The big idea: Everything that moves in a mechanical system is doing one of four things: linear, rotary, oscillating or reciprocating motion.
Name them by the path a point traces, not by the example you memorised.
Each motion drawn as the path it traces. Step through and watch what separates the two that reverse.
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| Motion | The path | Where you have met it |
|---|---|---|
| Linear | A straight line, in one direction only — it starts, travels and stops | A drawer opening, a lift rising, a conveyor belt, a paper trimmer blade |
| Rotary | A complete circle about a fixed centre, round and round without reversing | A wheel, a drill bit, a fan, a washing machine drum, a clock hand |
| Oscillating | Backwards and forwards along an ARC, swinging about a pivot | A windscreen wiper, a pendulum, a metronome, a swing, a hinged door |
| Reciprocating | Backwards and forwards along a STRAIGHT line, repeating | A piston, a sewing-machine needle, a jigsaw blade, a bicycle pump |
The one mix-up worth fixing now: Oscillating and reciprocating both go back and forth, so they get swapped in almost every class.
The difference is the path: a reciprocating part travels along a straight line, an oscillating part swings along an arc about a pivot.
A jigsaw blade reciprocates. A wiper oscillates. Both reverse; only one has a pivot.
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How a mechanisms question opens
Name the motion going IN
A hand turning a handle is rotary. A hand pushing a plunger is reciprocating. Write the motion, never the part.
Name the motion coming OUT
The needle goes up and down along a straight line, so it reciprocates. The wiper swings, so it oscillates.
The mechanism is whatever converts one into the other
Rotary in, reciprocating out is almost always a crank or a cam. Rotary in, linear out is a rack and pinion or a screw.
A motion, not a part: The commonest wasted mark in this topic is writing "the handle" where the question asked for the input motion.
A handle is not a motion. "Rotary motion applied by hand at the handle" is.
How this is tested — identifying the four types of mechanical motion in real products. It comes up two ways:
Paper 1 — multiple choice
- Name the motion shown in a labelled drawing.
- Choose the pair of motions a named mechanism converts between.
Paper 2 — analysing a product
- Identify the input and output motions of a product's mechanism.
- Explain which type of motion a named part undergoes, and why.
The trap: Calling a swinging part reciprocating because it goes back and forth. Reciprocating is a STRAIGHT line; oscillating is an arc about a pivot.
A hand-operated rotary whisk is turned by its handle. Identify the type of motion at the handle, at the drive wheel, at the beaters and at the mixture, justifying each.
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