Have you ever helped with gardening? Did you use a wheelbarrow? They can be very useful to carry all sorts of things - soil, weeds, grass, cuttings, bricks Why do you think is it easier to push things in a wheelbarrow than to carry them in your arms?
Levels of difficulty: Level 1 - Make a simple model of a wheelbarrow. Level 2 - As level 1, that can carry a book. Level 3 - As level 1, that can carry a house brick.
Equipment: KNEX set including small wheels, plus a paperback book and house brick.
Approach: What shape is a wheelbarrow? Where does the wheel go? Where can you put the objects you are carrying? What sort of handles does it have? For level 2, is your wheelbarrow large enough to carry the book? Will it take the weight without breaking or bending too much? For level 3, if your wheelbarrow breaks, how could you make it stronger?
The aim of the project is to build a working model, and use it to carry out an experiment with levers.
Possible solution: See photo.
Before starting: If possible, look at a real wheelbarrow.
Conclusion: Firstly, compare the different designs. Then, with the house brick in the wheelbarrow, try lifting the brick with the fingers at different positions along the handle. Is it easier to lift as your finger moves towards the end of the handle? Why is this? (the wheelbarrow is acting as a lever). Finally, if you have a spring balance, use to this to measure the force needed to lift the brick at different positions along the handle.