Gravity is an attractive force that exists between any two objects that have mass. The Earth pulls you down. You pull the Earth up (but it's so massive you can't notice!). Even two apples on a table attract each other — just incredibly weakly.
💡 Key insight: Double the distance → Force becomes 4× weaker (inverse-square law!)
On a planet's surface, all objects fall with the same acceleration g regardless of mass. This is the surface gravitational acceleration.
Where M is the planet mass and R is its radius. On Earth, g = 9.8 m/s²
When an object falls freely (no air resistance), it accelerates downward at rate g. The equations of motion apply:
| Planet | g (m/s²) | Compared to Earth |
|---|---|---|
| ☿ Mercury | 3.7 | 0.38× |
| ♀ Venus | 8.87 | 0.9× |
| 🌍 Earth | 9.8 | 1× |
| 🌙 Moon | 1.62 | 0.17× |
| ♂ Mars | 3.72 | 0.38× |
| ♃ Jupiter | 24.79 | 2.53× |
| ♄ Saturn | 10.44 | 1.07× |
| ⛢ Uranus | 8.69 | 0.89× |
| ♆ Neptune | 11.15 | 1.14× |