Arch
An arch is a curved shape that spreads weight to its sides so it can hold heavy loads.
A curved row of blocks forms an arch shape over a gap between two stacks. Each block pushes against the ones beside it, spreading the load sideways to the stacks below. The stacks carry that weight down to the table. Pull any one block out and the whole row falls — the curved shape is what holds it all up.
Explaining arch by grade level
Think about an eggshell. It curves over like a tiny dome. Press down on top and the shell pushes out to the sides. That curved shape is what makes it strong, not thick walls.
Projects that explore arch
A curved shape spreads weight to its sides instead of letting it push straight down. Eggshell domes work the same way. The curved shell pushes force outward and downward, so four small domes can support a tall stack of books without cracking.
An arch's height relative to its width determines how well it spreads weight to its sides. When you bend plywood strips of increasing length into arches that all stand 100 mm tall, the wider arches produce a flatter curve. Hanging a pail from each arch's center and adding sand in 100-gram increments reveals the difference: wider arches collapse under less weight. A flatter curve weakens the sideways force distribution, confirming that increasing a dome's diameter without raising its height weakens the structure.
An arch spreads weight along its curve, but the shape of that curve matters. When plywood strips of different lengths bend between two pillars set 500 mm apart, a 650 mm strip creates the strongest arch and holds 16.5 kg. Shorter strips and longer strips both break under less weight, showing that an arch needs the right curve to spread force well.