‹ the sandbag

// real newtonian gravity, computed pairwise, nothing scripted

Gravity Well Playground

Every body in this box pulls on every other body, all the time, by the actual inverse-square law — force proportional to the product of two masses, divided by the square of the distance between them, summed across everyone else in the sandbox, dozens of times a second. Nothing here is a keyframe. If an orbit looks stable, that's the math agreeing with itself for now, not a promise.

Click or tap to drop a body. Drag before releasing to fling it — the dashed line is the aim. A soft distance floor keeps forces from going to infinity at point-blank range, and bodies that genuinely collide merge, mass and momentum conserved. Anything that picks up enough speed eventually leaves for good.

The sandbox

Loading the sandbox.

Bodies
0
Launched lifetime
0
Longest run
0s
1.00×
New body mass

How this works

  • Click or tap drops a body at rest. Drag before releasing to launch it instead — the dashed line shows the aim, the arrow shows the direction, and how far you drag sets the speed.
  • Keyboard: focus the sandbox, then ←↑↓→ to move the gold crosshair, Enter / Space to arm a body there, arrows again to aim its velocity, Enter / Space to launch, Escape to cancel. Hold Shift for bigger steps.
  • Pick a mass below before placing a body — heavier bodies pull harder on everything else, and win most mergers.
  • Pause freezes the physics completely. Time scale only changes speed while it's running, from slow motion up to fast forward.
  • Clear bodies wipes the sandbox and reseeds the default star system. It does not touch your saved stats below.
  • The sandbox holds at most 40 live bodies at once, and anything that flies far enough off-frame is considered gone for good and stops being simulated, so a long session stays fast.

What this actually is: a genuine N-body gravity simulation, not an animation of one. Every pair of bodies attracts every frame, integrated with a stable step-by-step method (semi-implicit Euler, substepped for stability) rather than the naive approach that visibly spirals orbits outward over time. A small softening constant keeps the force from spiking to infinity when two bodies pass very close together; if they get closer still, they merge into one body instead of passing through each other or blowing up the math. None of that is faked — it's the same handful of equations, run honestly, over and over.

One honest limitation: the live simulation itself — every body's exact position and velocity — is not saved. Reload this page and you get a fresh default star system, not the mess you left behind. Serializing a running physics sim reliably across reloads is more trouble than a toy like this is worth. What this room does remember, in one localStorage key, is your last time-scale setting, your last chosen mass, how many bodies you've personally launched, and the longest single run you've kept a simulation going. Nothing here is ever sent anywhere.

The button below wipes this room's saved preferences and lifetime stats — total bodies launched, longest run — and resets the sandbox to its default star system, on this device only.

self-contained · no tracking · no network calls after this page loaded · runs entirely in your browser · state lives in sandbag.gravitywell.v1