Topic
Forces
10 articles
Physics
Why does a spinning coin whirr before it stops?
A coin flattens out, the rattle climbs to a frantic buzz, and then everything ends at once with a slap. Something in there is speeding up while the coin itself is slowing down, and the mathematics says it reaches infinity.
Physics
Why does a whip crack?
A person swings their arm at walking pace and something at the far end of a leather rope breaks the sound barrier. Nothing is exploding and nothing is hitting anything. The bang is a small piece of air being shoved out of the way faster than it can move.
Biology
Why don’t birds fall off branches when they sleep?
The famous answer is that a bird's toes lock shut automatically when it settles down. Somebody tested that properly, and birds whose locking tendons had been removed altogether slept on their perches perfectly well.
Physics
Why is ice slippery?
Wet glass is not slippery. Cold metal is not slippery. Ice is, even at temperatures where nothing on it could possibly be melting, and the explanation everybody was taught turns out to be far too feeble to work.
Technology
Why did the Millennium Bridge wobble?
A brand new footbridge opened across the Thames, swayed alarmingly under the crowd, and shut two days later for two years. Nothing was broken and nothing was undersized. The people crossing it were the missing part of the design.
Astronomy
Why can’t we feel the Earth spinning?
Standing still at the equator, you are being carried eastward faster than an airliner flies. Nothing rattles, nothing leans, and a dropped ball falls straight down. For two thousand years that was the best argument against the Earth moving at all.
Chemistry
Why does wet sand hold its shape?
Dry sand cannot even hold up a small heap. Add a splash of water and the same grains will stand in a tower with vertical walls. Add more water and it collapses into soup, so there is a right amount, and it is far smaller than anybody guesses.
Physics
Why does a bicycle stay up?
Standing still it falls over the moment you let go. Rolling, it will cross a field on its own with nobody on it. Everybody has heard the explanation involving spinning wheels, and somebody built a bicycle specifically to test it.
Physics
Why does toast land butter-side down?
Everybody assumes this is a joke about bad luck, or that the butter weighs one side down. Ten thousand drops by a thousand schoolchildren say it is real, and the culprit turns out to be the height of your kitchen table.
Physics
Why doesn’t a spinning top fall over?
Set a top going and it leans at an angle that would flatten anything else on the table, then stays there, drifting round in a slow circle. Nothing is propping it up, and gravity has certainly not let go of it.