Question 10
A hydraulic jack connected to a motor is used to raise an entire car. The mass of the car is 1500 kg. The motor uses 1.2 kW of power to raise the car.
What is the time taken to raise the car 500 mm vertically, rounded to two decimal places?
Not quite. $1200\div(1500\times0.5)=1.60$ turns $P=W/t$ upside down and leaves out $g$ as well. Time is the work divided by the power. In 2025, 2.9% of Queensland students chose (A).
That is it. Step through the hints below to see why. In 2025, 77.5% of Queensland students chose (B).
Not quite. 6.25 s comes from using 10 for $g$, which makes the work 7500 J. The formula book gives 9.8, and with two options this close the shortcut costs you the mark. In 2025, 13.3% of Queensland students chose (C).
Not quite. $1500\times9.8\div1200=12.25$ leaves out the height. The work is the weight of the car times the 0.5 m it rises. In 2025, 5.7% of Queensland students chose (D).
The motor lifts the car through 500 mm, which is 0.5 m. The work done is the weight of the car times that height.
Leave out the height and you land on (D). Use 10 for $g$ and you land on (C).
Power is work per second, $P=W/t$. The question gives no efficiency, so all 1.2 kW, which is 1200 W, goes into lifting the car. Rearrange for the time.
$$t=\frac{W}{P}=\frac{7350}{1200}=6.125\text{ s}$$
Flip the fraction and the answer is not a time at all. (A) does that and leaves out $g$ as well.
The question asks for two decimal places. 6.125 sits exactly halfway, and halfway rounds up.
$$6.125\approx6.13\text{ s}$$
This is why (B) and (C) sit so close together. QCAA put the 10 for $g$ slip only 0.12 s from the answer.
Find the work needed to lift the car, then divide by the power. Convert the millimetres and the kilowatts first.
The most common wrong answer was (C), from using 10 for $g$. It sits only 0.12 s from the answer, so a rough value of $g$ is enough to lose the mark.
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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