---
title: "Newton's Second Law with Numbers"
description: "Treating F = ma as a working equation: the resultant force sets the acceleration, and mass measures how strongly an object resists that change. Mass, not weight, is what appears in the calculation."
canonical: https://lightmysky.com/learn/science/newtons-second-law-with-numbers-mt_Nn2MVJL7bO
source: https://lightmysky.com/learn/science/newtons-second-law-with-numbers-mt_Nn2MVJL7bO.md
retrieved: 2026-09-12
---

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# Newton's Second Law with Numbers

Treating F = ma as a working equation: the resultant force sets the acceleration, and mass measures how strongly an object resists that change. Mass, not weight, is what appears in the calculation.

Subject: Science · Area: Forces & Motion · Ages 14 to 15
Page: https://lightmysky.com/learn/science/newtons-second-law-with-numbers-mt_Nn2MVJL7bO

## Ready when they can

- Calculates any one of resultant force, mass or acceleration when the other two are given
- Explains why a loaded trolley accelerates less than an empty one under the same push
- Uses the resultant of several forces, not one single force, in the calculation

## Lesson: Force sets the speeding

Newton's second law fits in one line: resultant force equals mass times acceleration. Push twice as hard and you get twice the acceleration. Keep the push the same but double the mass, and the acceleration halves.

Resultant means the single force left after every push and pull is combined, so always combine first and calculate second. Draw the object with every force as an arrow, pick a direction to call positive, then feed the total into F equals m a, with mass in kilograms.

**Example.** A loaded trolley accelerates less than an empty one under the same push, since more mass resists the change. Friction and drag join the arrows too, so the resultant can be far smaller than the push you apply. Say the forces in words first; if the words make sense, the sum usually follows.

**Tip.** Watch the units and the signs. Mass goes in as kilograms and acceleration comes out in metres per second squared. A quick sketch stops sign mistakes before they happen, especially when friction points against the motion.

**Recap.** Combine forces into one resultant, then let F equals m a do the rest.

## Practice

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

## Needs first

- [Newton's First & Second Laws](https://lightmysky.com/learn/science/newtons-first-and-second-laws-mt_tXxxCFl32J)

## Opens up

- [Terminal Velocity](https://lightmysky.com/learn/science/terminal-velocity-mt_3eNYHV-m-c)
- [Momentum and Its Conservation](https://lightmysky.com/learn/science/momentum-and-its-conservation-mt_Nn5_DMbK1_)
- [Newton's Second Law on a Slope](https://lightmysky.com/learn/science/newtons-second-law-on-a-slope-mt_OR_sKFelnP)
- [Circular Motion: Angular Speed and Centripetal Force](https://lightmysky.com/learn/science/circular-motion-angular-speed-and-centripetal-force-mt_PW1G6nYStq)