Module 2 · Falling around: orbits
Inertia, and falling and missing
Interactive 7 min
Switch the Sun off and the Earth would coast in a straight line at about 29.8 km/s [1]. Switch it on and gravity bends that straight line into a closed loop. An orbit is a body falling toward the Sun all the time and always missing it, because it also moves sideways.
Try this. Compare the green arrow (velocity) with the red one (gravity). Which one points along the path and which one points at the Sun?
Sources: [1] Earth Fact Sheet (NASA GSFC NSSDC)
Step 1 of 2
A moving body keeps moving
Now remove the Sun entirely. With no force acting on it, the Earth obeys Newton’s 1st law: it drifts in a perfectly straight line at a constant 29.8 km/s, forever (green arrow = its velocity). This is inertia. On its own, motion makes a straight line — never a curve, never a circle. Something has to bend the path. Keep this drifting Earth in mind for the next step.
Text version of this interactive
A moving body keeps moving
Now remove the Sun entirely. With no force acting on it, the Earth obeys Newton’s 1st law: it drifts in a perfectly straight line at a constant 29.8 km/s, forever (green arrow = its velocity). This is inertia. On its own, motion makes a straight line — never a curve, never a circle. Something has to bend the path. Keep this drifting Earth in mind for the next step.
Why the Earth doesn’t fall into the Sun
Put it together. The Sun’s gravity (red arrow) pulls the Earth straight toward it the whole time — so why no collision? Because the Earth is also moving sideways (green arrow) at 29.8 km/s. Each moment it does fall toward the Sun, but its sideways speed carries it past — it keeps missing. The dashed line shows where inertia alone would send it; gravity bends that straight path into a closed loop. An orbit is simply falling, continuously, and always missing.
About this interactive
- Licence
- MIT
- Credits
- © 2026 Mateusz Wojczal. Earth texture: Solar System Scope, CC BY 4.0. Three.js (MIT). Inter and Roboto Mono (SIL OFL 1.1).
- Source
- https://github.com/ulams-dev/ulams/tree/main/demo-content/gravity