Description

5 Questions total

need to follow the instructions

could provide the lecture note if need

ENGM2022: Assignment 3: Winter Term 2021

NOTE: To receive any points, follow what the problem says regarding

how solutions are to be constructed. Use our 4-step MUD (Method of

Undetermined Coefficients) for inhomogeneous problems after problem 1

– Step 1: Get homogeneous solution yh , Step 2: Get particular solution

yp , Step 3: Satisfy conditions on y = yh + yp , Step 4: Sketch y. Show your

work. The sketch featues always need to be justified and this is spelled

out in each problem as clearly as possible.

1. (20 points) Find the solution of yÃŒË† + 2cyÃŒâ€¡ + 2y = 0, y(0) = y0 , yÃŒâ€¡(0) = v0 . In your

solution clearly label the following steps: Step 1: Characteristic Equation, Step

2: General Solution, Step 3: Satisfy conditions, Step 4: Sketch.

(a) (1 pt) Choose c > 0 so that the model is critically damped.

(b) (3 pt) Solve the critically-damped model, but do not sketch, for y(0) = y0 ,

yÃŒâ€¡(0) = v0 and show that

y(t) = (y0 + (v0 + cy0 )t)) eÃ¢Ë†â€™ct

(1)

(c) (3 pt) Now sketch critically-damped y(t) assuming that y0 = 1 and v0 =

Ã¢Ë†â€™2. In your sketch youÃ¢â‚¬â„¢ll need to determine if y can go below y = 0. To

receive any sketch points use pure logic without calculus to show whether y

goes below zero. Complete the sketch with that single piece of information

– and the facts we outlined in class about the number of zero crossings

available to damped solutions of second order differential equations.

(d) (4 pt) Assume that c is large enough to make the model over-damped.

Find the solution y(t) and do not sketch. Show that

(y0 ÃŽÂ»2 Ã¢Ë†â€™ v0 )eÃŽÂ»1 t + (v0 Ã¢Ë†â€™ y0 ÃŽÂ»1 )eÃŽÂ»2 t

ÃŽÂ»2 Ã¢Ë†â€™ ÃŽÂ»1

Ã¢Ë†Å¡

Ã¢Ë†Å¡

where ÃŽÂ»1 = Ã¢Ë†â€™c + c2 Ã¢Ë†â€™ 2 and ÃŽÂ»2 = Ã¢Ë†â€™c Ã¢Ë†â€™ c2 Ã¢Ë†â€™ 2.

y=

(2)

(e) (2 pt) Show that the over-damped solution satisfies the conditions y(0) =

y0 , yÃŒâ€¡(0) = v0 .

(f) (1 pt) Refer to the over-damped solution and find a formula to choose y0

and v0 so that the solution goes to zero as rapidly as possible – this will

require the coefficient of the slowest part of y to be zero.

(g) (6 pt) Assume that c is small enough to make the model under-damped.

Show that

q

y(t) = eÃ¢Ë†â€™ct y02 + ((v0 + cy0 )/Ãâ€°)2 cos(Ãâ€°t Ã¢Ë†â€™ Ãâ€ )

(3)

Ã¢Ë†Å¡

where Ãâ€° = 2 Ã¢Ë†â€™ c2 and tan Ãâ€ = ((v0 + cy0 )/Ãâ€°)/y0 . In your sketch please

draw the solution envelope and identify the psuedo-period.

2. (10 points) Consider Problem #4 Assignment 2. Assume that the person in the

question is not a woman being tested for gestational diabeters but is a child

that repeatedly asks for a sugary drinks in a periodic fashion in response to

Ã¢â‚¬â„¢hunger pangsÃ¢â‚¬â„¢. In this case, assume that GÃ¢Ë†Å¾ (t) = G1 cos(Ãâ€°t).

(a) (1 point) If the child asks for a sugary drink once per hour and we work

in hours as our time unit (e.g. not seconds) then what is Ãâ€°?

(b) (1 point) Just For Fun Ã¢â‚¬â€œ approximating is everywhere and very helpful.

There are about 30g of sugar in one 340ml (about 12oz – typical pop can

size) of typical orange juice. If a 22kg (about 50lb) child drinks two 340ml

cans of juice/day, each containing about 30g of sugar, in how many days

will they consume their body weight in sugar?

(c) (2 points) Write down and solve the auxiliary problem Y (t) in this problem.

Show that the auxiliary solution is

Y (t) =

eat

k+a

(4)

where a = iÃâ€°.

(d) (4 points) Follow the 4-step MUD and use the auxiliary solution Y (t) to

construct the particular solution in Step 2. Omit Step 4: Sketch. Show

that the solution satisfying the condition is

G(t) = k1 G1

k cos(Ãâ€°t) + Ãâ€° sin(Ãâ€°t)

+ AeÃ¢Ë†â€™kt

k2 + Ãâ€°2

(5)

with A = G0 Ã¢Ë†â€™ k1 kG1 /(k 2 + Ãâ€° 2 ).

(e) (2 points) Use the magnitude of the auxiliary problem solution to construct

the transfer function and sketch the transfer function.

