Body A
At rest in the laboratory
Position x′
Velocity v′
NEWTON’S FIRST LAW · GALILEAN COORDINATES
A body keeps
its velocity.
With negligible net force, a body stays at rest or moves uniformly in a straight line. Watch both happen at once.
IDEAL ISOLATION · NET FORCE = 0
Drag to orbit · scroll / pinch to zoom · right-drag to pan
Running · markers every 2 seconds · observation pauses at 20 seconds.
Body B
Moving uniformly in the laboratory
Position x′
Velocity v′
Equal time. Equal distance.
Body B covers 2 m every 2 s. Its trail is straight and its markers are equally spaced. Body A’s markers overlap at rest.
What counts as isolated?
This model sets the net external force to zero. In practice, negligible net force means nearly constant velocity over the time observed. Distance from other bodies only approximates isolation: gravity can still act across space.
Rest depends on the frame.
A Galilean frame moves at constant velocity u relative to another inertial frame. Here, x′ = x − ut, v′ = v − u, and t′ = t. Switch frames: Body B can be at rest while Body A moves. The net force and acceleration remain zero. The animation pauses at 20 s to keep the bodies in view; their physical motion would continue.