D. Structures and Mechanisms · Systems in Action

Design Challenge: A Lifting System with Mechanical Advantage

A1.3 A1.2 A1.4 A1.5 A3.2 D2.5 D2.6 D2.7About 60 minutes

Learning goals

I can use the engineering design process to design, build, test and improve a lifting system.
I can measure input and output forces and distances, and calculate mechanical advantage and efficiency.
I can follow safety procedures while building and testing.
I can communicate my design and results clearly.

Words to know

engineering design process
A cycle engineers use: define the problem, research and brainstorm, plan, build a prototype, test, evaluate, improve and communicate.

criteria
What a successful design must do.

constraints
Limits on the design, such as materials, cost, time and safety rules.

prototype
A first working model used to test an idea.

iterate
To repeat the design cycle, improving the design each time.

force meter
A tool that measures force, such as a spring scale. A luggage scale can be used if you convert its reading to newtons.

The challenge

Your community garden needs a way to lift heavy watering cans onto a high shelf in the shed without hurting anyone's back. Your job: build a model lifting system that uses mechanical advantage.

Criteria:

Lift a full 500 mL water bottle (load about 5 N) at least 20 cm straight up.
The input force must be less than the load (actual MA greater than 1). Bonus: actual MA of 2 or more.
Calculate the IMA, actual MA and efficiency of your design.

Constraints:

Use household materials: string, empty thread spools or tape rolls (as pulley wheels), pencils or dowels (as axles), cardboard, tape, a broom handle resting across two chairs (as a support), paper clips.
About 60 minutes to build, test and improve.
Safety rules below must be followed (A1.4).

A rope runs over a pulley on a garden shed ceiling beam, with one end tied to a metal bucket, below a shelf of watering cans. — A single fixed pulley like this changes the direction of the pull. How could you add pulleys to make the input force smaller than the load? AI-generated image

Safety first (A1.4)

Build with an adult nearby. An adult should do any cutting and bend any wire.
Keep the load low and never put your head, hands or feet under the hanging bottle.
Make sure the broom handle and chairs are stable before you hang anything.
Elastic bands can snap: stretch them only a little and point them away from faces.
Wipe up spilled water right away so no one slips.

Measuring force at home

You need to measure the input force. Use one of these:

A luggage scale or spring scale. If it reads in kilograms, multiply by 9.8 to get newtons.
A homemade elastic-band force meter. Hang one full 500 mL bottle (about 5 N) from an elastic band taped beside a ruler and mark the stretch. Hang two bottles in a bag (about 10 N) and mark again. Use these marks to estimate other forces.

Also measure distances with a ruler: how far the load rises (output distance) and how much string you pull (input distance).

The engineering design process

Define the problem, criteria and constraints.
Research and brainstorm: sketch at least two ideas, for example a movable pulley, a two-pulley block and tackle, a lever (a ruler on a fulcrum), or a winch (wheel and axle made from a spool on a pencil).
Plan: choose one, draw a labelled diagram and predict its IMA.
Build a prototype.
Test three times. Record input force, input distance, load and load distance.
Evaluate: calculate actual MA and efficiency. Did it meet the criteria?
Improve (iterate): reduce friction, add a pulley, change arm lengths. Test again.
Communicate: present your design and results (A1.5).

The engineering design process as a flow. The loop between testing, evaluating and improving is where most of the engineering happens; you leave it only when the design meets the criteria.

Worked example

Example: analysing test data

A student builds a movable pulley from a thread spool on a pencil axle, with the string tied to the broom handle. Test results: load 5 N lifted 0.2 m; input force 3 N, string pulled 0.4 m. Find the IMA, actual MA, work in, work out and efficiency.

IMA: 2 rope segments hold the movable pulley, so IMA = 2. (Check: input distance ÷ output distance = 0.4 ÷ 0.2 = 2. ✓)
Actual MA = 5 N ÷ 3 N ≈ 1.67.
Work out = 5 N × 0.2 m = 1.0 J. Work in = 3 N × 0.4 m = 1.2 J.
Efficiency = 1.0 ÷ 1.2 × 100% ≈ 83%. (Check: 1.67 ÷ 2 ≈ 0.83. ✓)
Improve: the student slides a short piece of drinking straw onto the pencil as a smooth sleeve for the spool and rubs a little soap on it to reduce friction. New input force: 2.8 N. Work in = 2.8 × 0.4 = 1.12 J; efficiency = 1.0 ÷ 1.12 × 100% ≈ 89%.

IMA = 2, actual MA ≈ 1.67, efficiency ≈ 83%, improved to about 89% by reducing friction.

