| English | Resultant force |
|---|---|
| Bahasa Melayu | Daya paduan |
| 中文 | 合力 |
Definition
The single force that has the same effect as two or more forces acting together on an object.
What you need to know
A resultant force is the single force that has the same overall effect on an object as two or more forces acting on it together. When forces act along the same straight line, forces pointing in the same direction are added, while forces pointing in opposite directions are subtracted, to find the size and direction of the resultant force.
The resultant force is linked to motion through Newton's second law, written as F_net = ma, where F_net is the resultant force in newtons (N), m is the mass of the object in kilograms (kg), and a is the acceleration produced in metres per second squared (m s⁻²). For the same mass, a larger resultant force always produces a larger acceleration, and the acceleration always acts in the same direction as the resultant force.
When the resultant force on an object is zero, the object is in equilibrium: it stays at rest or keeps moving at a constant velocity in a straight line. This special case, where a = 0 m s⁻², connects resultant force directly to Newton's first law of motion.
Worked example
A trolley of mass 2 kg is pushed across a bench with a force of 15 N to the right, while friction acts on it with a force of 5 N to the left. Because the two forces act along the same line but in opposite directions, they are subtracted to find the resultant force.
Resultant force, F_net = 15 N − 5 N = 10 N, directed to the right, in the same direction as the larger applied force.
Applying Newton's second law, F_net = ma, the acceleration of the trolley is a = F_net ÷ m = 10 N ÷ 2 kg = 5 m s⁻², directed to the right. Notice that every quantity carries its correct SI unit throughout the calculation: force in newtons, mass in kilograms, and acceleration in metres per second squared.
If the pushing force were reduced until it exactly balanced the 5 N frictional force, the resultant force would become 0 N and the trolley would move at constant velocity instead of accelerating.
How it is examined
In Paper 1, objective questions often ask candidates to calculate a resultant force from two or more given forces acting along a line, or to identify which diagram shows an object in equilibrium. In Paper 2, structured and essay questions commonly ask candidates to define resultant force, state the equation connecting force, mass and acceleration, and calculate resultant force and acceleration for a described situation, sometimes requiring a free body diagram.
In Paper 3, practical questions may ask candidates to describe an experiment showing how acceleration varies with the resultant force applied to a trolley, and to determine values from a table or graph of results.
A common mistake is forgetting to subtract opposing forces before applying F_net = ma, or mixing up mass and weight. Candidates also lose marks for omitting units or for writing a direction without stating it clearly alongside the numerical answer.
Another frequent mistake is describing an equilibrium situation as having no force acting at all, rather than explicitly stating that the resultant force equals 0 N, which is the phrasing examiners look for when awarding full marks.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)