteria will double every (100•2.5849625) = 258.49625 minutes
Thomas
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
yeah, it does. why do we attempt to gain all of them one side or the other?
Melissa
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
Spiro; thanks for putting it out there like that, 😁
Melissa
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
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Hinderson 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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