Newton's Cradle
Five suspended spheres, one transfer of momentum, and an instructive refusal of common sense to substitute for mechanics. Pull a sphere aside and release it. Then change the laws of the experiment.
What the instrument demonstrates
For an ideal elastic collision, both momentum and kinetic energy are conserved. With equal masses, the familiar result is an impulse travelling through the stationary middle spheres and sending the sphere at the far end outwards. Unequal masses redistribute energy and velocity in less familiar ways.
Each sphere moves as a pendulum, approximately following θ̈ = −(g/L) sin θ. The programme resolves contact by applying impulses along the horizontal collision normal and enforces the pendulum constraint. Restitution below 1, or the modest simulated air resistance, gradually removes mechanical energy. At zero gravity a displaced ball will not begin swinging unless given a velocity.
This is a numerical teaching model, not a rigid-body laboratory metrology package. Spheres remain the same visible size when mass is changed, so that geometry is not confused with mass. Rapid simultaneous collisions and long-term contact are approximated by iterative numerical resolution.