Formula and method
Newton’s second law is F = ma for constant mass in an inertial frame.
Worked example
A 10 kg mass accelerating at 2 m/s² requires a net force of 20 N along that direction.
Weight W = mg on Earth and other worlds
Weight is the gravitational force on a mass: W = mg. Mass stays the same everywhere, but g changes. Weight mode uses standard gravity g₀ = 9.80665 m/s² for Earth, or the mean surface gravity from the NASA NSSDCA planetary fact sheets (GM ÷ mean radius²): Moon 1.62 m/s², Mars 3.73 m/s² and Jupiter 25.92 m/s² (at the 1-bar pressure level, since Jupiter has no solid surface). Gravity at a planet's equator is slightly lower than this mean, because rotation makes the planet bulge there. You can also enter a custom g.
Example: a 70 kg person weighs 70 × 9.80665 = 686.47 N on Earth, 70 × 1.62 = 113.4 N on the Moon and 70 × 3.73 = 261.1 N on Mars. To go the other way, m = W ÷ g: a 686.4655 N weight under standard gravity is a 70 kg mass.
| Body | g (m/s²) | Weight (N) |
|---|---|---|
| Earth (standard) | 9.80665 | 686.47 |
| Moon | 1.62 | 113.4 |
| Mars | 3.73 | 261.1 |
| Jupiter | 25.92 | 1,814.4 |