The big idea: Let go of your phone and it drops straight down — the Earth is pulling on it, as it pulls on every mass nearby. That invisible pull filling the space around a mass is a gravitational field.
The field strength g measures how strong the pull is per kilogram (N kg⁻¹) — stronger for bigger masses and closer in.
A gravitational field points inward, towards the mass M, because gravity always pulls. The lines get further apart as you move out — the field is weaker further away.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Spot it: Gravity is always attractive — the field lines point inward, towards the mass.
The lines spread out as you move away, so the field gets weaker the further out you go.
Free preview
This is the free notes preview
You're reading the free notes. Aimnova Pro unlocks the full study experience — and you can try it free for 7 days:
- FlashcardsLock in vocabulary and key terms with spaced repetition.
- Practice questionsAnswer exam-style questions and get instant AI marking.
- Mock exams & past-paper vaultSit full mocks and see exactly how examiners award marks.
- Personalised study planA daily plan built around your exam date and weak areas.
Newton's law of gravitation gives the pull between any two masses. The force grows with the masses and shrinks with the square of the distance between them:
- gravitational force between the masses (N)
- gravitational constant, 6.67 × 10⁻¹¹ N m² kg⁻²
- the two masses (kg)
- distance between their centres (m)
The gravitational field strength g is the force per kilogram on a small mass placed in the field. Dividing Newton's law by that small mass m gives a neat form that only needs the big mass M and the distance:
- gravitational field strength (N kg⁻¹), also the free-fall acceleration
- gravitational force on the small mass (N)
- the small mass placed in the field (kg)
- mass of the planet or star making the field (kg)
- distance from the centre of M (m)
F = m g. Cover the one you want: m and g side by side → multiply (F = m g); F above g → divide (m = F ÷ g); F above m → divide (g = F ÷ m).
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
g is also the acceleration of free fall: Because F = mg and F = ma, the field strength g equals the acceleration a falling mass would have.
That is why two different masses dropped at the same place fall with the same acceleration — g does not depend on the falling mass m.
A planet has mass 6.0 × 10²⁴ kg and radius 6.4 × 10⁶ m. Find the gravitational field strength at its surface. (G = 6.67 × 10⁻¹¹ N m² kg⁻².)
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Memorize terms 3x faster
Smart flashcards show you cards right before you forget them. Perfect for definitions and key concepts.
How this is tested — gravitation and field strength turn up across the Fields theme:
Paper 1A
- Quick inverse-square reasoning — how g changes at a different distance (three times farther → one ninth).
- Comparing the accelerations of masses falling from different heights.
Paper 2
- Calculate g = GM/r² at a planet's surface.
- Or a star's field at an orbital distance.
The classic trap: Forgetting the square — moving three times farther divides g by 3² = 9, not by 3.
The inverse-square shortcut: Because g is proportional to 1/r², you don't always need G and M.
If the distance is multiplied by a number n, the field strength is divided by n². So r ×2 → g ÷4, and r ×3 → g ÷9.
Animated graph
Watch the graph build step by step in study mode.
The gravitational field strength a distance r from the centre of a planet is 8.1 N kg⁻¹. Find the field strength at a point three times as far from the centre (a distance 3r).
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Two small balls, one of mass 2.0 kg and one of mass 5.0 kg, are released from rest at the same point above the planet's surface. Compare their initial accelerations.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.