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Why a small ball on a big ball shoots up nine times higher: the two-step bounce
Drop a tennis ball on top of a basketball and the small ball can fly far above the height it fell from. Nothing is cheating: it is a two-step bounce. The floor…
Why salt makes ice cream freeze: ice melting below 0 °C pulls out the heat
Ice is already at 0 °C, so how can a handful of rock salt make it colder? Because salt makes ice want to melt below 0 °C, and melting costs energy: about 334…
Why a boat's wake is always a 39° V: Kelvin's wedge
Behind every boat spreads a V of ripples, and it is always about 39° wide, whether the boat crawls or speeds. Lord Kelvin worked out why in 1887: only waves…