---
title: "Newton's Law of Gravitation and Gravitational Field Strength"
description: "Any two masses attract with a force that falls off as the square of their separation. Dividing that force by the test mass leaves a field strength that belongs to the source alone."
canonical: https://lightmysky.com/learn/science/newtons-law-of-gravitation-and-gravitational-field-strength-mt_uUYXl3L0oW
source: https://lightmysky.com/learn/science/newtons-law-of-gravitation-and-gravitational-field-strength-mt_uUYXl3L0oW.md
retrieved: 2026-09-12
---

> **Agent view.** This is the Markdown twin of the page, for tools and assistants.
> When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md
> API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md
> Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt
> Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json
> Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`.

# Newton's Law of Gravitation and Gravitational Field Strength

Any two masses attract with a force that falls off as the square of their separation. Dividing that force by the test mass leaves a field strength that belongs to the source alone.

Subject: Science · Area: Forces & Motion · Ages 16 to 17
Page: https://lightmysky.com/learn/science/newtons-law-of-gravitation-and-gravitational-field-strength-mt_uUYXl3L0oW

## Ready when they can

- Uses F = GMm over r squared with r measured centre to centre
- Shows that surface g follows from GM over R squared, and predicts g at a stated height
- Sketches the radial field of a single mass and says what the spacing of the lines shows

## Lesson: Gravity between every pair of masses

Every mass attracts every other mass along the line joining them. The force grows with each mass and shrinks with the square of the distance between centres.

**Example.** The same pull holds an apple on its tree and the Moon in its orbit. Each body pulls the other equally hard in opposite directions, even when one mass is far larger.

Field strength g is the force per unit mass, in newtons per kilogram. Near a planet it follows g equals G M over R squared. A radial field sketch uses inward lines, spaced wider where the field is weaker.

**Tip.** Always measure r from centre to centre, not from the surface. Doubling the separation quarters the force, since the falloff goes as one over r squared.

**Recap.** Masses pull along their joining line with a force that falls as one over r squared.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Universal Gravitation](https://lightmysky.com/learn/science/universal-gravitation-mt_oLjz18CxDg)

## Opens up

- [Gravitational Potential and Escape Speed](https://lightmysky.com/learn/science/gravitational-potential-and-escape-speed-mt_drqTwCD9W8)
- [Coulomb's Law and the Electric Field of a Point Charge](https://lightmysky.com/learn/science/coulombs-law-and-the-electric-field-of-a-point-charge-mt_p7FuzIsQch)
- [Orbits: Period, Speed and Kepler's Third Law](https://lightmysky.com/learn/science/orbits-period-speed-and-keplers-third-law-mt_PLKLykGF8Z)