Weight, mass and gravitational field strength
The weightThe force acting on an object due to the pull of gravity from a massive object like a planet. The force acts towards the centre of the planet and is measured in newtons (N). of an object is the force on it caused by the gravityThe force of attraction between all objects. The more mass an object has, the larger the force of gravity it exerts. due to the planet. The weight of an object and the gravitational fieldThe space or area around a body where another body would experience gravitational attraction. strength are directly proportional. For a given mass, the greater the gravitational field strength of the planet, the greater its weight.
Weight can be calculated using the equation:
weight = mass × gravitational field strength
\(W = m \ g\)
This is when:
- weight (W) is measured in newtons (N)
- mass (m) is measured in kilograms (kg)
- gravitational field strength (g) is measured in newtons per kilogram (N/kg)
Gravity on other objects in space
| Space object | g, gravitational field strength (N/kg) |
| The Sun (star) | 293.0 |
| Mercury | 3.7 |
| Venus | 8.8 |
| Earth | 9.8 |
| Moon (satellite) | 1.7 |
| Mars | 3.7 |
| Ceres (dwarf planet) | 0.27 |
| Jupiter | 24.7 |
| Saturn | 10.5 |
| Uranus | 9.0 |
| Neptune | 11.7 |
| Pluto (dwarf planet) | 0.49 |
| Space object | The Sun (star) |
|---|---|
| g, gravitational field strength (N/kg) | 293.0 |
| Space object | Mercury |
|---|---|
| g, gravitational field strength (N/kg) | 3.7 |
| Space object | Venus |
|---|---|
| g, gravitational field strength (N/kg) | 8.8 |
| Space object | Earth |
|---|---|
| g, gravitational field strength (N/kg) | 9.8 |
| Space object | Moon (satellite) |
|---|---|
| g, gravitational field strength (N/kg) | 1.7 |
| Space object | Mars |
|---|---|
| g, gravitational field strength (N/kg) | 3.7 |
| Space object | Ceres (dwarf planet) |
|---|---|
| g, gravitational field strength (N/kg) | 0.27 |
| Space object | Jupiter |
|---|---|
| g, gravitational field strength (N/kg) | 24.7 |
| Space object | Saturn |
|---|---|
| g, gravitational field strength (N/kg) | 10.5 |
| Space object | Uranus |
|---|---|
| g, gravitational field strength (N/kg) | 9.0 |
| Space object | Neptune |
|---|---|
| g, gravitational field strength (N/kg) | 11.7 |
| Space object | Pluto (dwarf planet) |
|---|---|
| g, gravitational field strength (N/kg) | 0.49 |
Example
An apple has a mass of 100 g. Calculate its weight on Mars (g = 3.7 N/kg).
100 g = 100 ÷ 1000 = 0.1 kg
\(W = m \ g\)
\(W = 0.1 \ kg \times 3.7 \ N/kg\)
\(W = 0.37 \ N\)
Question
Calculate the weight of a 30 kg dog on the Moon (g = 1.7 N/kg).
\(W = m \ g\)
\(W = 30 \ kg \times 1.7 \ N/kg\)
\(W = 51 \ N\)