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Q1
Practice Application Task
VCE Specialist Mathematics, Unit 3 - SAC 2
50 marks. 70 minutes working time. CAS permitted.
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Practice Application Task
Offshore Wind Farm Engineering
Area of Study:Vectors & CalculusTask format:Application TaskWorking time:70 minutesReading time:10 minutesTotal marks:50 marksCAS calculator:Permitted
Instructions: Answer all questions in the spaces provided. Unless otherwise stated, an exact answer is required. Where more than one mark is allocated, appropriate working must be shown. Diagrams are not drawn to scale.
This is a practice paper. Your school's SAC may differ in length, marks, and time allocation. The format and difficulty are calibrated to be representative of a typical Unit 3 Application Task covering vectors and calculus.
Scenario: Offshore Wind Farm Engineering
Engineers design an offshore wind farm in Bass Strait. They use vectors for turbine positioning and force analysis, and calculus for power curve modelling and structural load calculations. This task involves optimising turbine placement, analysing forces on structures, and predicting power output and structural loads.
Part A: Turbine Positioning
12 marks
Three turbines are positioned at points A(2, 5, 0), B(8, 3, 0), and C(5, 9, 0) (coordinates in km). These positions define the corners of a triangular platform in the xy-plane.
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Question 14 marks
a.
Find vector \(\overrightarrow{AB}\) and calculate its magnitude.
[2 marks]
b.
Find the midpoint M of AC and the distance from B to M.
[2 marks]
Have you genuinely attempted this question on paper first?
a. [2 marks]
\[\overrightarrow{AB} = B - A = (8-2, 3-5, 0-0) = (6, -2, 0)\]
Area of triangle ABC: \[\text{Area} = \frac{1}{2}|\overrightarrow{AB} \times \overrightarrow{AC}| = \frac{1}{2} \cdot 30 = 15 \text{ km}^2\]
Unit normal vector: \[\hat{\mathbf{n}} = \frac{(0, 0, 30)}{|(0, 0, 30)|} = \frac{(0, 0, 30)}{30} = (0, 0, 1) = \mathbf{k}\]
Area = 15 km\(^2\), Unit perpendicular vector = \((0, 0, 1)\) or \(\mathbf{k}\)
1 mark: correct area. 1 mark: correct unit normal vector.
Part B: Force Analysis
12 marks
Support cables and wind loads act on the turbine structure. We model these using vectors in three dimensions.
Question 44 marks
a.
A support cable runs through point A(2, 5, 0) in the direction \(\mathbf{d} = 2\mathbf{i} - \mathbf{j} + 3\mathbf{k}\). Write the vector equation of this cable line.
[1 mark]
b.
A second cable runs through point B(8, 3, 0) in the direction \(\mathbf{e} = \mathbf{i} + 2\mathbf{j} - \mathbf{k}\). Show that these two lines are skew (do not intersect).
[3 marks]
Have you genuinely attempted this question on paper first?
For intersection, we need: \(2 + 2t = 8 + s\), \(5 - t = 3 + 2s\), \(3t = -s\)
From equation 3: \(s = -3t\)
Substitute into equation 1: \(2 + 2t = 8 - 3t \Rightarrow 5t = 6 \Rightarrow t = 1.2\)
Then \(s = -3.6\). Check in equation 2: \(5 - 1.2 = 3.8\) and \(3 + 2(-3.6) = 3 - 7.2 = -4.2\)
Since \(3.8 \neq -4.2\), the system has no solution.
The lines do not intersect (and are not parallel since directions \((2,-1,3)\) and \((1,2,-1)\) are not scalar multiples). Therefore the lines are skew.
1 mark: second line equation. 1 mark: setting up system. 1 mark: correct conclusion with reasoning.
Question 54 marks
a.
A wind force \(\mathbf{F} = 120\mathbf{i} + 80\mathbf{j} - 40\mathbf{k}\) (in Newtons) acts on a turbine blade oriented along direction \(\mathbf{u} = 3\mathbf{i} + 4\mathbf{j}\). Find the component of \(\mathbf{F}\) parallel to \(\mathbf{u}\) (the vector projection).
[2 marks]
b.
Find the perpendicular component of \(\mathbf{F}\) to \(\mathbf{u}\).
