Presets
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Physics
Balls
Model checks
The science
Impulse model
The 3D lab and this page use the same rules. Between impacts each ball falls with a = −g. A wall hit reverses the normal velocity and multiplies it by the wall restitution ew. For two balls with masses m1, m2 (mass ∝ r³, the volume of a sphere) and relative normal speed vn the impulse is J = −(1 + e) vn / (1/m1 + 1/m2) applied along the contact normal, so total momentum is conserved and the kinetic energy lost equals ½ (1 − e²) μ vn² with reduced mass μ.
Bounce envelope
A ball dropped from height h0 rebounds to hn = ew²n h0 after n floor hits, the dashed envelope in the third graph.
Honest simplifications: frictionless circles (no spin), a 2D box with mass proportional to r³ kept for consistency with the 3D page, mass-weighted overlap correction instead of continuous collision detection, no air drag.
What to try
- Use Perfectly elastic: kinetic plus potential energy stays constant.
- Use Zero gravity gas: the speed histogram relaxes towards a thermal-like shape.
- Drop the tagged ball and compare the rebound peaks with the red markers.