Gravity and orbits

Drop a cup and it falls. The same pull keeps the Moon going round the Earth, the Earth round the Sun, and decides how a ship gets to Mars. This topic starts with things falling down and climbs, level by level, to Newton's law of gravity, orbits, escape speed and the half-ellipse path that takes 259 days to reach Mars.

On Mars, gravity is a little over a third of Earth's. That one number changes how you walk, jump, build and launch, and it comes back in almost every lesson here.

8 levels, 48 lessons. The ages are only a rough guide for parents and teachers; anyone can start at any level.

Level	Rough age	Lessons
Level 1: Up and down	5-6	Why things fall down; Heavy and light; Throwing and catching; Round planets; Jumping on Mars; The Moon goes round
Level 2: Weight and pull	7-8	What weight is; Mass and weight; Weight on other worlds; The balancing point; Newton's cannon; Days, years and seasons
Level 3: Falling, measured	9-10	Timing a fall; Air gets in the way; Parachutes; The shape of a throw; A world at 0.38 g; Tides
Level 4: Gravity everywhere	11-12	The apple and the Moon; Weaker with distance; Slower farther out; Weightless in orbit; Phobos and Deimos; Escaping
Level 5: Equations of falling	12-13	Distance fallen; The speed of a fall; Projectiles; The Mars jump, worked out; Terminal velocity; Gravity with height
Level 6: Newton's law of gravity	13-14	The law of gravity; g from a planet's size and mass; Orbital speed; Kepler's third law; Areostationary orbit; Weighing planets
Level 7: Energy and orbits	15-16	Gravitational energy; Escape speed; Ellipses; Vis-viva; Hohmann transfer; Launch windows
Level 8: Orbits for mission design	17 and beyond	Delta-v budgets; Gravity assists; Lagrange points; Orbits that drift; The fate of Phobos; Relativity and orbits

The numbers for Mars and Earth come from NASA's Mars fact sheet and Earth fact sheet.