[2 marks]
Have you genuinely attempted this question on paper first?
a.
Three forces act on a junction point:
\(\mathbf{F}_1 = 500\mathbf{i} + 300\mathbf{j}\) N
\(\mathbf{F}_2 = -200\mathbf{i} + 400\mathbf{j} + 100\mathbf{k}\) N
\(\mathbf{F}_3 = -100\mathbf{i} - 500\mathbf{j} + 200\mathbf{k}\) N
Find the resultant force \(\mathbf{R}\).
[1 mark]
b.
Find the magnitude of \(\mathbf{R}\).
[1 mark]
c.
Find the angle that \(\mathbf{R}\) makes with the horizontal plane (the xy-plane).
[2 marks]
Have you genuinely attempted this question on paper first?
The power output \(P(v)\) (in kilowatts) of a turbine as a function of wind speed \(v\) (in m/s) is modelled by:
\[P(v) = 800\, \sin^{-1}\left(\frac{v}{25}\right) \quad \text{for } 0 \leq v \leq 25\]
Wind speed at time \(t\) hours is given by: \(v(t) = 20 - 5\cos\left(\frac{\pi t}{12}\right)\)
Question 74 marks
a.
Find \(\dfrac{dP}{dv}\).
[2 marks]
b.
Evaluate \(\dfrac{dP}{dv}\) when \(v = 15\) m/s and interpret this result.
[2 marks]
Have you genuinely attempted this question on paper first?
\(\dfrac{dP}{dv}\bigg|_{v=15} = 40\) kW/(m/s). This means that at a wind speed of 15 m/s, the power output is increasing at a rate of 40 kilowatts for each additional 1 m/s increase in wind speed.
Since \(\dfrac{dP}{dv} > 0\) for all valid \(v\), \(\dfrac{dP}{dt}\) is maximum when \(\sin\left(\dfrac{\pi t}{12}\right) = 1\), i.e., when \(\dfrac{\pi t}{12} = \dfrac{\pi}{2}\)
\[t = 6 \text{ hours}\]
Power output changes most rapidly at \(t = 6\) hours (6 hours after the start of the day).
1 mark: recognising that maximum occurs when \(\sin\left(\dfrac{\pi t}{12}\right) = 1\). 1 mark: correct time.
Question 94 marks
a.
The blade pitch angle \(\theta\) and power coefficient \(C\) are related implicitly by:
\[C^3 + 2C\theta - \theta^2 = 1\]
Use implicit differentiation to find \(\dfrac{dC}{d\theta}\) in terms of \(C\) and \(\theta\).
[2 marks]
b.
Given that when \(\theta = 1\), \(C = 1\), find the value of \(\dfrac{dC}{d\theta}\) at this point.
[1 mark]
c.
Interpret the sign of \(\dfrac{dC}{d\theta}\) at the point \((\theta, C) = (1, 1)\).
[1 mark]
Have you genuinely attempted this question on paper first?
The derivative is zero, meaning that the power coefficient is at a stationary point (local maximum or minimum) with respect to pitch angle. A small change in pitch angle has no first-order effect on the power coefficient at this operating point.
1 mark: correct interpretation of zero derivative in context.
Part D: Structural Load Analysis
13 marks
The structural loads and stress distribution on the turbine foundations require integration to find total quantities.
Question 104 marks
a.
The stress on a support beam at distance \(x\) from the base is: \(\sigma(x) = \dfrac{600x}{x^2 + 9}\)
Using the substitution \(u = x^2 + 9\), find \(\int \sigma(x)\, dx\).
[2 marks]
b.
Find the total stress from \(x = 0\) to \(x = 4\).
[2 marks]
Have you genuinely attempted this question on paper first?
a. [2 marks]
Let \(u = x^2 + 9\), then \(du = 2x\, dx\), so \(x\, dx = \dfrac{1}{2}du\)
\[\int \frac{600x}{x^2 + 9}\, dx = \int \frac{600}{u} \cdot \frac{1}{2}\, du = 300 \int \frac{1}{u}\, du = 300\ln|u| + C\]
\[= 300\ln(x^2 + 9) + C\]
\(\int \sigma(x)\, dx = 300\ln(x^2 + 9) + C\)
1 mark: correct substitution and setup. 1 mark: correct integration.
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