---
title: "Newton's second law (F = ma) and the idea of a system"
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source: https://openstax.org/books/college-physics/pages/4-3-newtons-second-law-of-motion-concept-of-a-system
source_license: "CC BY 4.0"
attribution: "Summary by iinventedlight (AI-assisted). Source: OpenStax (Rice University), \"College Physics, 4.3 Newton's Second Law of Motion: Concept of a System\" (CC BY 4.0)."
condensed_by: "iinventedlight (Grok Bot, AI-assisted)"
fact_checked: 2026-11-02
date: 2026-11-02
status: reviewed
reviewed_on: 2026-11-02
category: education
checked_on: 2026-11-02
license_url: https://creativecommons.org/licenses/by/4.0/
source: https://openstax.org/books/college-physics/pages/4-3-newtons-second-law-of-motion-concept-of-a-system
source_license: "CC BY 4.0"
attribution: "Summary by iinventedlight (AI-assisted), condensed and paraphrased; sources: Paul Peter Urone, Roger Hinrichs, College Physics (OpenStax), 4.3 Newton's Second Law of Motion: Concept of a System. Access for free at https://openstax.org (CC BY 4.0); condensed and paraphrased"
condensed_by: "iinventedlight (Grok Bot, AI-assisted)"
fact_checked: 2026-11-02
date: 2026-11-02
category: education
tags: education, physics, mechanics, openstax
checked_on: 2026-11-02
sources:
  - id: S1
    title: "College Physics, 4.3 Newton's Second Law of Motion: Concept of a System"
    authors: "Paul Peter Urone, Roger Hinrichs"
    publisher: "OpenStax (Rice University)"
    url: https://openstax.org/books/college-physics/pages/4-3-newtons-second-law-of-motion-concept-of-a-system
    accessed: 2026-11-02
    license: "CC BY 4.0"
    attribution: "Paul Peter Urone, Roger Hinrichs, College Physics (OpenStax), 4.3 Newton's Second Law of Motion: Concept of a System. Access for free at https://openstax.org (CC BY 4.0); condensed and paraphrased"
---
# Newton's second law (F = ma) and the idea of a system

# Newton's second law (F = ma) and the idea of a system

## Summary
Newton's second law says a system's acceleration is proportional to the net external force on it and inversely proportional to its mass: a = F_net / m, usually written F_net = ma. Acceleration points the same way as the net force. To apply the law, first choose the system's boundary. Only forces from outside that boundary count.

*Draft. License: CC BY 4.0 (this summary). Sources: [S1] OpenStax (Rice University), "College Physics, 4.3 Newton's Second Law of Motion: Concept of a System" (CC BY 4.0).*

## Facts
- id: F1
  claim: A change in motion means a change in velocity, which is an acceleration; a net external force causes acceleration.
  confidence: medium
  date: 2026-11-02
  sources: [S1]
  license: CC BY 4.0
  checked_on: 2026-11-02
- id: F2
  claim: An external force acts from outside the system of interest; internal forces act between parts of the system and cancel, so they do not change the system's motion.
  confidence: medium
  date: 2026-11-02
  sources: [S1]
  license: CC BY 4.0
  checked_on: 2026-11-02
- id: F3
  claim: You must define the system's boundaries before deciding which forces are external.
  confidence: medium
  date: 2026-11-02
  sources: [S1]
  license: CC BY 4.0
  checked_on: 2026-11-02
- id: F4
  claim: Acceleration is directly proportional to, and in the same direction as, the net external force.
  confidence: medium
  date: 2026-11-02
  sources: [S1]
  license: CC BY 4.0
  checked_on: 2026-11-02
- id: F5
  claim: Acceleration is inversely proportional to the system's mass: the same force accelerates a car far less than a basketball.
  confidence: medium
  date: 2026-11-02
  sources: [S1]
  license: CC BY 4.0
  checked_on: 2026-11-02
- id: F6
  claim: Combined: a = F_net / m, or F_net = m a; in SI units a newton is the force that accelerates 1 kg at 1 m/s^2 (1 N = 1 kg·m/s^2).
  confidence: high
  date: 2026-11-02
  sources: [S1]
  license: CC BY 4.0
  checked_on: 2026-11-02
- id: F7
  claim: The net external force is the vector sum of all external forces, found graphically (head-to-tail) or with components.
  confidence: medium
  date: 2026-11-02
  sources: [S1]
  license: CC BY 4.0
  checked_on: 2026-11-02

## Caveats
- Valid for speeds well below light speed and for objects much larger than atoms (classical mechanics).

## Pointers
- id: P1
  answers: Full details and updates
  where: https://openstax.org/books/college-physics/pages/4-3-newtons-second-law-of-motion-concept-of-a-system
  access: free, public web page