Cavendish Torsion Balance (2D)

Two big masses twist a torsion arm by micro-radians; read the laser spot and recover G. Same engine as the 3D laboratory.

Live readout

Numerical checks

The science

Four Newtonian pairs give the torque tau(theta) = sum over pairs (x F_z - z F_x) with F = G m M / r^2. The arm obeys I theta'' = tau - kappa theta - c theta' with I = 2 m L^2, a natural period T = 2 pi sqrt(I / kappa) and, once settled, G = kappa theta / tau_1 where tau_1 is the torque for G = 1.

Time is accelerated (time-scale slider); real runs take hours and need vibration, temperature and air-current control.
Simplifications: point masses, a massless fibre and arm, a flat horizontal plane, no sensor noise in this 2D view.

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