The movable pulley from the example (not to scale). Two string segments hold up the spool, so you pull 0.4 m of string to lift the bottle 0.2 m.

Test	Load (N)	Load distance (m)
1	5	0.2
2	5	0.2
3	5	0.2
Average	5	0.2

Copy this data table to record your tests. Repeat each test 3 times and average the input force (A1.2).

Hands-on activity

Build, test and improve your lifting system

You need: a full 500 mL water bottle, string (about 3 m), 2 empty thread spools or tape rolls, 2 pencils or dowels, tape, a broom handle and two sturdy chairs of equal height, a ruler or measuring tape, a luggage scale or an elastic-band force meter, paper and pencil, an adult helper.

Rest the broom handle across the backs (or seats) of two chairs as your support. Check that it is stable.
Build your chosen design. For a movable pulley: tie one end of the string to the broom handle, run it down under the spool (with a pencil through it as an axle), and back up over the broom handle. Hang the bottle from the pencil axle with a loop of string.
Pull the free end of the string with your force meter. Measure the input force while the bottle rises slowly and steadily.
Measure how far the bottle rises and how much string you pull.
Fill in the data table and calculate actual MA and efficiency.
Make one change to improve efficiency or MA. Test again (keep everything else the same so the test is fair).
Present your design: a labelled diagram, your data table, your calculations and what you would change next.

What to look for: a working system that lifts the bottle 20 cm with an input force less than 5 N; a correct IMA; actual MA below IMA; efficiency below 100%; one clear improvement tested fairly; a presentation that uses the words input force, output force, mechanical advantage, work and efficiency. Rubbing the string over a broom handle causes a lot of friction, so a second spool as a fixed pulley usually raises efficiency.

Just for fun

A law every engineer learns: your prototype will work perfectly every single time... until someone is watching. That is one more reason to test three times.

Your turn

What is the difference between criteria and constraints?

Why do engineers build a prototype?

A luggage scale reads 0.3 kg while pulling. What is the force in newtons?

A design lifts a 5 N load with 2.5 N of input force. What is the actual MA?

Why should you repeat each test three times?

Name one way to reduce friction in a homemade pulley.

Name two safety rules for this challenge.

More practice: warm-up

What does iterate mean?

What is the IMA of a single movable pulley?

About how much does a full 500 mL water bottle weigh in newtons?

Which step comes right after testing?

In a fair test, how many things should you change at a time?

More practice: on your own

A team's winch lifts a 5 N load 0.2 m while they pull with 1.5 N over 1.0 m. Find the actual MA, work in, work out and efficiency.

The same winch has an IMA of 5 (input distance ÷ output distance = 1.0 ÷ 0.2). Check the efficiency using actual MA ÷ IMA.

A student's lever lifts the bottle with 2 N, but the bottle only rises 5 cm. Does it meet the criteria? What could they change?

Explain why a design with an IMA of 4 might be less useful than one with an IMA of 2 for this challenge.

Write a one-sentence conclusion for the example test data in this lesson.

Challenge

Team A: load 5 N, rises 0.2 m, input 2 N over 0.6 m. Team B: load 5 N, rises 0.2 m, input 3 N over 0.4 m. Which has the greater actual MA? Which is more efficient?

Use math (A3.2): if your design has an efficiency of 80% and an IMA of 3, what input force will lift a 6 N load?

Describe how a real lifting system, such as a crane on a construction site, uses the same engineering design process and safety thinking as your model.

Check your understanding

Quiz

Which is a constraint in this design challenge?

A design lifts 5 N with 2 N of input force. Its actual MA is...

Work out is 1.0 J and work in is 1.25 J. Efficiency is...

To make a fair test of an improvement, you should...

What is the main purpose of a prototype?

Which is the safest practice when testing a lifting system?

For parents and teachers

Big idea (A1.3 and D2.5 to D2.7): Students use an engineering design process ... to design, build and test devices, models, structures and/or systems, applying work, mechanical advantage and efficiency. They also practise fair testing and repeated measurements (A1.2), safety (A1.4), math connections (A3.2) and communicating results (A1.5).

How to help: Supervise the build, especially the support and any cutting. A luggage scale makes the measurements much easier. Ask your child to predict before each test and to explain any surprise. Praise good iteration: a design that improves from test 1 to test 2 shows real engineering thinking.

Common difficulties:

Measuring force while the load is jerking or stopped. Pull slowly and steadily and read the force while it moves.
Expecting actual MA to equal IMA. Friction always lowers it.
Changing several things at once. Change one variable per test.