Question 10
A diagram of a steam-driven engine is shown.
When the steam engine is in motion, the movement of the piston rod is
Not quite. Oscillating motion swings back and forth through an arc about a pivot, like a pendulum. The piston rod goes back and forth along a straight line. The connecting rod is the part that swings. In 2022, 10.7% of Queensland students chose (A).
That is it. Step through the hints below to see why. QCAA printed the option as “reciprocal”, meaning reciprocating. In 2022, 63.2% of Queensland students chose (B).
Not quite. Rotary motion turns about an axis, which is what the crank and the flywheel do. The piston rod never turns. In 2022, 2.7% of Queensland students chose (C).
Not quite. The rod does move along a straight line, but linear motion keeps going one way. The piston rod reverses every half turn of the crank, and back and forth along a straight line is reciprocating. In 2022, 22.9% of Queensland students chose (D).
The question asks about the piston rod only. It is fixed to the piston and slides through the end of the cylinder, so it can only move along its own axis.
The crank and flywheel turn, and the connecting rod does something in between. Ignore them, and watch the labelled part.
The syllabus names four types of motion. Sort them by path and by direction:
Linear runs along a straight line in one direction.
Rotary turns about an axis.
Reciprocating goes back and forth along a straight line.
Oscillating swings back and forth along an arc about a pivot.
The piston rod moves along a straight line and reverses at each end of its stroke, once every half turn of the crank. Straight line plus back and forth is reciprocating.
QCAA printed option (B) as “reciprocal”. In maths a reciprocal is one over a number. The syllabus word is reciprocating, and that is the word to write in a short response. QCAA used “reciprocal” again in 2024 Question 3.
The piston rod slides back and forth along one straight line and reverses at each end of the stroke. That is reciprocating motion, which QCAA printed as “reciprocal”. The crank turns it into the rotary motion of the flywheel.
The most common wrong answer was (D), linear. Linear and reciprocating motion both run along a straight line, and reciprocating is the one that keeps reversing.
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 2022 QCAA Engineering external assessment, © State of Queensland (QCAA) 2022, 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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