The big idea: A mechanical system takes an input motion, does something to it in a process, and delivers an output motion.
The input and the output are always motions. The process is the mechanism that converts one into the other.
The model, then the same three boxes filled in for a tin opener, a bicycle pump and a windscreen wiper.
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| What changes | What that buys | A product that does it |
|---|---|---|
| The TYPE of motion | A motor can only spin, but a product may need something to rise, slide or swing | A crank turning rotary into reciprocating in a sewing machine |
| The SPEED | Slower usually means stronger, and faster usually means weaker | A worm gear slowing a wiper motor from 3,000 to 50 revolutions a minute |
| The DIRECTION | The output can be reversed, or turned through a corner to reach where it is needed | Bevel gears in a hand drill, or a crossed belt on a small conveyor |
| The FORCE | A hand cannot produce enough force on its own for many tasks | The long handles of bolt cutters closing a short, hard jaw |
One mechanism, one change: A well-designed mechanism usually changes one of those four things.
When a product needs several changed, mechanisms are chained — which is why a sewing machine has five of them rather than one clever part.
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Three boxes, three rules
Input — a motion and where it comes from
"Rotary motion applied by hand at the winding key." Naming the part alone is the most common wasted mark in this topic.
Process — the mechanism, named
"A pair of spur gears, then a lever." Not "the inside of the tin opener", and not the product's own name.
Output — a motion and what it does
"Rotary motion at the cutting wheel, slow enough to shear the rim." The consequence is what turns a label into an explanation.
Feedback is worth a sentence: Some mechanical systems sense their own output and correct it — a governor on an engine, a thermostatic valve, a spring return that reloads a mechanism.
That is feedback, and naming it where it exists lifts an answer above a description of the chain.
How this is tested — describing inputs, processes and outputs in mechanical systems. It comes up two ways:
Paper 1 — multiple choice
- Identify the process in a described mechanical system.
- Choose the output motion a named mechanism produces.
Paper 2 — analysing a product
- Describe the input, process and output of a product's mechanism.
- Explain what the process changes and why the product needs it changed.
The trap: Writing the product in the process box. "The tin opener" is not a process; "a pair of spur gears driving a cutting wheel" is.
A hand-operated garlic press is squeezed to force cloves through a perforated plate. Describe the input, process and output of this mechanical system.
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