Input, Process & Output in Mechanical Systems

This lesson applies the systems approach (Input → Process → Output) to mechanical systems, showing how force, motion and energy flow through mechanisms. This is a core concept in AQA GCSE Design and Technology (8552), Section 3.1.5.

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The Mechanical Systems Approach

Just like electronic systems (covered in Section 3.1.4), mechanical systems can be described using the Input → Process → Output model:

Stage	Function in a Mechanical System	Example (Bicycle)
Input	The force or motion applied to the system	Rider pushes the pedals (human effort)
Process	The mechanism that changes, transmits or amplifies the force or motion	Chain and sprocket system, gears
Output	The resulting force, motion or action	Rear wheel turns, bicycle moves forward

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Input: Forces and Energy Sources

The input to a mechanical system is the effort — the force or energy that drives the system.

Common Input Sources

Input Source	Description	Example
Human effort	Force applied by a person	Turning a handle, pushing a lever, pedalling
Electric motor	Converts electrical energy to rotary motion	Motor in a washing machine, power drill
Engine	Burns fuel to produce rotary motion	Petrol engine in a car, diesel engine in a truck
Spring	Stores potential energy and releases it	Clockwork mechanism, spring-loaded toy
Gravity	Weight of an object provides a downward force	Pendulum clock, water wheel
Pneumatic/hydraulic	Compressed air or pressurised fluid provides force	Pneumatic drill, hydraulic press

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Process: Mechanisms

The process stage is the mechanism itself. It changes the input in one or more of the following ways:

What the Mechanism Changes	Explanation	Example
Type of motion	Converts one motion type to another	Cam converts rotary to reciprocating
Direction of motion	Changes the direction of movement	Bevel gear changes axis of rotation by 90°
Speed of motion	Increases or decreases the speed	Gear reduction slows the output but increases torque
Magnitude of force	Amplifies or reduces the force	A lever multiplies the input force
Distance / range of motion	Changes how far the output moves	A lever can increase or decrease the distance moved

Key Concept: Trade-Off Between Force and Distance

A fundamental principle of mechanics is that you cannot get something for nothing. When a mechanism multiplies force, it reduces distance (or speed), and vice versa. This is based on the conservation of energy:

$\text{Energy input} = \text{Energy output} + \text{Energy lost to friction}$Energy input=Energy output+Energy lost to friction

$\text{Force} \times \text{Distance (input)} = \text{Force} \times \text{Distance (output)}$Force×Distance (input)=Force×Distance (output)

Example — A Lever: If a lever has a mechanical advantage of 3, the output force is 3 times the input force, BUT the input must move 3 times further than the output.

AQA Exam Tip: Whenever you describe a mechanism multiplying force, always mention that the distance moved (or speed) is reduced as a trade-off. This shows understanding of the underlying physics and targets the higher mark bands.

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Output: Work Done

The output is the useful work performed by the system — the result that the mechanism was designed to achieve.

Common Outputs