What is friction?
Friction is the rubbing force that slows things down. When two things rub, friction tries to stop them. Rubber tyres use friction to grip the floor, but a wheel rubbing on a beam wastes energy. Good engineers add friction where they want grip and remove it where they want speed.
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Push a wheel tight against its beam, then leave a small gap. Which one spins longer?
Build it: 8 builds that teach friction
- 🌉 Speed-Cancelling Gear Bridge: A 40–8–40 gear train: the middle turns five times, but the ends stay together. (level 2 of 5, about 15 minutes, 5 steps)
- 🌀 Coasting Wheel Bench: Spin a retained tabletop wheel gently, then measure how it slows down. (level 2 of 5, about 15 minutes, 8 steps)
- 🛒 Push Buggy: A sturdy buggy with a push bar held up by super-strong triangles. (level 2 of 5, about 15 minutes, 12 steps)
🏁 Pull-back Racer: Pull it back, let it go — ZOOM! A racer with a spring for an engine. (level 2 of 5, about 12 minutes, 10 steps)- 🔬 Compound Counting Bench: Two five-to-one stages multiply: count 25 input turns for one output turn. (level 2 of 5, about 20 minutes, 7 steps)
- 📏 Rack Travel Bench: Turn a small gear and measure the straight travel of a captured rack. (level 3 of 5, about 25 minutes, 13 steps)
- 🪢 Belt-Drive Buggy: No gears touching: a loop carries the power from the motor down to the wheels. (level 3 of 5, about 35 minutes, 11 steps)
- 🔄 Reverse-to-Turn Car: Only one motor — but it can still turn! Drive backwards and the wheels swing round. (level 4 of 5, about 50 minutes, 24 steps)
In the classroom
- E1 Push, Pull, Roll (Explorers, Foundation)
- I7 Pulleys & Belts (Inventors, Intermediate)
Common engineering questions
How can I try friction at home?
Push a wheel tight against its beam, then leave a small gap. Which one spins longer?
Which builds teach friction?
8 MightyCog builds teach it: Speed-Cancelling Gear Bridge, Coasting Wheel Bench, Push Buggy, Pull-back Racer and Compound Counting Bench and more. Start with Speed-Cancelling Gear Bridge: 5 steps, about 15 minutes.