Grade by Grade
D. Structures and Mechanisms · Systems in Action

Purpose, Inputs and Outputs of Systems

D2.2 A1.5About 40 minutes
Learning goals
  • I can describe the purpose of a system.
  • I can identify the inputs, process and outputs of mechanical and social systems.
  • I can tell the difference between intended (useful) outputs and unintended outputs.
  • I can explain how feedback helps a system control itself.

Words to know

input
Anything that goes into a system: materials, energy, force, information, money or people's time.
process
What the system does to the inputs to change them into outputs.
output
Anything that comes out of a system.
intended output
The useful result the system was designed to produce. Also called a useful or desired output.
unintended output
A result the designers did not want, such as waste heat, noise, pollution or garbage. Also called a side effect.
feedback
Information about the output that goes back into the system so it can adjust, like a thermostat sensing the room temperature.

Hook: what goes in, what comes out

Think about a toaster. You put in bread. You plug it in so electrical energy can flow. You choose a setting. A minute or two later out comes toast. But that is not all that comes out. The kitchen gets a little warmer, the wires glow orange, and the house smells like toast.

Every system takes in inputs, does something to them (a process) and gives out outputs. Some outputs are what we want. Others are not.

A stainless steel toaster with toasted bread popped up, beside a loaf of bread on a kitchen counter.
Bread and electrical energy go in; toast comes out. The warm air rising above the toaster is an output too, just not the one we want. AI-generated image
INPUTSPROCESSOUTPUTSbread sliceselectricalenergyyour settingwires glowand heat thebread; timerpops it uptoast (useful)thermal energyinto the roomlight, smell
An input, process, output (IPO) diagram for a toaster. Toast is the intended output. Thermal energy released into the room, light and smell are unintended outputs.

Purpose, inputs, process, outputs

  • Purpose: the job the system is designed to do. A toaster's purpose is to brown bread.
  • Inputs: what goes in. Inputs can be materials (bread, water, snow), energy (electricity, fuel, food for your muscles), force (your push on pedals), information (a setting, a schedule) or, in social systems, people, time and money.
  • Process: what happens inside the system to change the inputs.
  • Outputs: what comes out. Intended outputs match the purpose. Unintended outputs are side effects, like noise, waste heat, exhaust, garbage or wear on parts.

A well-designed system makes lots of intended output and as little unintended output as possible.

SystemPurposeInputsIntended outputsUnintended outputs
Bicyclecarry a riderrider's force on the pedalsmotion of bike and riderthermal energy from friction, sound, wear on tires and chain
Gas snowblowerclear snowgasoline, snow, the operator's controla cleared drivewayexhaust gases, noise, thermal energy
Washing machineclean clothesdirty clothes, water, detergent, electrical energyclean clothesdirty water, sound, thermal energy, lint
Schooleducate studentsstudents, teachers, funding, books, timestudents with new knowledge and skillspaper waste, traffic at drop-off time
Inputs and outputs of four systems. A social system (the school) has inputs and outputs too.

Feedback: systems that check themselves

Some systems use part of their output to control themselves. This is called feedback.

  • A furnace with a thermostat: the thermostat senses the room temperature (an output of the heating). When the room reaches the setting, it tells the furnace to turn off. When the room cools, it turns the furnace back on.
  • You riding a bike: your eyes see that you are drifting toward the curb (output), and your brain tells your arms to steer back (adjusting the input).
  • A school: report cards and tests are feedback. They show whether students are learning, so teachers can change how they teach.
THERMOSTATcompares room temp with settingFURNACEswitches ON or OFFOUTPUTwarm air heats the roomSENSOR READINGroom temperature nowsignalfeedbackgoes back in
A feedback loop in a home heating system. The thermostat keeps checking the result (the room temperature) and uses it to switch the furnace on or off.
Worked example

Let's describe a system: a city public transit system

Describe the purpose, inputs, process and outputs of a city bus system. Include at least one unintended output.

  1. Purpose: move many people around the city reliably and at low cost.
  2. Inputs: buses, fuel or electricity, drivers and mechanics, routes and timetables, money from fares and taxes, and the passengers themselves.
  3. Process: buses follow routes on a schedule, stop to pick up and drop off riders, and are cleaned and repaired at night.
  4. Intended outputs: people arrive at work, school, appointments and stores. Fewer cars are on the road.
  5. Unintended outputs: exhaust from diesel buses, noise, and crowding at rush hour.
  6. Feedback: riders' complaints and counts of how many people board each route help planners change routes and timetables.

