Lesson 01 · 3 min · 5 things to do
Falling, and missing
Explain why something in orbit is not beyond gravity.
A space station orbits 400 km up. How strong is Earth's gravity there?
- Yes.400 km is a small addition to Earth's 6,371 km radius, so gravity is barely reduced. Astronauts float for a completely different reason.
- Not quite.If gravity were zero the station would travel in a straight line and leave. It is gravity that keeps bending its path into a circle.
- Not quite.Gravity falls with the square of distance from Earth's CENTRE, and 400 km is only 6% further out than the surface.
Astronauts float because they are falling, and so is everything around them. Orbit is a permanent fall that keeps missing the ground.
Fire a cannonball horizontally from a very tall mountain. Slide the speed.
1 km/s5 km/s7.8 km/sHow far it travels200 kmEarth's circumference40,000 km1 km/sIt arcs and lands a long way off. Falling, and hitting.
How far it travels5,000 kmEarth's circumference40,000 km5 km/sA huge arc, and still it comes down. The curve of its fall is sharper than the curve of the Earth.
How far it travels40,000 kmEarth's circumference40,000 km7.8 km/sIt falls at exactly the rate the ground curves away. It never lands.
At the last speed, what is the cannonball doing?
- Yes.This is the whole idea of an orbit, and it is Newton's own thought experiment. Nothing is holding it up — it is simply moving sideways fast enough that its fall never arrives.
- Not quite.It is fully inside gravity, and being turned by it constantly. Escape needs a much higher speed and a different path.
- Not quite.There is no upward force. There is only sideways motion and a continuous fall.
Move the control to see what changes.
A rocket goes straight up to 400 km and stops. What happens?
- Yes.Altitude alone is not orbit. Sub-orbital flights reach space and return within minutes, because they never got the sideways speed that makes a fall miss.
- Not quite.Nothing holds it. Space is not a shelf.
- Not quite.An orbit needs sideways motion. Without it, the object's path is a straight line up and back down.
Which of these are true of something in orbit?
It is being pulled by gravity almost as strongly as you are.
It is falling all the time.
It has escaped Earth's pull.
It moves sideways fast enough to keep missing the ground.
It is weightless because there is no gravity up there.
Yes.Weightlessness is the feeling of falling freely, not the absence of gravity. Anyone who has been over the top of a rollercoaster has felt a second of the same thing.A low orbit takes about 90 minutes to go round. How many sunrises does someone on board see in 24 hours?
sunrisesYes.24 × 60 ÷ 90. Sixteen orbits, sixteen sunrises — which is a direct consequence of moving fast enough for a fall to keep missing.
Lesson complete
Orbit is not being beyond gravity. It is falling and missing the ground.
Next: Sideways is the whole problem →