Free Fall & Orbit
ISS orbit — objects float but gravity is there. Builds on Newton's Three Laws.
Astronauts on the International Space Station float around, apparently weightless. The ISS orbits at about 400 km altitude.
Why do astronauts float?
The ISS is falling toward Earth every second — but it's also moving sideways so fast (28,000 km/h) that Earth's surface curves away beneath it. The result: it falls in a circle. Everything inside falls together, so there's no relative motion — that's weightlessness.
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Fire a cannonball from a mountaintop. Increase speed until it falls AROUND the Earth instead of hitting the ground.
v_orbit = √(gR)
Newton imagined a cannon on a mountain firing faster and faster. At ~7.9 km/s, the cannonball falls at the same rate Earth curves — it orbits! This was 300 years before Sputnik.
The Moon falls toward Earth every second — but its sideways speed is just right to keep it in orbit. It's been falling for 4.5 billion years and hasn't hit us yet.
Before you open the parachute, you're in free fall — briefly weightless. Your stomach 'drops' because your organs are falling at the same rate as the rest of you.
GPS satellites orbit at 20,200 km altitude. They're in free fall too, just much higher and slower. Their clocks tick differently due to both speed AND gravity — Einstein was right.
“Newton realized that the same force making an apple fall also keeps the Moon in orbit. The only difference is speed. Throw a ball fast enough, and it becomes a satellite.”
Orbital Launcher
Launch projectiles from a mountain at increasing speeds. Watch them transition from falling, to suborbital arcs, to full orbits, to escape trajectories.
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Find the exact speed where the projectile completes one full orbit. Then increase just a bit more — watch it become an ellipse. Go even faster to escape Earth entirely!
There are exactly three outcomes: crash (too slow), orbit (just right), or escape (too fast). The boundary between orbit and escape is called escape velocity: v_esc = √(2gR).