What mistakes do students make with Resultant forces?

Always subtracting the two forces regardless of which way they point. Giving a size with no direction. Adding a 400 N driving force and 400 N of drag to get 800 N instead of zero.

What are the exam tips for Resultant forces in Physics AQA GCSE?

Write the positive direction at the top of your working; it costs one line and prevents sign errors. If the object is at constant speed, you already know the resultant is zero, which can give you the missing force. State the direction in words, such as 'forwards' or 'to the left', rather than relying on a minus sign alone.

AQA · GCSE · Physics

Course version: 8463 · Version from Sept 2016

Students should be able to calculate the resultant of two forces that act…

Full specification wording

Resultant forces: Students should be able to calculate the resultant of two forces that act in a straight line.

For two forces on the same straight line, the resultant is found by simple addition and subtraction once you have fixed a positive direction.

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

Point 3 of 7
2 key terms

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

The correct subtraction of the opposing force, such as 4200 - 3000.
Adding forces acting the same way and subtracting the one acting the opposite way.
The magnitude of the resultant in newtons.
The direction of the resultant, named in words or shown by the sign.

Common mistakes and how to put them right

Always subtracting the two forces regardless of which way they point.
Giving a size with no direction.
Adding a 400 N driving force and 400 N of drag to get 800 N instead of zero.
Forgetting that the resultant is zero when the object moves at a constant speed.
Changing the positive direction between two parts of the same question.

Exam tips

Write the positive direction at the top of your working; it costs one line and prevents sign errors.
If the object is at constant speed, you already know the resultant is zero, which can give you the missing force.
State the direction in words, such as 'forwards' or 'to the left', rather than relying on a minus sign alone.

Try the lesson

The start of the Resultant forces: Students should be able to calculate the resultant of two forces that act in a straight line. lesson, made from checked teaching notes for this specification point.

Have a go first

A box is pushed to the right with a force of 30 N. A friction force of 12 N acts to the left. What is the resultant force on the box?

Tip: press 1 to 4 to choose

Listen

For two forces on the same straight line, the resultant is found by simple addition and subtraction once you have fixed a positive direction. Start by writing down which way you are calling positive. Forces pointing that way count as positive, forces the other way as negative, and the resultant is the total. A car with a 4200 N driving force and 3000 N of drag has a resultant of 1200 N forwards. Two equal opposing forces give a resultant of zero, which is what happens once a car reaches maximum speed, when the driving force and the resistive forces are equal. Always finish with both the magnitude in newtons and the direction, because force is a vector. A strong answer earns marks for these points. the correct subtraction of the opposing force, such as 4200 - 3000. adding forces acting the same way and subtracting the one acting the opposite way. the magnitude of the resultant in newtons. the direction of the resultant, named in words or shown by the sign. Exam tip: Write the positive direction at the top of your working; it costs one line and prevents sign errors.

See it

A top-down flowchart. The first box says 'Write down the positive direction' and branches to two boxes: 'Forces pointing that way count as positive' and 'Forces pointing the other way count as negative'. Both of these lead into a box saying 'Add the forces to get the total'. From that box three arrows lead to 'Resultant: magnitude in newtons', 'Resultant: direction in words or by sign', and 'Two equal opposing forces give a resultant of zero'. The last of these leads to a final box saying 'Zero resultant happens at maximum speed'. — It shows the steps for finding the resultant of two forces in a straight line, from fixing a positive direction to stating magnitude and direction, including the zero-resultant case.

Key words

Say what each word means before you turn the card.

positive direction

Try answering aloud or in your head before turning the card.

adding and subtracting forces

Try answering aloud or in your head before turning the card.

See every definition

positive direction: The way you decide to call positive before calculating, so forces pointing that way count as positive and forces the other way as negative.
adding and subtracting forces: The simple method of finding the resultant of two forces on the same straight line once a positive direction is fixed.

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. The first box says 'Write down the positive direction' and branches to two boxes: 'Forces pointing that way count as positive' and 'Forces pointing the other way count as negative'. Both of these lead into a box saying 'Add the forces to get the total'. From that box three arrows lead to 'Resultant: magnitude in newtons', 'Resultant: direction in words or by sign', and 'Two equal opposing forces give a resultant of zero'. The last of these leads to a final box saying 'Zero resultant happens at maximum speed'.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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