medium difficulty5 min

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.

In one line: Arrows first, arithmetic next.

Practice This Concept

Understand it

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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1A crate has 120 N to the east and 45 N to the west.
2Net force = 120 - 45 = 75 N east.

Method you can reuse

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

Sketch it in your answer

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

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

Think first, then show answer

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

Common wrong answer

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

Don't mix this up

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

Exam tip

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