Tag Archives: Year 12 Specialist Mathematics

September 13, 2026 · 3:15 am

Simple Harmonic Motion Question

An object in simple harmonic motion passes through the central point $x=0$ at time $t=0$ and every second thereafter.

Find an equation for the motion if $v(0)=-4.$

Because the particle passes through the central point $x=0$ when $t=0$ , it is natural to use a sine function: $x=A\sin(\omega t).$

The particle passes through the central point every second.

However, passing through the centre occurs twice during each complete cycle — once moving in one direction and once moving in the other.

Therefore, the time from one central crossing to the next is half a period: $\frac{T}{2}=1.$

Hence, $T=2\text{ seconds}.$

For simple harmonic motion, $\omega=\frac{2\pi}{T}.$

Since $T=2$ , $\omega=\frac{2\pi}{2}=\pi.$

Our equation is therefore $x=A\sin(\pi t).$

We still need to determine $A$ .

For simple harmonic motion,

$v^2=k^2(A^2-x^2)$

When $t-0, x=0$ and $v=-4$

$16=\pi^2(A^2-0)$

$A=\frac{4}{\pi}$

Hence,

$x(t)=\frac{4}{\pi}sin(\pi t)$

Partial Fractions

Integration Using Partial Fraction Decomposition

Partial fraction decomposition is the process of taking a rational function and decomposing it into simpler rational expressions which are easier to integrate.

We only use partial fractions if the rational function is proper. If

$f(x)=\frac{p(x)}{q(x)}$

then the degree of $p(x)$ must be less than the degree of $q(x)$ .

Types of Partial Fraction Decompositions

Factor	Term in Partial Fraction
$ax+b$	$\frac{A}{ax+b}$
$(ax+b)^n$	$\frac{A_1}{ax+b} + \frac{A_2}{(ax+b)^2} +\cdots+ \frac{A_n}{(ax+b)^n}$
Irreducible quadratic $ax^2+bx+c$	$\frac{Ax+B}{ax^2+bx+c}$

Example

$\int \frac{3x+5}{x^2-x-2}\,dx$

Find

Factorise the denominator:

$x^2-x-2=(x-2)(x+1)$

Write

$\frac{3x+5}{(x-2)(x+1)} = \frac{A}{x-2} + \frac{B}{x+1}$

Multiplying through by

$(x-2)(x+1)$ :

$3x+5=A(x+1)+B(x-2)$

Let $x=-1$ :

$2=-3B$

$B=-\frac23$

Let $x=2$ :

$11=3A$

$A=\frac{11}{3}$

Hence

$\frac{3x+5}{x^2-x-2} = \frac{11}{3(x-2)} - \frac{2}{3(x+1)}$

Integrating:

$\int\frac{3x+5}{x^2-x-2}\,dx = \int \left( \frac{11}{3(x-2)} - \frac{2}{3(x+1)} \right) dx$

$= \frac{11}{3}\ln|x-2| -\frac23\ln|x+1| +C$

Example

Find

$\int\frac{3x+2}{(x-1)^2}\,dx$

Write

$\frac{3x+2}{(x-1)^2} = \frac{A_1}{x-1} + \frac{A_2}{(x-1)^2}$

Multiplying through by $(x-1)^2$ :

$3x+2=A_1(x-1)+A_2$

Let $x=1$ :

$5=A_2$

Let $x=0$ :

$2=-A_1+5$

$A_1=3$

Therefore

$\int\frac{3x+2}{(x-1)^2}\,dx = \int \left( \frac3{x-1} + \frac5{(x-1)^2} \right) dx$

$= 3\ln|x-1| -\frac5{x-1} +C$

Example

Find

$\int \frac{2x-5} {(x-1)(x^2+1)} \,dx$

Write

$\frac{2x-5} {(x-1)(x^2+1)} = \frac{A}{x-1} + \frac{Bx+C}{x^2+1}$

Multiplying through:

$2x-5 = A(x^2+1) + (Bx+C)(x-1)$

Let $x=1$ :

$-3=2A$

$A=-\frac32$

Let $x=0$ :

$-5=-\frac32-C$

$C=\frac72$

Let $x=-1$ :

$-7=-3+\left(-B+\frac72\right)(-2)$

$-4=2B-7$

$B=\frac32$

Hence

$\int \frac{2x-5} {(x-1)(x^2+1)} \,dx = \frac12 \int \left( -\frac3{x-1} + \frac{3x}{x^2+1} + \frac7{x^2+1} \right) dx$

$= \frac12 \left( -3\ln|x-1| +\frac32\ln(x^2+1) +7\arctan(x) \right) +C$

$= -\frac32\ln|x-1| +\frac34\ln(x^2+1) +\frac72\arctan(x) +C$

Logistic Growth Worked Example

A brumby is a free-roaming wild horse found in large number in parts of Australia. The culling of brumbies was banned in the year 2000. At this time the estimated population of brumbies in
Kosciuszko National Park was 1600. Scientists have modelled the population, P(t), of brumbies in
Kosciuszko National Park t years since the ban, by

$\begin{equation*}P(t)=\frac{18000}{10.25e^{0.15t}+1}\end{equation}$