MYP Design Stability and Strength Notes

Stability Is About Where the Weight Sits

1. The centre of gravity is the single point an object would balance on, the point all of its weight appears to act through.
2. Find it on a flat shape by hanging it from two different corners and drawing a plumb line each time, because the two lines cross at the centre of gravity.
3. The base area is the shape you get by joining up every point touching the floor, so a four legged stool has a rectangular base even though only four small tips are in contact.
4. An object stays upright while the vertical line down from its centre of gravity lands inside that base area.
5. Tilt it far enough that the line falls outside the base and gravity finishes the job without any help from you.
6. Only two moves make anything more stable: lower the centre of gravity, or widen the base.

Why a Tall Narrow Product Tips

1. Height on its own does not cause tipping, and a tall product with a wide heavy foot is perfectly safe.
2. The higher the centre of gravity sits above a given base, the smaller the tilt angle needed before the weight line escapes the base.
3. A desk lamp with a cast iron foot and a 500 mm arm stays put because nearly all its mass is in the bottom 40 mm.
4. Loading a product high up undoes your work, which is why heavy books belong on the bottom shelf of a bookcase and not the top.
5. Flat pack furniture ships with a wall strap because a shallow base saves floor space, so the wall is doing the stability job instead.
6. A toddler pulling on an open drawer applies force high up and far out from the base, which is why toppling furniture is a real safety hazard rather than a theory question.

Triangulation: The Only Shape That Cannot Fold

1. Push the corner of a square frame with pivoting joints and it flops into a parallelogram, because all four sides keep their length while the angles change.
2. A triangle cannot do that, since changing its angles would mean changing the length of at least one side.
3. Adding one diagonal across a square frame splits it into two triangles and stops the flop dead.
4. That diagonal is pulled in tension if the frame leans one way and squashed in compression if it leans the other.
   1. A cable diagonal only fights the tension case, so cable bracing is normally fitted as a cross of two cables.
5. The same trick shows up in the thin back panel of a wardrobe, the frame of a bike, an electricity pylon, a roof truss and the corner brace inside a laser cut box.

Webs, Gussets and Flanges: Material Where It Earns Its Keep

1. In a beam that is bending, the top and bottom surfaces do nearly all the work and the middle does almost none.
2. The flanges are those top and bottom strips, and the web is the thin upright piece holding them apart.
3. An I section spends most of its material on the flanges, so it resists bending far better than a solid bar of the same mass.
4. A gusset is a triangular plate fixed across a corner to stop the two members rotating against each other, which is triangulation built into a joint.
5. In a foamboard model a 40 mm card triangle glued into each corner turns a wobbly frame into a rigid one for a couple of grams.
6. A cardboard box uses the same layout lying down: two flat liners acting as flanges with a fluted middle acting as the web.

Fold It, Corrugate It, Roll It

1. A flat sheet is floppy because every scrap of its material sits in one plane, right next to the neutral axis.
2. Bending a single 20 mm lip along the edge of a 1 mm aluminium sheet makes it several times stiffer along that edge, with no extra material at all.
3. That lip is why a baking tray, the side of a filing cabinet and the base of a laptop all have turned over edges.
4. Corrugating means folding a sheet into repeated ridges, which stiffens it along the ridges while leaving it easy to bend across them.
5. Corrugated roofing sheet and corrugated cardboard both work this way, and both are noticeably floppy in the other direction.
6. Rolling a sheet into a tube is the strongest trick available, because every part of the material now sits far from the centre and resists bending from any direction.
7. Roll a sheet of A4 paper into a 30 mm tube, tape the seam, and four of them will hold a stack of textbooks clear of the desk.

Strength to Weight Ratio Decides What You Build It From

1. Strength to weight ratio compares how much load a material carries against how much it weighs, and it decides every product that has to be carried, worn or flown.
2. Balsa is weak in absolute terms but extremely light, so a balsa bridge model regularly beats a pine one in a load per gram competition.
3. Aluminium is about a third the density of steel, which is why bike frames, drone arms and aircraft skins use it even though steel is stronger.
4. Carbon fibre composite beats both, but it costs far more, needs specialist tooling and is hard to recycle, so it rarely suits a school project.
5. Compare fairly by testing samples of equal mass rather than equal size, otherwise the denser material wins simply for being heavier.
6. Put the ratio into your specification as a number, such as holds 5 kg while weighing under 300 g, so you have something measurable to judge your solution against.