MATH2048 Mathematics for Engineering and the Environment Part II SEMESTER 1 EXAMINATION 2022/23
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SEMESTER 1 EXAMINATION 2022/23
MATH2048 Mathematics for Engineering and the Environment Part II
Section A: All students
A1. [Total for this question: 20 marks]
(a) [4 marks] Explain why the following are incorrect and then correct the error.
(i) [2 marks] Let f(t) = δ(t + 3π).
Therefore, the Laplace transform off(t) is
∞
L[f(t)] = l δ(t + 3π)e−stdt = e3πs
0
for s > 0.
(ii) [2 marks] Let the Laplace Transform of g(t) be ˜(g)(s) =
e −2s |
(s + 1)2 + 4 |
Therefore, the original function, g(t), is given as
g(t) = H(t − 2)etsin(2t)
(b) [16 marks] Use the method of Laplace transforms to solve the following initial-value problem:
¨(y) − ˙(y) − 2y = δ(t − 2π)
where δ(t) is the Dirac delta function.
A2. [Total for this question: 25 marks]
The forced heat equation with unit diffusion constant is
The domain is x ∈ [0, 1]. The boundary conditions are
The forcing term is
F(x,t) = exp(−t)(1 − x).
(a) [10 marks] Show that the solution to the homogeneous problem
∂y ∂2y
∂t ∂x2
subject to the boundary condition in equation (2) is
∞
y(x,t) = T0 (t) + Tn (t)cos(nπx).
You do not need to compute the explicit form of Tn (t) or T0 (t).
(b) [10 marks] Using the information from part (a), and that the Fourier cosine series for 1 − x is
1 − x (1 − ( − 1)n )cos(nπx), (6)
show that the general solution to the forced heat equation in equation (1) subject to the boundary condition in equation (2) is
y(x,t) = C0 −t +
(1 − ( − 1)n ) + Cne− (nπ)2t] cos(nπx), (7)
where C0 and Cn are constants.
(c) [5 marks] Set the initial data to be
y(x,0) = 0. (8)
Show, using (7) from (b), that the solution to the original PDE satisfying the initial condition is
1 − e −t)+ ( 1(n)) (e −t − e − (nπ)2t)] cos(nπx).
Section B: All students except Civil Engineering
B1. [Total for this question: 35 marks]
(a) [16 marks] Define the following:
f(x,y, z) = ze1 −xy ,
g(x,y, z) = sin (x2 yz, ,
v(x,y, z) = y(z − 1)i − x(1 − z)j + (1 + xy − 2z)k,
where i, j , k are the three unit vectors of the Cartesian axes.
Showing all your working, evaluate the following or explain why it is not possible.
(i) Find the Gradient (Δ) off(x,y, z).
(ii) Find the Gradient (Δ) of v(x,y, z).
(iii) Find the Divergence (Δ·) of g(x,y, z).
(iv) Find the Divergence (Δ·) of u(x,y, z).
(v) Find the Curl (Δ×) of g(x,y, z).
(vi) Find the Curl (Δ×) of v(x,y, z).
(vii) Find the Laplacian (Δ2) off(x,y, z).
(viii) Find the Laplacian (Δ2) of u(x,y, z).
(b) [5 marks] Define the conservative vector field u as
u = ze1 −zi − e1 −zj + e1 −z (x + y − xz)k
Find a scalar potential for u.
(c) [4 marks] Find the work done by a particle subject to the force
F = (2x − 3y)i + (y − z)j − x2yk
when moving from t = 0 to t = 1 along a paramaterized curve, C, defined as r = (t − 1,t,1 − t).
(d) [5 marks] A closed curve C has coordinates
sin(t)i + cos(t)j + 2cos(t)k
where t ∈ [0, 2π] is a parameter. A vector field F is given by
F = ysin(z)i + xsin(z)j + xy cos(z)k.
Using Stoke’s Theorem compute the line integral
1C F·dr.
(e) [5 marks] A cylinder encloses the region V where x2 + y2 ≤ 1 and 0 ≤ z ≤ 1. A vector field F is given by
F = −yzi + xzj + (xy + z)k.
Use the divergence theorem to find the flux of the vector field through the surface of the cylinder,
ll∂V F·dS.
Section C: Civil Engineering students only
C1. [Total for this question: 15 marks]
(a) An aircraft manufacturer makes two commercial airliners called B2B and B4B.