Purpose: move people. Inputs: buses, energy, workers, schedules, money, riders. Outputs: people delivered (intended), exhaust and noise (unintended).

Worked example

Let's spot the feedback

A smart sprinkler system waters a lawn only when a soil moisture sensor says the soil is dry. Identify the input, the intended output and the feedback.

  1. Inputs: water, electrical energy and the sensor's readings.
  2. Intended output: a healthy lawn without wasted water.
  3. Feedback: the soil moisture sensor measures the result of the watering (wet or dry soil) and sends that information back to the controller, which turns the water on or off.

The moisture sensor's reading is the feedback. It lets the system adjust itself so it does not overwater.

Just for fun

What if your toaster had no timer and no feedback at all? It would keep going until your toast became charcoal, an extremely unintended output. Luckily the smoke alarm would then give you some very loud feedback.

Hands-on activity

IPO charts for three home systems

You need: paper, a pencil.

  1. Choose three systems in your home: one mechanical (for example a bicycle, a manual can opener or an egg beater), one electrical (for example a fridge, dishwasher or washing machine) and one social (for example how your family plans and cooks dinner).
  2. For each, draw an IPO diagram like the toaster diagram: three boxes with arrows.
  3. Under Outputs, list intended outputs and unintended outputs separately.
  4. Circle any feedback you can find (a sensor, a timer, a person checking the result).
  5. Pick one unintended output and suggest one way to reduce it.

What to look for: for a fridge, inputs are electrical energy and warm food; the intended output is cold food; unintended outputs include thermal energy released at the back of the fridge and a humming sound; the thermostat inside is feedback. For family dinner, inputs include groceries, time, energy for cooking and people's skills; outputs include a meal (intended) and dirty dishes and food scraps (unintended); asking "Did everyone like it?" is feedback.

Your turn

1

Name the inputs of a gas lawn mower.

2

Name one intended and one unintended output of a lawn mower.

3

What is the purpose of a dishwasher?

4

What does feedback mean in a system?

5

A hockey team practises, plays games and watches game videos to improve. What is the feedback?

6

What are the inputs of a school library?

7

Why do engineers try to reduce unintended outputs?

More practice: warm-up

1

Fill in the blanks: inputs → ____ → outputs.

2

Is toast an intended or unintended output of a toaster?

3

Is noise usually an intended or unintended output of a blender?

4

Name one input of a bicycle.

5

What senses the room temperature in a home heating system?

More practice: on your own

1

Describe the purpose, two inputs and two outputs of a car wash.

2

Some outputs are unintended but useful. Give an example.

3

Draw or describe an IPO diagram for a recycling collection program in your town.

4

Explain how feedback works in a car's cruise control.

5

How could you change a family's grocery shopping system to reduce one unintended output?

Challenge

1

A hospital emergency department is a system. Describe its purpose, three inputs, two intended outputs and one unintended output.

2

Explain why the same output can be intended in one system but unintended in another. Use thermal energy as your example.

3

A town wants its snow-clearing system to use less road salt. Suggest an input change and a feedback component that could help.

Check your understanding

Quiz

1.

In an IPO diagram, what does P stand for?

2.

Which is an unintended output of a car?

3.

Which is an input of a school?

4.

A thermostat turns a furnace off when the room is warm enough. This is an example of...

5.

What is the purpose of a snowblower?

6.

Which output of a washing machine is intended?

For parents and teachers

Big idea (D2.2): Students describe the purpose, inputs and outputs of various systems. The IPO model (input, process, output) works for mechanical, electrical and social systems. Grade 8 adds unintended outputs (waste heat, noise, pollution) and feedback, which prepare for the lessons on efficiency and impacts.

How to help: At home, point to an appliance and ask "What goes in? What comes out? What comes out that we don't want?" Try it on a social system too, such as getting everyone out the door in the morning.

Common difficulties:

  • Mixing up inputs and outputs (calling fuel an output). Ask "Does it go in or come out?"
  • Forgetting energy as an input. Every machine needs an energy input: electricity, fuel or muscle power.
  • Listing only intended outputs. Push for side effects; they matter for efficiency and the environment.