Gravitational Field Strength

Gravitational field strength ($g$) is the force of gravity acting per unit mass at a point in a gravitational field. On the surface of Earth, $g \approx 9.8\ \text{N/kg}$.

What does g = 9.8 N/kg mean?

It means every kilogram of mass near Earth’s surface experiences a gravitational force of 9.8 N. A 2 kg book experiences $2 \times 9.8 = 19.6\ \text{N}$ of gravitational force (its weight).

How does g vary?

Location	g (N/kg)
Earth’s surface	9.8
Moon	1.6
Mars	3.7
Jupiter	24.8

The value of $g$ depends on the mass and radius of the planet or moon. Larger, denser bodies have stronger gravitational fields.

$g$ also decreases with altitude above a planet’s surface. At the top of a tall mountain, $g$ is slightly less than 9.8 N/kg, but the difference is too small to matter in IGCSE calculations.

The dual role of g

Gravitational field strength (N/kg) and acceleration due to gravity (m/s²) have the same numerical value. This is because the units are equivalent:

$\text{N/kg} = \frac{\text{kg m/s}^2}{\text{kg}} = \text{m/s}^2$

So $g = 9.8\ \text{N/kg}$ is the same as saying an object in free fall near Earth accelerates at $9.8\ \text{m/s}^2$.

Common errors and how to correct them

• Using g = 10 N/kg without checking the question. The syllabus states $g \approx 9.8\ \text{m/s}^2$. Use a different value only when the question gives one.
• Assuming g is the same everywhere. On the Moon, $g$ is about one-sixth of Earth’s value. Always check which planet or moon the question refers to.
• Confusing g with G. $g$ (lowercase) is gravitational field strength at a surface. $G$ (uppercase) is the universal gravitational constant. IGCSE only requires $g$.

How to apply this in an exam

If the question does not state a value for $g$, use 9.8 N/kg (or 9.8 m/s²). If it provides a different value, use that instead. Always write the value you used in your working.