Gravitational field strength — CIE A-Level Physics
CIE A-Level Physics · 9702 · 25 min read
1. Definition of gravitational field strength★★☆☆☆⏱ 8 min
Gravitational field strength is a vector quantity: its direction is always towards the mass that creates the field, matching the direction of the attractive gravitational force. Rearranging the definition gives the gravitational force on mass $m$ at a point where field strength is $g$: $F = mg$.
2. Gravitational field strength in radial fields★★★☆☆⏱ 10 min
A point mass or uniform spherical mass (like a planet) produces a radial gravitational field, where field strength depends on distance from the centre of the mass. We can derive the formula for $g$ directly from Newton's law of universal gravitation:
3. Uniform gravitational fields★★☆☆☆⏱ 7 min
When we are close to the surface of a large planet like Earth, the change in $r$ (distance from Earth's centre) is tiny compared to $r$ itself. This means $g = \frac{GM}{r^2}$ is approximately constant, and the radial field lines are approximately parallel and equally spaced, forming a uniform field.
Common Pitfalls
Why: $r$ is defined as the distance from the centre of the mass creating the field, not the surface
Why: g is a vector, so direction must be accounted for when combining
Why: CIE examiners expect you to use the definition-based unit N kg⁻¹ for field strength
Why: The inverse square law only applies to points outside the mass creating the field