3. (8 points) Refer to Problem #1 in this assignment. Consider the model in #1(a)

with external input F0 t exp(Ã¢Ë†â€™ct) and initial conditions: y(0) = 1, yÃŒâ€¡(0) = Ã¢Ë†â€™4.

Use the 4-step MUD method to solve this problem and omit Step 4: Sketch.

Step 1: (1 point), Step 2: (5 points), Step 3: (2 points), Step 4: omit. Show

that

y(t) = (E + F t)eÃ¢Ë†â€™ct +

F0 3 Ã¢Ë†â€™ct

te

6

(6)

Ã¢Ë†Å¡

where E = 1 and F = c Ã¢Ë†â€™ 4 and c = 2. Note: To receive full points in

Ã¢â‚¬â„¢Step 2 Get yp Ã¢â‚¬â„¢: (i) you will need to use our Ã¢â‚¬â„¢parallelÃ¢â‚¬â„¢ algebra technique that

you were shown during class – you will need this method in Laplace Transform

switching problems, (ii) clearly

show why the guess must be yp (t) = t2 (At +

Ã¢Ë†Å¡

B) exp(Ã¢Ë†â€™ct), where c = 2 to avoid duplication, and (iii) use our recursive

method of differentiating exponential * polynomial functions when finding yÃŒâ€¡p

and yÃŒË†p (the dot refers to d/dt and is a common notation that we also use in our

notes). All of these are fully explained in the Week5Class2 WedFeb3 video. It

will

Ã¢Ë†Å¡ be convenient, if you wish, to leave in the parameter Ã¢â‚¬â„¢cÃ¢â‚¬â„¢ instead of writing

2 and onceÃ¢Ë†Å¡the last step is complete in the parallel method you can replace

Ã¢â‚¬â„¢cÃ¢â‚¬â„¢ with c = 2 and the cancellations will occur as expected.

4. (5 points) Consider the problem in Case 3 underdamped, Week4Class3 FriJan29.

LetÃ¢â‚¬â„¢s apply an external input to that problem of the form A cos(ÃŽÂ±t) + B sin(ÃŽÂ²t).

(a) (1 point) Write down the auxiliary problem Y (t) and solve for Y (t) and

show that

eat

(7)

Y (t) = 2

a + 2a + 5

where a = iÃâ€° and we will be setting Ãâ€° = ÃŽÂ± and Ãâ€° = ÃŽÂ² later in this problem.

(b) (2 points) Use the auxiliary problem to construct the particular solution

yp (t) and show that

yp (t) =

A cos(ÃŽÂ²t Ã¢Ë†â€™ ÃŽÂ¦)

A cos(ÃŽÂ±t Ã¢Ë†â€™ Ãâ€ )

+

2

2

2

(5 Ã¢Ë†â€™ ÃŽÂ± ) + 4ÃŽÂ±

(5 Ã¢Ë†â€™ ÃŽÂ² 2 )2 + 4ÃŽÂ² 2

(8)

where tan(Ãâ€ ) = (2ÃŽÂ±)/(5 Ã¢Ë†â€™ ÃŽÂ±2 ) and tan(ÃŽÂ¦) = (5 Ã¢Ë†â€™ ÃŽÂ² 2 )/(Ã¢Ë†â€™2ÃŽÂ²)

(c) (1 point) Use the solution of the auxiliary problem to construct the transfer

function and note on your figure why this is sometimes referred to as a

Ã¢â‚¬â„¢band-passÃ¢â‚¬â„¢ amplifier. Use the variable Ãâ€° as the frequency of the external

input.

(d) (1 point) Use calculus to precisely location the input frequency associated

with the transfer function peak.

5. (7 points) Consider problem #1 of this assignment again. This time set the

damping c = 0 so that there is no damping (this is maximum under-damping

for this model).

Ã¢Ë†Å¡

(a) (4 points) Solve the problem for external input sin(Ãâ€°t) where Ãâ€° 6= 2.

Solve the problem using our 4-step MUD method and employ the auxiliary

problem Y (t) in finding the particular solution yp (t) in Step 2. Assume

zero initial conditions. Step 1: (1 point), Step 2: (2 points), Step 3: (1

point), Omit Step 4: Sketch. Show that

”

#

Ã¢Ë†Å¡

1

Ãâ€°

y(t) =

Ã¢Ë†â€™ Ã¢Ë†Å¡ sin( 2t) + sin(Ãâ€°t)

(9)

2 Ã¢Ë†â€™ Ãâ€°2

2

(b) (2 points)

Note that Ã¢Ë†Å¡

the solution is proportional to R(t) = sin(Ãâ€°t) Ã¢Ë†â€™

Ã¢Ë†Å¡

sin( 2t) when Ãâ€° Ã¢â€°Ë† 2. Use the trigonometry approach in the notes

to show that R(t) may

of the fast

Ã¢Ë†Å¡ be written as two times the product Ã¢Ë†Å¡

oscillation cos{(Ãâ€° + 2)t/2} and the slow oscillation sin{(Ãâ€° Ã¢Ë†â€™ 2)t/2}.

(c) (1 point) Sketch R(t) by using the result in (5b). Show the slow envelope

surrounding the fast oscillation and note the period of each on your sketch

as we did during class.

Purchase answer to see full

attachment