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Why a ship rolls back upright, until the cargo goes too high: the metacentre and the righting arm A loaded ship that leans over usually rolls back upright, because the water's upward push shifts sideways and twists her straight. But raise the cargo high… Engineering4 min read1 Nov Why there are two high tides a day: the Moon pulls the near sea harder and the far sea softer The sea rises twice a day, not once, and the second high tide happens on the side of the Earth facing away from the Moon. Why? The Moon pulls the near ocean a… Earth & Space4 min read27 Oct Why planets speed up near the Sun: Kepler's equal-areas law Earth moves at 30.29 km/s near 3 January and 29.29 km/s near 4 July, with nothing pushing it along. The rule behind the change is Kepler's equal-areas law: a… Earth & Space3 min read20 Oct Why a falling ruler slips through your fingers: gravity as a reaction-time stopwatch Drop a ruler without warning and you will not catch it at the top. The place where your fingers close is a stopwatch reading: the distance it fell is ½ g t²… Mind4 min read18 Oct Why rockets drop their tanks on the way up: the rocket equation and staging Orbit isn't about height. It's about speed: roughly 7.8 km/s sideways, and about 9 to 10 km/s of total speed gain once gravity and air drag take their cut. The… Earth & Space4 min read16 Oct Why a cannonball fired hard enough never lands: Newton's cannon and the orbit Fire a cannonball horizontally from a mountaintop and it lands some distance away. Fire it harder and it lands farther. Fire it hard enough and the Earth's… Earth & Space4 min read13 Oct Why the bottom of a dropped Slinky hangs in mid-air: a wave has to bring the news Hold a Slinky by its top, let it stretch, and let go. The top rushes down while the bottom coil hovers, apparently ignoring gravity, until the collapsing coils… Physics5 min read11 Oct Why the Moon always shows us the same face: tidal locking The Moon does spin. It turns exactly once for every trip around the Earth, so the same face always points our way. Walk around a chair while always facing it… Earth & Space3 min read7 Oct Why astronauts float: the space station never stops falling Astronauts on the International Space Station look weightless, but at 400 km up, Earth's gravity is still about 90% as strong as on the ground. The trick is… Earth & Space4 min read28 Sep Why we only ever see one side of the Moon The Moon always shows us the same face, so it's easy to assume it doesn't spin. It does. It turns exactly once for every orbit, about every 27.3 days, so the… Earth & Space6 min read26 Sep