The two aircraft were developed in parallel with the result that they are powered by the same type of engine. The B2B has two engines and can still take-off if at least one of its engines remains operational throughout the take-off. The B4B has four engines and can still take-off if at least two of its engines remains
operational.
For a given aircraft, the engines are assumed to operate independently of each other. Let pbe the probability of engine failure for this type of engine during
take-off.
(i) [2 marks] Show that the probability of the B2B taking off successfully is given by
P(B2B takes off) = 1 − p2 .
(ii) [4 marks] It can be shown that the probability of the B4B taking off successfully is given by
P(B4B takes off) = 1 − 4p3 + 3p4 .
If a successful take-off was your only concern, for what values of p would you prefer to travel on the B2B than the B4B?
(iii) [2 marks] Do you think the assumption that the engines operate independently of each other is reasonable? Justify your answer.
(b) [4 marks] Two methods, A and B, are available for teaching a certain industrial skill. The assessment failure rates for A and B are 20% and 10%, respectively. However, B is more expensive and hence is only used 30% of the time (A is
used the other 70%). A worker was taught the skill by one of the methods but failed the assessment. What is the probability that they were taught by method A?
(c) The time to failure (in 100s of hours) of a certain electronic component is a random variable Y with cumulative distribution function (cdf) given by
F(y) = 1 − exp( −y2 ),
for y > 0.
(i) [1 mark] Find the probability density function (pdf).
(ii) [2 marks] Find the probability that a randomly selected component operates for at least 200 hours.
C2. [Total for this question: 20 marks]
(a) The strength of concrete depends, to some extent, on the method used for
drying. An experiment was performed to compare two different drying methods. Seventeen specimens of concrete were mixed and then one of the two drying
methods was chosen for each specimen at random. After 10 days, the strength of the 17 specimens was measured (in psi). The results are shown in the table below.
Method I |
Method II |
|
Number of specimens Sample mean Sample standard deviation |
n1 = 7 ¯(y)1 = 3250 s1 = 210 |
n2 = 10 ¯(y)2 = 3240 s2 = 190 |
(i) [2 marks] Calculate the combined sample variancesc(2) .
(ii) [4 marks] Test at the 5% significance level the null hypothesis that the mean concrete strength under the two drying methods are the same. Assume that the variance of the observations under the two drying methods are the same.
Hint: You may find some of the R code and output useful.
> qt(0.975,df=15)
[1] 2.13145
> qt(0.975,df=16)
[1] 2.119905
> qt(0.975,df=17)
[1] 2.109816
(b) The combined fuel efficiency (Y ; measured in km/l) and engine volume (X;
measured in l) of nine cars were measured. It is thought that Y is related to X according to a simple linear regression model. The results of fitting this model are shown below. However some of the output is missing.
Call:
lm(formula = eff ~ vol)
Residuals:
Min 1Q Median 3Q Max
-2.0394 -0.8490 0.2110 0.6831 2.2756
Coefficients:
Estimate Std . Error t value Pr(>|t|)
(Intercept) 36.730 2.416 ####### #######
vol -5.518 1.211 ####### #######
---
Signif . codes: 0 ’***’ 0.001 ’**’ 0.01 ’*’ 0.05 ’.’ 0.1 ’’ 1
Residual standard error: 1.36 on 7 degrees of freedom
Multiple R-squared: 0.7478,Adjusted R-squared: 0.7117 F-statistic: 20.75 on 1 and 7 DF, p-value: 0.002619
(i) [2 marks] Interpret the sign of the estimated slope parameter.
(ii) [4 marks] Calculate a 95% confidence interval for the slope of the regression line. Hence test the null hypothesis that the slope is equal to zero at the 5% significance level.
(iii) [2 marks] Give an estimate of the response variance.
(iv) [2 marks] Interpret the meaning of Adjusted R-squared: 0.7117.
(v) [2 marks] Describe two graphical assessments of the adequacy of the model.
(vi) [2 marks] Predict the fuel efficiency for a car with a 2l engine.
Hint: You may find some of the R code and output useful.
> qt(0.975,df=7)
[1] 2.364624
> qt(0.975,df=8)
[1] 2.306004
> qt(0.975,df=9)
[1] 2.262157
2024-01-19