Did you solve it? Do you have the brain of an engineer?

The answers to today’s puzzles

Earlier today I set four engineering puzzles. Here they are again with solutions. (Do check out the hundreds of great comments posted below the line in the original story.)

The cranberries

In a cranberry sorting facility, thousands of cranberries are dropped every minute onto a board. Good cranberries are firm, and bounce, while bad cranberries are soft, and don’t bounce.

What simple trick allows sorters to catch all of the good ones and none of the bad?

Solution

The board is at an angle. Good berries bounce off onto a conveyor belt. Bad ones fall down into a discard box.

Caps off

In the US, medicine bottle caps are designed to be secure against only 85 per cent of children under five.

Why is this percentage not higher? Eighty-five seems quite low.

Solution

If the child-resistance rate was 100 per cent, the mechanism would also be hard for weak or arthritic hands. If the bottle is too hard for an elderly person to open, they may leave the cap off entirely, which is more dangerous than the small percentage of clever or strong kids who’d have taken it off anyway.

The standard isn’t “as safe as possible” — it’s tuned to the point where making it safer for kids would make it less safe overall.

Slats entertainment

In 2020, engineers replaced the pedestrian handrail on the Golden Gate Bridge. The old handrail rested on metal balusters in the shape of an H, so the support looked like “HHHH…” where the H’s horizontal section was 10cm wide. The new handrail was supported only by thin vertical slats, making it look like “IIII…” and where each slat was only 0.6cm wide.

The aim of the new handrail was to make it less resistant to wind, and hence safer during high winds. But it brought two serious unintended consequences.

What were they?

Solution

1) The wind level on the bridge was much higher, to the great annoyance of cyclists.

2) During certain winds, the whole bridge began to make an unbearable whistling noise that could be heard for miles. Essentially, the new slats behaved like reeds, and the bridge turned into in a gigantic harmonica.

Under pressure

Cabin air pressure in a plane is controlled to a level that is comfortable to humans. When a plane flies, the air pressure outside the plane drops to much lower than this level, causing the air pressure in the cabin to push outwards against the frame with great force.

In the early 1950s, there were some major plane crashes when the fuselage was ripped apart mid flight.

UK manufacturer de Havilland found a way to pressure test its plane, the Comet, the world’s first commercial jet, without the plane leaving the ground.

What did they do?

Solution

Engineers at Farnborough built a giant water tank at and submerged the plane in it. Filling the cabin with water and increasing the pressure let engineers safely find the limit at which the structure would fail. Air at that same pressure would have exploded destructively, destroying the evidence along with the plane. Here’s a picture of the water tank from 1954, with the Comet’s wings sticking out.

Today’s puzzles were adapted from How to Keep a Plane in the Sky, by Alex Davies. The book tells fascinating stories about test engineers – those geniuses whose work underpins our trust that objects do not crack, break or explode. You can buy the book at the Guardian Bookshop and other retailers.

I hope you had fun. I’ll be back in two weeks.

I’ve been setting a puzzle here on alternate Mondays since 2015. I’m always on the look-out for great puzzles. If you would like to suggest one, email me.