Helps you understandanother area
Phases of the MoonGravity is what keeps the Moon in the orbit that produces the monthly cycle of phases.
Gravity
Why does everything fall down — and what is the Moon falling towards?
The force that pulls an apple to the ground is the same one that keeps the Moon circling Earth. Meet mass and weight, free fall, orbits and why astronauts float.
5 depths≈ 220 min of reading70 practice questions
The ladder
Choose how deep to go
Every depth is open. Start where it feels interesting — you can always move back.
- Discoverabout 35 minWhy does everything fall down?Start with a dropped pencil and end with galaxies. Discover what a force is, why heavy things do not fall faster, how air changes everything, the real difference between mass and weight, and the true reason astronauts float.Owner-accepted · not line-by-line reviewed
- Understandabout 45 minHow gravity works: weight, falling and orbitsTurn the story into rules you can use: weight = mass × g, distance = ½ g t², why mass cancels in free fall, how drag sets terminal velocity, Newton’s universal law in words, and the real reason astronauts float.Owner-accepted · not line-by-line reviewed
- Investigateabout 45 minTest it: predictions, ramps, pendulums and Newton’s own proofTurn gravity into hands-on science: rebuild Galileo’s ramp, design fair tests for mass and shape, weigh the Earth with a pendulum, check whether Newton’s law survives the trip to the Moon, hunt for orbital speed by binary search, and see how ISRO climbs to the Moon and Mars one burn at a time.Owner-accepted · not line-by-line reviewed
- Go deeperabout 50 minThe mathematics behind every number in this topicMeet Newton’s law with its constant G, derive orbital and escape speed from scratch, redo Newton’s Moon test in full, explore why gravitational and inertial mass are equal, see why g is not uniform on Earth, and look at the mechanics behind ISRO’s orbit-raising missions.Owner-accepted · not line-by-line reviewed
- Extendabout 45 minCurved spacetime, black holes and the questions nobody has answered yetGo beyond Newton to Einstein: gravity as curved spacetime, the rubber-sheet picture and its flaws, the tests that confirmed general relativity, black holes, gravitational waves, orbital puzzles from tidal locking to dark matter, and open questions with real projects.Owner-accepted · not line-by-line reviewed
Practise and apply
Question bank · 70 questions
9 sections from Foundation to Challenge, each with hints and a worked solution — or play a 10-question round for points and stars.
Foundation 16Core 24Stretch 20Challenge 10
The web
Connected ideas
Follow a connection when you’re curious — some lead into completely different areas.
Gravity
Helps you understandanother area
TidesTides are gravity made visible: the Moon pulls the near ocean harder than the far ocean.
Helps you understandanother area
EclipsesEclipses happen only because the Sun, Earth and Moon move on fixed gravitational paths we can predict.
Related toanother area
Body systems and how they connectBones, muscles and blood pressure are all built for a life spent pulling against Earth's gravity — which is why astronauts weaken in orbit.
Used inanother area
Four operationsWeight on another world is your mass times that world's gravity — multiplication with an astonishing answer.
Words to know
44 words are explained in these lessons
ForceGravityMassWeightNewton (N)GBalanced forcesAir resistanceFree fallTerminal velocityOrbitMicrogravity+32 more