How mechanisms trade one thing for another. Gears trade speed for turning force; levers and pulleys trade distance for force, by the same factor; a design brief decides which trade is right.
Each lesson works two ways. Students can work through the online tutorial at their own pace (it checks their answers and saves their progress in their browser), or you can teach it to the whole class from the slides, then send students into the tutorial for the Modify and Make steps.
Every lesson guide has plain-language notes, the answers to every check, a solution for every task, and slides and a worksheet for Waypoints Prepare. These guide pages are not linked from student pages, but they are not secret either: keep printed Teacher editions for answers you do not want students to find.
Whole-course cheat sheet: Open to print (every command, grouped by unit: a handout for the final challenge)
Gear ratios, choosing a pair to meet a brief, then idlers and compound trains for bigger ratios.
Two gears that mesh push each other round, tooth by tooth. The gear that is turned by the motor is the driver; the gear it turns is the driven gear. The gear ratio is driven teeth ÷ driver teeth.
When a small gear drives a big one (ratio more than 1), the big one turns slower but with more turning force, by the same factor: 8 teeth driving 24 is a ratio of 3, so the output turns 3 times slower and can lift 3 times as much. Swap them round and it turns 3 times faster but is 3 times weaker. Nothing is free: a gear trades speed for force.
A design brief sets criteria (what it must do: lift 450 g) and constraints (limits: within 15 s, with this motor and these gears). A design passes only if it meets every criterion. Sometimes no design does, and the constraints have to be talked about.
A gear between two others is an idler: its teeth cancel out (8 → 40 → 24 has the ratio of 8 → 24), so it changes only the direction. Each mesh reverses the turn. Two gears on one axle turn together, so the next stage can start from a small gear again: a compound train, whose stage ratios multiply (8 → 24 = 8 → 40 is 3 × 5 = 15). Big ratios come in stages so no gear has to be huge.
gear, driver, driven gear, gear ratio, turning force (torque), stall, criteria, constraints, trade-off, idler, compound train, stage.
Cheat sheet: Open to print (A4, with board pictures) · Markdown
Trading force for distance with a bar on a pivot and with ropes that share a load.
A lever is a bar that turns on a pivot. A force's turning effect (its moment) is the force × its distance from the pivot. A lever balances when the turning effects on both sides are equal: a 600 N rock 0.5 m from the pivot balances a 200 N push 1.5 m away.
Pushing far from the pivot makes the push stronger, by the same factor that the hand moves farther: 3 times the force, 3 times the distance. There are three kinds: the pivot in the middle (class 1: crowbar, scissors), the load in the middle (class 2: wheelbarrow, nutcracker) and the push in the middle (class 3: tweezers, your forearm). Class 3 always needs more force than the load, and gains speed and reach.
A single fixed pulley changes the direction of a pull, not its size (like an idler gear). Ropes that hold up a moving block share the load: with 4 of them you pull a quarter of the load, and pull 4 times as much rope.
Gears, levers and pulleys all make the same trade: a bigger force for a longer distance (or a lower speed). With no friction, work in = work out (force × distance).
lever, pivot (fulcrum), load, effort (push or pull), turning effect (moment), mechanical advantage, fixed pulley, block and tackle, work.
Cheat sheet: Open to print (A4, with board pictures) · Markdown
Friction, efficiency, and why more pulleys stop helping.
Cheat sheet: Open to print (A4, with board pictures) · Markdown