Question 8
An object has a mass of 1500 kg. It starts from rest and accelerates uniformly at 0.4 m/s2 for a horizontal distance of 20 m.
What is the object’s kinetic energy?
That is it. Step through the hints below to see why. In 2025, 79.2% of Queensland students chose (A).
Not quite. $1500\times16=24\,000$ J leaves out the ½ in $\text{KE}=\tfrac12mv^2$. In 2025, 7.7% of Queensland students chose (B).
Not quite. $\tfrac12\times1500\times16^2=192\,000$ J squares the 16 a second time. The 16 from $v^2=u^2+2as$ is already $v^2$, so the speed is 4 m/s. In 2025, 7.6% of Queensland students chose (C).
Not quite. $1500\times9.8\times20=294\,000$ J is $mgh$ with the 20 m used as a height. The object moves horizontally, so it gains no potential energy, and the question asks for kinetic energy. In 2025, 4.8% of Queensland students chose (D).
Kinetic energy needs $v^2$, and the question does not give you $v$. List what you do know. “Starts from rest” gives $u=0$, and there is no time.
$$u=0,\quad a=0.4\text{ m/s}^2,\quad s=20\text{ m}$$
$$\begin{aligned}v^2&=u^2+2as\\&=0+2\times0.4\times20\\&=16\end{aligned}$$
Stop there. You never need $v$ itself.
Put $v^2=16$ straight into the formula book kinetic energy.
$$\text{KE}=\tfrac12mv^2=\tfrac12\times1500\times16$$
$$\text{KE}=12\,000\text{ J}=12\text{ kJ}$$
Leaving out the ½ gives (B). Squaring the 16 again gives (C).
Check with work. The net force does work on the object, and all of that work becomes kinetic energy.
$$\begin{aligned}F&=ma=1500\times0.4\\&=600\text{ N}\end{aligned}$$
$$\begin{aligned}W&=Fs=600\times20\\&=12\,000\text{ J}\end{aligned}$$
The track is level, so nothing goes into potential energy. That rules out (D).
$$\begin{aligned}v^2&=u^2+2as\\&=0+2\times0.4\times20\\&=16\end{aligned}$$
$$\text{KE}=\tfrac12mv^2=\tfrac12\times1500\times16$$
$$\text{KE}=12\,000\text{ J}=12\text{ kJ}$$
With no time given, $v^2=u^2+2as$ hands you $v^2$ directly, and that goes straight into $\text{KE}=\tfrac12mv^2$. The work done by the net force, $W=Fs=600\times20$, gives the same 12 kJ.
The most common wrong answer was (B), from leaving out the ½, with (C) just behind it. Both come from rushing the last line after the motion equation was done correctly.
Only mark this done when you could do it without help. Reading the solution does not count.
Question wording and any figures are from the 2025 QCAA Engineering external assessment, © State of Queensland (QCAA) 2025, licensed under CC BY 4.0, and have been adapted, except where a third-party source is named. Tangent Tuition is not affiliated with the QCAA.
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