Edexcel IAL Physics · Fields & Forces
Gravitational Fields
Any two masses in the universe pull on each other — and the closer or heavier they are, the stronger that pull, following one elegant inverse-square rule that governs everything from a dropped apple to a satellite in orbit.
Quick Overview
- A gravitational field is a region where any mass feels a force — field lines point towards the mass causing the field because gravity is always attractive.
- Gravitational field strength g = F/m tells you the force per kg at a point.
- Newton's Law of Gravitation: F = GMm/r² — the force between two masses, obeying an inverse-square law.
- Near a point mass (or spherical planet), g = GM/r² — this also follows inverse-square (g ∝ 1/r²).
- Gravitational potential V = −GM/r is always negative and follows a 1/r relationship, not 1/r².
- Gravity and electric fields are mathematically similar (both inverse-square), but gravity is mass-based and always attractive, while electric fields are charge-based and can attract or repel.
- For orbits, gravitational force provides centripetal force, giving v² = GM/r and T² = 4π²r³/GM (T² ∝ r³).
1. What Is a Gravitational Field?
Think of a gravitational field as an invisible "zone of influence" that surrounds anything with mass. Step into that zone with your own mass, and you'll feel a pull — even if nothing is touching you. The Earth's field is why you stay glued to the ground; the Sun's field is why the Earth doesn't go flying off into deep space.
The direction of a gravitational field at any point is shown using field lines (arrows). Because gravity only ever attracts — it never pushes things apart — every field line points towards the centre of the mass creating the field. If you dropped a test mass at any point in the field, the field line tells you which way it would accelerate.
Radial fields
Around a single point mass (or any spherical object, like a planet), the field is radial — the lines spread out symmetrically in all directions, like the spokes of a wheel, all converging on the centre.
↖ ↑ ↗
\ | /
←----( M )----→ All arrows point INWARD
/ | \ toward the mass M
↙ ↓ ↘
Why this makes sense
Imagine standing anywhere around a planet — no matter which side you're on, "down" always means "toward the centre." That's exactly what a radial field is describing.
Practice Question 1
Why do gravitational field lines always point towards a mass, and never away from it?
2. Gravitational Field Strength (g)
Gravitational field strength answers a very specific question: "If I put a 1 kg mass at this exact point, how much force would it feel?" That's literally what the formula says — force per unit mass.
A crucial thing to internalise: an object's mass never changes, no matter where it is in the universe. What changes from planet to planet is g — and since weight = mg, your weight changes even though your mass doesn't. Stand on Earth (g = 9.81 N kg⁻¹) and you weigh a certain amount; stand on Jupiter (g ≈ 25 N kg⁻¹) and that same mass would feel roughly 2.5× heavier — potentially not even able to stand upright!
g vs G — Don't Mix Them Up
g
changes
G
What affects g at a planet's surface?
mass
radius
force
derive