Gravitational Field Strength

Gravitational field strength explains how strongly a planet (or the Sun) pulls on things. It tells you the weight of 1 kilogram at a place.

Definition and key ideas

Gravitational field strength, g, is the force per unit mass at a point:

$$g = \frac{W}{m}$$

- W is weight (a force, in newtons, N).
- m is mass (in kilograms, kg).
- Units of g are newtons per kilogram (N/kg). These are the same as metres per second squared (m/s²): $$1\,\text{N/kg} = 1\,\text{m s}^{-2}$$

Near Earth’s surface, $g \approx 9.8\,\text{N/kg}$. In IGCSE calculations you may use $g \approx 10\,\text{N/kg}$ unless told otherwise. Because $W = mg$, g is also the acceleration of free fall (ignoring air resistance).

Mass and weight

• Mass is the amount of matter in an object (kg). It does not change when you move planets.
• Weight is the gravitational force on that mass (N). It changes if g changes.

Relationship: $W = mg$.

What affects g?

• Planet’s mass: more massive planets have stronger g at their surfaces (e.g., Jupiter > Earth > Moon).
• Distance from the planet: g decreases as you move farther from the planet’s centre. Around the Sun, g gets weaker with distance, so far-out planets move more slowly in their orbits.

Think of a planet’s gravity like an invisible “slope” pulling you inward. The closer you are and the bigger the planet, the steeper the slope.

Common misconceptions

• “Mass changes on different planets.” Mass stays the same; weight changes.
• “Astronauts are in zero gravity.” In orbit they are in continuous free fall; g is smaller but not zero.
• “N/kg and m/s² are different.” They are equivalent units for g.