# 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 $\text{\hspace{0.17em}}\frac{\omega }{2\pi }=\frac{\pi }{2\pi }=\frac{1}{2}.$
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 $\text{\hspace{0.17em}}\frac{1}{4}.$

## 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.

is precalculus needed to take caculus
It depends on what you already know. Just test yourself with some precalculus questions. If you find them easy, you're good to go.
Spiro
the solution doesn't seem right for this problem
what is the domain of f(x)=x-4/x^2-2x-15 then
x is different from -5&3
Seid
how to prroved cos⁴x-sin⁴x= cos²x-sin²x are equal
Don't think that you can.
Elliott
how do you provided cos⁴x-sin⁴x = cos²x-sin²x are equal
What are the question marks for?
Elliott
Someone should please solve it for me Add 2over ×+3 +y-4 over 5 simplify (×+a)with square root of two -×root 2 all over a multiply 1over ×-y{(×-y)(×+y)} over ×y
For the first question, I got (3y-2)/15 Second one, I got Root 2 Third one, I got 1/(y to the fourth power) I dont if it's right cause I can barely understand the question.
Is under distribute property, inverse function, algebra and addition and multiplication function; so is a combined question
Abena
find the equation of the line if m=3, and b=-2
graph the following linear equation using intercepts method. 2x+y=4
Ashley
how
Wargod
what?
John
ok, one moment
UriEl
how do I post your graph for you?
UriEl
it won't let me send an image?
UriEl
also for the first one... y=mx+b so.... y=3x-2
UriEl
y=mx+b you were already given the 'm' and 'b'. so.. y=3x-2
Tommy
Please were did you get y=mx+b from
Abena
y=mx+b is the formula of a straight line. where m = the slope & b = where the line crosses the y-axis. In this case, being that the "m" and "b", are given, all you have to do is plug them into the formula to complete the equation.
Tommy
thanks Tommy
Nimo
0=3x-2 2=3x x=3/2 then . y=3/2X-2 I think
Given
co ordinates for x x=0,(-2,0) x=1,(1,1) x=2,(2,4)
neil
"7"has an open circle and "10"has a filled in circle who can I have a set builder notation
x=-b+_Гb2-(4ac) ______________ 2a
I've run into this: x = r*cos(angle1 + angle2) Which expands to: x = r(cos(angle1)*cos(angle2) - sin(angle1)*sin(angle2)) The r value confuses me here, because distributing it makes: (r*cos(angle2))(cos(angle1) - (r*sin(angle2))(sin(angle1)) How does this make sense? Why does the r distribute once
so good
abdikarin
this is an identity when 2 adding two angles within a cosine. it's called the cosine sum formula. there is also a different formula when cosine has an angle minus another angle it's called the sum and difference formulas and they are under any list of trig identities
strategies to form the general term
carlmark
consider r(a+b) = ra + rb. The a and b are the trig identity.
Mike
How can you tell what type of parent function a graph is ?
generally by how the graph looks and understanding what the base parent functions look like and perform on a graph
William
if you have a graphed line, you can have an idea by how the directions of the line turns, i.e. negative, positive, zero
William
y=x will obviously be a straight line with a zero slope
William
y=x^2 will have a parabolic line opening to positive infinity on both sides of the y axis vice versa with y=-x^2 you'll have both ends of the parabolic line pointing downward heading to negative infinity on both sides of the y axis
William
y=x will be a straight line, but it will have a slope of one. Remember, if y=1 then x=1, so for every unit you rise you move over positively one unit. To get a straight line with a slope of 0, set y=1 or any integer.
Aaron
yes, correction on my end, I meant slope of 1 instead of slope of 0
William
what is f(x)=
I don't understand
Joe
Typically a function 'f' will take 'x' as input, and produce 'y' as output. As 'f(x)=y'. According to Google, "The range of a function is the complete set of all possible resulting values of the dependent variable (y, usually), after we have substituted the domain."
Thomas
Sorry, I don't know where the "Â"s came from. They shouldn't be there. Just ignore them. :-)
Thomas
Darius
Thanks.
Thomas
Â
Thomas
It is the Â that should not be there. It doesn't seem to show if encloses in quotation marks. "Â" or 'Â' ... Â
Thomas
Now it shows, go figure?
Thomas
what is this?
i do not understand anything
unknown
lol...it gets better
Darius
I've been struggling so much through all of this. my final is in four weeks 😭
Tiffany
this book is an excellent resource! have you guys ever looked at the online tutoring? there's one that is called "That Tutor Guy" and he goes over a lot of the concepts
Darius
thank you I have heard of him. I should check him out.
Tiffany
is there any question in particular?
Joe
I have always struggled with math. I get lost really easy, if you have any advice for that, it would help tremendously.
Tiffany
Sure, are you in high school or college?
Darius
Hi, apologies for the delayed response. I'm in college.
Tiffany
how to solve polynomial using a calculator
So a horizontal compression by factor of 1/2 is the same as a horizontal stretch by a factor of 2, right?