The experiment
The revision rung: four steps of an experiment that changed something, put in order. It does not ask for the result, it asks for the route, because the route is what separates physics from a well-argued opinion. And how slow it was is usually the surprise: Eratosthenes measured the Earth with a shadow and a well two centuries BC, and Cavendish weighed the planet with two lead balls hanging from a wire, in a shed, going to the trouble of looking through a telescope from outside so that his own body heat would not spoil the reading.
Where this sits
Exercises from this rung
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Torricelli invents the vacuum
Answer He fills a long sealed tube with mercury, He upends it into a dish with no air getting in, The mercury drops and stops at about 76 cm, A gap with nothing in it is left at the top
Why Torricelli invents the vacuum, 1643. What holds the column up is the weight of the air outside. The barometer was born from that leftover gap.
easierFoucault shows the Earth turns
Answer He hangs a heavy ball from a 67-metre wire, He releases it carefully so it swings in one plane, He lets the swings leave a mark in sand, Hours later the plane of the marks has turned
Why Foucault shows the Earth turns, 1851. The pendulum did not turn: the floor beneath it did. It is the first visible proof without looking at the sky.
in betweenCavendish weighs the planet
Answer He hangs a rod with two small balls from a wire, He brings two huge lead balls near the ends, The rod turns a little and the wire twists, He measures the turn by telescope, from outside
Why Cavendish weighs the planet, 1798. He watched from outside so his own body heat would not stir the air and spoil the reading.
harder