Does an orbit stay put?
Try: choose Sun and planets, leave Euler off in More controls, then play.
Why: Leapfrog balances its half-step velocity updates on a calm orbit, so energy drift stays near zero and the planets keep circling.
LAB / GRAVITY PLAYGROUND
Gravity playground.
Drag empty space to fling a body, tap to drop one into orbit, or drag a body to move or throw it. Pick a scenario to start.
An educational 2D Newtonian gravity model in illustrative units, not general relativity or a simulator of record. Tilt and lensing are visual effects. Breakups, radiation, evaporation and explosions use simplified rules. The bounded launch preview follows moving bodies, gravity, collisions and wormholes, but omits those special events and is skipped in crowded scenes.
Bodies 0Time 0Energy drift —
An animated view of the chosen scenario. With a pointer, drag on it to throw the selected body, or tap to place it in orbit. Press Space while it has focus to play or pause. With the keyboard, use the arrow keys to move a cursor; press L with a throwable tool to aim, use arrows to pull back, and press Enter to launch. Escape cancels. Enter alone places the selected tool. The square brackets pick a body and M carries it; press the question mark for the full list. Two fingers or Shift-drag turn the view, and the dotted line is the predicted path. The readouts below describe what is happening.
How to play
Pick a body under Throw, then drag on the canvas to throw it, or tap to drop it into orbit. Tap a body to inspect it. Choose a scenario and press Play. Show more controls has the rewind timeline, and Undo takes back your last change.
Show keyboard controls
Click the canvas first, then: arrow keys move the cursor, L starts a launch aim with a throwable tool, arrows pull back, and Enter launches; Escape cancels. Enter alone places the selected tool. [ and ] choose a body, M carries it, Space plays or pauses, and ? announces the keyboard controls. The scene pauses while you carry or aim.
Throw
Show more controls speed, look, mass, special tools, camera, Future paths, rewind, sharingHide more controls
Speed
Special
View
tap a body to pick it first
Show
Timeline
press Play first to record history
Physics
Bodies
Try a scenario
Does the method matter?
Try: choose Figure-eight (Three-body), play, and press Euler integrator in More controls. Reset keeps Euler on; switch it off to compare.
Why: Euler updates position and velocity in sequence, so its energy error accumulates and the delicate eight comes apart. Speed changes viewing pace, not the physics step.
Can a planet speed a probe up?
Try: choose Slingshot and play.
Why: the moving planet pulls the probe while it passes; the probe gains speed as the planet gives up a little momentum. The readout compares its speeds in illustrative units.