Edexcel iGCSE Physics -1.15 Resultant Forces- Study Notes- New Syllabus

Resultant Force Along a Line

When two or more forces act on an object along the same straight line, they can be combined to find a single force called the resultant force.

The resultant force determines how the object moves.

What Is Resultant Force?

The resultant force is the single force that has the same effect as all the forces acting together.

• If the resultant force is not zero, the object will accelerate.
• If the resultant force is zero, the object is in equilibrium.

Forces Acting in the Same Direction

When forces act in the same direction along a line, they are added together.

\( \mathrm{Resultant\ force = F_1 + F_2 + F_3 + \dots} \)

• The resultant force acts in the same direction.
• The magnitude increases.

Forces Acting in Opposite Directions

When forces act in opposite directions along a line, they are subtracted.

\( \mathrm{Resultant\ force = larger\ force – smaller\ force} \)

• The resultant force acts in the direction of the larger force.
• If forces are equal, the resultant force is zero.

Choosing a Direction

To avoid confusion:

• Choose one direction as positive (e.g. right or upwards).
• Forces in the opposite direction are negative.
• Add the forces algebraically.

Key Idea

• Only forces acting along the same line can be added directly.
• Direction is always important.
• The resultant force determines acceleration.

Important Points to Remember

• Always include direction with your answer.
• Forces are measured in newtons (N).
• A zero resultant force does not mean no forces are acting.