Build It Strong: Structures and Mechanisms

You need: A rectangular kitchen sponge per pair · A permanent marker · A ruler · A pool noodle (optional)

1. Draw straight lines across the sponge every 1 cm, like a ladder.
2. Squash it between your palms. Sketch the lines: are they closer or further apart?
3. Stretch it gently from the ends and sketch again.
4. Bend it into a smile. Look at the top and the bottom. Which side is squashed and which is stretched?
5. Twist it. Sketch the lines now. This is torsion.
6. Push the top one way and the bottom the other way. This is shear.
7. Label your four sketches with the words compression, tension, torsion and shear.

How you'll know: You can point to a beam or shelf at school and say which side is in compression and which is in tension when it is loaded.

Go further: Bend a pool noodle with lines on it. Where is there no squash or stretch at all? Engineers call this the neutral axis.

You need: 8 paper straws · Split pins or pipe cleaners for joints · A hole punch or skewer to make holes · A small weight or a stack of coins

1. Make a square frame from four straws, joined at the corners with split pins.
2. Make a triangle frame from three straws.
3. Stand each frame up and press down on the top corner. What happens to each shape?
4. Add one diagonal straw to the square. Test it again.
5. Try the same test on a pentagon. How many diagonals does it need before it stops moving?
6. Record: shape, number of straws, did it change shape (yes/no).

How you'll know: Your braced square holds its shape, and you can explain that a triangle can only change shape if one of its sides changes length.

Safety: Take care with skewers and split pins; push them through on a cutting board, not towards your hand.

Go further: Find three places in a photo of the Auckland Harbour Bridge where a square panel has been turned into two triangles.

You need: 20 paper straws · 1 m of masking tape · Two desks or book stacks 30 cm apart · A paper cup hung from string · Washers or coins · A ruler

1. Plan on paper first. Sketch a side view with triangles along the length.
2. Build two identical side trusses, then join them with cross pieces.
3. Lay the bridge across the 30 cm gap. Hang the cup from the middle.
4. Add washers one at a time. After every five, measure how far the middle has sagged in millimetres.
5. Keep going until it fails. Record the load and film or photograph the failure.
6. Look at the broken bridge. Was it a straw that buckled (compression) or a joint that pulled apart (tension)?

How you'll know: You have a table of load and sag, and you can point to the exact part that failed and name the force that broke it.

Safety: Keep feet out from under the load. Stop before the cup is heavier than 1 kg.

Go further: Build a second bridge with the same materials but a different truss pattern (all triangles pointing up, then alternating). Which carries more per straw?

You need: Three A4 sheets of the same paper · Sticky tape · A stack of books of similar weight · A kitchen scale

1. Roll one sheet into a round tube, fold one into a triangular prism and one into a square prism. Each is the full height of the page.
2. Tape each seam with the same amount of tape.
3. Stand a column up and gently stack books on top, one at a time, keeping them level.
4. Weigh the books the column held just before it crumpled.
5. Repeat for all three shapes. Make it fair: same paper, same height, same tape.
6. Draw a bar graph of your results.

How you'll know: You can explain which column held most and suggest why, using the words buckle and compression.

Go further: Test whether four round columns under a board can hold your own body weight, with a teacher spotting.

You need: Two cotton reels or bottle-cap pulleys on pencils · String (2 m) · A 1 kg bag of rice · A spring balance or kitchen luggage scale · A strong broom handle across two chairs · A metre ruler

1. Lift the rice straight up with the spring balance. Record the force.
2. Hang one pulley from the broom handle. Run the string over it and lift the rice. Record the force and which way you pull.
3. Now make a block and tackle: one fixed pulley on the broom and one moving pulley attached to the rice.
4. Lift again. Record the force.
5. Measure how much string you pull to lift the rice 20 cm in each setup.
6. Fill a table: setup, force needed, string pulled. What pattern do you see?

How you'll know: Your results show that the two-pulley system needs less force but more string, and you can explain that as a trade: less force, more distance.

Safety: Keep the load below knee height and feet clear. Tie off the string when not pulling.

Go further: Design a system with three pulleys and predict the force before you test it.

You need: Card gears from a printable template (or bottle caps with notched edges) · Split pins · A corrugated card base · A cotton reel, pencil and string for the winch · A small toy passenger

1. Pin a big gear and a small gear on the card so their teeth mesh.
2. Mark a dot on each gear. Turn the big gear once. Count how many times the small gear turns.
3. Swap which gear you turn. What changes: speed, force or direction?
4. Push a pencil through a cotton reel to make an axle. Tape a card crank handle to one end.
5. Tie string to the reel and to the toy passenger. Turn the handle to reel the passenger in.
6. Sketch your gear train and label input, output and direction of turning.

How you'll know: You can predict how many turns the output gear makes, and your crank winch pulls a small load along a line.

Go further: Add a third gear. What happens to direction? Use gears to make the winch easier to turn.

You need: A baking tray on four tennis balls or 12 marbles inside a larger tray · String tied to the top tray · A tower of straws, card or paper at least 60 cm tall · A 100 g 'rooftop' load (a bag of rice) · A timer and phone for slow-motion video

1. Tape your tower to the top tray. Put the rooftop load on top.
2. Agree a fair shake: pull the string 5 cm back and forth, once per second, for 20 seconds.
3. Run the test. Record whether it stands, leans or falls, and when.
4. Now add base isolation: rest the tower on a card platform sitting on four rubber erasers or marbles in bottle caps.
5. Run exactly the same shake. Compare.
6. Film both tests in slow motion and watch where the tower bends most.

How you'll know: You have evidence from two matched tests and can explain what the isolators changed.

Safety: Test on the floor, away from walls and desks. Clear up marbles straight away so no one slips.

Go further: Measure how far the top of the tower sways in millimetres against a ruler taped behind it, with and without isolators.

You need: Two towers from dowel, rulers or EPro8 rods (at least 40 cm tall) · String for the main line and guy ropes · A 500 g weight on a hook · A protractor · Masking tape

1. Stand the towers 1 m apart and tie the main line tight between their tops.
2. Hang the 500 g weight from the middle. Observe what the towers do.
3. Measure the angle of sag at one tower with a protractor.
4. Loosen the line so it sags more. Hang the weight again. Which setup tips the towers more?
5. Tie guy ropes from the back of each tower to a desk leg. Test again.
6. Draw a force diagram: arrows showing the weight pulling down and the line pulling the tower inward.

How you'll know: Your towers stay upright with the load, and your force diagram shows why a tight line pulls harder on the towers than a saggy one.

Safety: Wear goggles when tightening lines. Keep heads away from the line in case it snaps.

Go further: Lower the finishing tower so the seat rolls down by gravity, then design a way to stop it gently at the bottom.