What mistakes do students make with Newton's Second Law?

Writing that acceleration is proportional to mass rather than inversely proportional to it. Using the driving force rather than the resultant force in the calculation. Substituting the weight of an object on a horizontal surface as the resultant force.

What are the exam tips for Newton's Second Law in Physics AQA GCSE?

When a question changes two quantities at once, deal with the force first, then the mass. Show the subtraction that gives the resultant force as a separate line of working. If the acceleration is negative, say the vehicle is decelerating rather than dropping the sign.

AQA · GCSE · Physics

Course version: 8463 · Version from Sept 2016

Newton’s Second Law: The acceleration of an object is proportional to the…

Full specification wording

Newton's Second Law: Newton’s Second Law: The acceleration of an object is proportional to the resultant force acting on the object, and inversely proportional to the mass of the object.

Newton's Second Law joins two proportional relationships.

Checked against the AQA 8463 specification (opens in a new tab) on 22 September 2026.

Point 1 of 11

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

Identifying the resultant force before applying the law.
A proportional statement such as doubling the resultant force doubles the acceleration.
Stating that doubling the mass at constant force halves the acceleration.
A correct calculation using resultant force = mass × acceleration with consistent units.

Common mistakes and how to put them right

Writing that acceleration is proportional to mass rather than inversely proportional to it.
Using the driving force rather than the resultant force in the calculation.
Substituting the weight of an object on a horizontal surface as the resultant force.
Reading 'inversely proportional' as simply 'decreases', then predicting any smaller value.

Exam tips

When a question changes two quantities at once, deal with the force first, then the mass.
Show the subtraction that gives the resultant force as a separate line of working.
If the acceleration is negative, say the vehicle is decelerating rather than dropping the sign.

Try the lesson

The start of the Newton's Second Law: Newton’s Second Law: The acceleration of an object is proportional to the resultant force acting on the object, and inversely proportional to the mass of the object. lesson, made from checked teaching notes for this specification point.

Have a go first

A trolley is pushed with a resultant force of 6 N and it accelerates at 3 m/s². Using Newton's Second Law, what is the mass of the trolley?

Tip: press 1 to 4 to choose

Listen

Newton's Second Law joins two proportional relationships. At fixed mass, acceleration is directly proportional to the resultant force, so trebling the force trebles the acceleration. At fixed resultant force, acceleration is inversely proportional to mass, so a trolley loaded to twice its mass accelerates at half the rate. This is why a fully laden lorry pulls away far more slowly than an empty one with the same engine. In symbols, a = F ÷ m, usually written F = m a. The word resultant matters: if a 900 N driving force acts against 300 N of drag on a 200 kg quad bike, the resultant is 600 N and the acceleration is 3 m/s², not 4.5 m/s². Because acceleration is the ratio F ÷ m, a graph of acceleration against resultant force at fixed mass is a straight line through the origin. A strong answer earns marks for these points. identifying the resultant force before applying the law. a proportional statement such as doubling the resultant force doubles the acceleration. stating that doubling the mass at constant force halves the acceleration. a correct calculation using resultant force = mass × acceleration with consistent units. Exam tip: When a question changes two quantities at once, deal with the force first, then the mass.

See it

A top-down flowchart. A single node labelled 'Resultant force and mass' branches into two nodes: 'Acceleration proportional to resultant force' and 'Acceleration inversely proportional to mass'. The first of these leads to 'Trebling force trebles acceleration', and the second leads to 'Doubling mass halves acceleration'. Both of these then lead into a node labelled 'a = F ÷ m, written F = m a', which leads to a final node reading 'Graph of acceleration against resultant force at fixed mass: straight line through the origin'. — It shows how Newton's Second Law joins the two proportional relationships and leads to the equation F = m a.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. A single node labelled 'Resultant force and mass' branches into two nodes: 'Acceleration proportional to resultant force' and 'Acceleration inversely proportional to mass'. The first of these leads to 'Trebling force trebles acceleration', and the second leads to 'Doubling mass halves acceleration'. Both of these then lead into a node labelled 'a = F ÷ m, written F = m a', which leads to a final node reading 'Graph of acceleration against resultant force at fixed mass: straight line through the origin'.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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