Span the gap. Load it. Break it on purpose.

Step 1. Mass to weight

Rule: weight in newtons = mass in kilograms × 10. (W = mg, g ≈ 10 N/kg; Gravity and weight.)

Step 2. Share the load

Hang 2 kg (20 N) from the middle of the deck. The bridge has two side trusses, so each carries half: 10 N. The load is in the middle, so each table pushes up on each truss with half again: 5 N. Check: 5 + 5 + 5 + 5 = 20 N up, 20 N down. Balanced (Balanced and unbalanced forces).

Step 3. The force in one triangle (scale drawing)

At the top of one triangle, the load pushes down and the two sloping sides push back up along their lengths. Draw it to scale (M8-14-2, M7-11-3):

• Draw a line 10 cm straight down: 1 cm = 1 N, so this is 10 N.
• From each end, draw a line at 30° to the vertical (the sides of an equilateral triangle slope at 60° to the ground, which is 30° to the vertical).
• The lines meet and close a triangle. Measure the two new sides: each is about 5.8 cm, so each sloping straw carries about 5.8 N, squeezed (compression).

Check: force in each side = load ÷ (2 × sin of the slope angle) = 10 ÷ (2 × 0.866) ≈ 5.8 N. Trig is Year 9 (M9-10-1); the scale drawing gets the same answer without it.

What the slope does:

Flat triangles make big forces. Steeper triangles share the load better.

Step 4. Span and depth

The load tries to bend the bridge. The top strip gets squeezed (compression) and the bottom strip gets stretched (tension). Rule for a load in the middle: force in the top or bottom ≈ load × span ÷ (4 × depth). (Midspan bending moment of a simply supported beam, M = PL/4, carried as a pair of equal and opposite chord forces F = M ÷ d; moments are Moments and turning forces.)

One truss, 10 N load, 60 cm span:

Twice as deep, half the force (M9-07-1). Real truss bridges are often about 10 times longer than they are deep; paper bridges do better deeper, because paper is weak.

Step 5. Use each material for its job

• Bottom strip (tension): string or a paper strip. Both are strong when pulled.
• Top strip and sloping sides (compression): straws or tightly rolled paper tubes. Keep them short: a straw half as long can take about four times the push before it bends sideways. (Euler buckling load ∝ 1/length²; not in our blocks yet.)
• Diagonal length before cutting: for a triangle 10 cm deep over a 10 cm half-bay, diagonal = √(10² + 10²) ≈ 14.1 cm (M9-08-1).

Step 6. Test to failure

Add 100 g at a time. After each step, measure the sag at the middle with a ruler standing on the floor. Stop when something gives.

Example numbers. Plot load against sag (M8-15-2). It starts as a straight line; when the line bends upwards, something is about to give. Write down where it broke and which kind of force broke it (a straw folding = compression; tape peeling or paper tearing = tension).

Step 7. Strength for its weight

The bridge weighs 80 g and held 1,100 g. Ratio: 1,100 : 80 = 55 : 4, or about 14 : 1. It holds about 14 g for every gram it weighs (M7-08-1, M7-08-3). That's the number to beat next time.