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Physics Tricks

Three experiments whose answer surprises, each worked from the physics, drawn in 3D and held to it by tests. The upside-down pendulum is Kapitza's: a stiff pendulum whose pivot shakes up and down, and the tests see it stand upside down shaken above the rate A^2 w^2 = 2 g l gives (31.5 Hz here) and fall below it, rocking upright at the slow rate averaging theory gives to within 1%. The cradle's steel balls hang as pendulums and press by Hertz's law for steel spheres while they touch: the tests keep its energy to 1 part in 10,000 and its momentum through a knock, and see one, two or three balls in send over 95% of the energy out in as many at the far end. The superball is Garwin's ultraelastic rough ball: at every bounce the tests see no energy lost and its angular momentum about the touching point kept, and thrown under the table it comes back out after the floor, the table and the floor, while a smooth ball goes straight through. Ours: every size, mass and throw, and no losses anywhere: a real cradle dies away, a real superball bounces a little lower each time.