Question 3

[1 mark] Simple familiar

Topic guidesPremiumTopic guide 2 · Four types of motion Topic guide 2 · Worked example: 2024 Question 3

The question

The crank moves continually in the direction shown.

A crank and slider mechanism. On the left, a disc turns on a driving shaft at its centre, and a curved arrow beside the disc shows it turning continually. A crank arm runs from the driving shaft to a pin near the rim. A long connecting rod joins that pin to a slider on the right. The slider sits between two hatched guides on a horizontal centre line, so it can only move along that line.

Identify the type of motion that best describes what is experienced by the slider.

Not quite. Oscillating motion swings back and forth through an arc about a pivot, like a pendulum. The slider is held in a straight guide, so its back and forth runs along a line. In 2024, 10.0% of Queensland students chose (A).

That is it. Step through the hints below to see why. In 2024, 70.4% of Queensland students chose (B).

Not quite. Rotary describes the crank, which turns about the driving shaft. The question asks about the slider, which never turns. In 2024, 7.8% of Queensland students chose (C).

Not quite. The slider does move along a straight line, but linear motion keeps going in one direction. The slider reverses at both ends of its stroke, every turn of the crank. In 2024, 11.5% of Queensland students chose (D).

Trace the path of

The question asks about the slider. The crank is the part that “moves continually”, and turning about the driving shaft is rotary motion. That describes the input, which rules out (C).

The slider sits between two guides. Whatever the crank does, the slider can only move along the centre line.

Watch the slider through one turn of the crank. It travels one way, stops, comes back and stops again. Each trip is one stroke.

$$\text{stroke}=2r$$

Back and forth along a straight line is reciprocating. Linear motion keeps going one way, like a package on a conveyor, which rules out (D).

Oscillating motion also goes back and forth, but through an arc about a pivot, like the pendulum. The slider’s path is straight, which rules out (A).

That leaves (B). QCAA printed it as “reciprocal”, the same word it used for the steam engine piston rod in 2022 Question 10. The syllabus word is reciprocating, so write that in a written answer.

LinearAlong a straight line, one way

RotaryTurns about an axis, like the crank

ReciprocatingBack and forth along a straight line, like the slider

OscillatingBack and forth through an arc about a pivot

The crank turns the rotary input into reciprocating output. The slider is held in a straight guide and reverses at each end of its stroke, so it reciprocates. QCAA’s option says “reciprocal”, its word for reciprocating.

(B)

How Queensland students answered in 2024

(A)

10.0%

(B)

70.4%

(C)

7.8%

(D)

11.5%

The most common wrong answer was (D), linear. That choice notices the straight guide and misses the reversal. Ask two questions of any moving part: is the path a line or an arc, and does it keep going or come back?

Only mark this done when you could do it without help. Reading the solution does not count.

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Question wording and any figures are from the 2024 QCAA Engineering external assessment, © State of Queensland (QCAA) 2024, 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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