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Forces Acting Together

When more than one force acts on an object, their combined effect is called the net force, and it determines the change in motion.

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Student-friendly explanation

Forces in the same straight line are added if they act in the same direction and subtracted if they act in opposite directions. The object accelerates in the direction of the net force.

How to write this in exams

  1. 1

    Start with the exact idea

    When more than one force acts on an object, their combined effect is called the net force, and it determines the change in motion.

  2. 2

    Then show how to use it

    Draw force arrows, group directions, add same-direction forces, subtract opposite-direction forces, name final direction.

  3. 3

    Add one concrete example

    If two people push a car forward with 100 N and 150 N, the net force is 250 N forward.

  4. 4

    Avoid this incomplete answer

    Writing only the magnitude without direction loses meaning because force is a vector.

Definition

When more than one force acts on an object, their combined effect is called the net force, and it determines the change in motion.

Example

If two people push a car forward with 100 N and 150 N, the net force is 250 N forward.

Rule to remember

Same direction: net force = F1 + F2. Opposite direction: net force = larger force - smaller force, towards the larger force.

Memory hook

Arrows first, arithmetic next.

Examples and method

Worked example

A crate has 120 N to the east and 45 N to the west. Net force = 120 - 45 = 75 N east.

Method to apply

Draw force arrows, group directions, add same-direction forces, subtract opposite-direction forces, name final direction.

Diagram support

Use collinear arrows on a block; same-direction arrows on one side and opposite-direction arrows on both sides.

How CBSE asks it

Exams present force diagrams or word problems and ask for net force, direction, and motion effect.

Avoid common mistakes

Common confusion

Students often add all forces without checking whether they act in the same or opposite directions.

Common wrong answer

Writing only the magnitude without direction loses meaning because force is a vector.

Exam tip

Draw arrows before doing calculation; direction is the main clue in net-force questions.

Quick check

Two forces of 30 N and 20 N act on a box in opposite directions. What is the net force?

The net force is 10 N in the direction of the larger 30 N force because opposite forces are subtracted.

Study the forces acting together diagram carefully

Use the labelled diagram to keep forces acting together clear in short answers and revision.

What this diagram makes clear

This diagram keeps the labels and direction of forces acting together in the right order.

Where this helps in exams

Use this for labelled diagram work and short exam answers on forces acting together.

Revision cue

Revise forces acting together through the labels before writing the answer.

Answer writing and exam use

1-mark answer

When more than one force acts on an object, their combined effect is called the net force, and it determines the change in motion.

2-mark answer

When more than one force acts on an object, their combined effect is called the net force, and it determines the change in motion. Same direction: net force = F1 + F2. Opposite direction: net force = larger force - smaller force, towards the larger force. If two people push a car forward with 100 N and 150 N, the net force is 250 N forward.

3-mark answer

Forces in the same straight line are added if they act in the same direction and subtracted if they act in opposite directions. The object accelerates in the direction of the net force. Same direction: net force = F1 + F2. Opposite direction: net force = larger force - smaller force, towards the larger force. A crate has 120 N to the east and 45 N to the west. Net force = 120 - 45 = 75 N east. Exams present force diagrams or word problems and ask for net force, direction, and motion effect. Writing only the magnitude without direction loses meaning because force is a vector.
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