Question 9
The work done by a jack to vertically raise a 1500 kg vehicle a distance of 180 cm is
Not quite. $1500\times1.8=2700$ multiplies the mass by the height. The jack pushes against the weight of the vehicle, $mg$, so you need the 9.8. In 2021, 18.8% of Queensland students chose (A).
Not quite. $14\,700\div1.8=8167$ divides the weight by the height. Work is force times distance, so the height multiplies. In 2021, 5.8% of Queensland students chose (B).
That is it. Step through the hints below to see why. In 2021, 73.3% of Queensland students chose (C).
Not quite. $14\,700\div0.18=81\,667$ makes two slips. It turns 180 cm into 0.18 m instead of 1.8 m, then divides by the height instead of multiplying. In 2021, 1.7% of Queensland students chose (D).
To lift the vehicle at a steady speed, the jack has to push up with a force equal to its weight.
The 1500 is a mass in kilograms. Multiply it straight by a distance and you land on (A).
Work needs the distance in metres. There are 100 cm in a metre.
$$s=180\text{ cm}=1.8\text{ m}$$
Dividing by 1000 instead gives 0.18 m, which is the slip inside (D).
Work is the force times the distance moved in the direction of the force. The jack pushes up and the vehicle moves up.
Work grows with distance, so it is never a force divided by a distance. That rules out (B) and (D). The 2019 formula book that 2021 students had printed the same formula as $W=Fd$.
The work done lifting a load is its weight times the height it rises. It is also the potential energy the vehicle gains.
The most common wrong answer was (A), from leaving out $g$. A mass times a distance is not a work. Turn the mass into a weight before you multiply by the height.
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 2021 QCAA Engineering external assessment, © State of Queensland (QCAA) 2021, 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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