🪑 ➡️ 🛞⚫⚫

A flat piece of cardboard with two bottle-cap wheels on a stick axle, halfway down a sloping plank.

🎒 What you need

4 bottle caps, a nail and a stone to punch holes
two straight sticks or old pen tubes for axles
a piece of cardboard, and a plank or board to make a slope

✋ Do this

Punch a hole in the exact middle of each bottle cap. Off-centre holes give you a wobbly ride — try one on purpose later to see.
Push a stick through two caps to make an axle. Make two axles.
Tape the axles under your cardboard so the wheels spin freely. If they cannot spin, it is a sledge, not a cart.
Prop your plank on a stone. Race your cart down. Then drag a plain piece of cardboard down the same slope.
Watch the difference with your own eyes — the wheels win, and you can point at exactly why: the caps roll, the cardboard scrapes.
Load a small stone on top and race again. Does weight help or hurt?

💡 Why it works

Dragging means the whole surface fights you the entire way — that fighting is called friction. A wheel only touches the ground at one small spot at a time, and that spot rolls instead of scraping. This is why a rover crossing a field, a matatu on the road and a robot in a warehouse all use wheels, and why a bent axle ruins everything.

🔥 Challenge

Build a cart that rolls in a curve instead of straight. Hint: what happens if one side's wheels are bigger than the other's?

📖 New words

axlethe rod a wheel spins around

frictionthe rubbing that slows things down

⚙️

Wheel Wright

Tap when you have finished this module.