What is the difference between balanced and unbalanced forces at N(A)-Level?

Forces are balanced when they add to a resultant of zero; the object then stays at rest or keeps moving at constant velocity. Forces are unbalanced when the resultant is not zero; the object then accelerates in the direction of the resultant, with acceleration given by $a = F/m$. A car moving at steady speed has balanced forces, while a car speeding up has an unbalanced forward resultant.

How do you use the formula F = ma?

First find the single resultant force by combining the forces along the line of motion, adding those in the direction of motion and subtracting those that oppose it. Then put the resultant force $F$ in newtons and the mass $m$ in kilograms into $a = F/m$ to get the acceleration in metres per second squared. Always reduce the forces to one resultant before using the formula.

What is a moment, and how do you calculate it?

A moment is the turning effect of a force about a pivot. It is calculated as the force multiplied by the perpendicular distance from the pivot to the line of action of the force, $\text{moment} = F \times d$, with the unit newton metre ($\text{N m}$). A spanner turns a nut more easily with a longer handle because the larger distance gives a larger moment for the same force.

What is the principle of moments?

The principle of moments states that for an object in equilibrium, the sum of the clockwise moments about any pivot equals the sum of the anticlockwise moments about that same pivot. It lets you find an unknown force or distance on a balanced beam or seesaw by setting clockwise moments equal to anticlockwise moments and solving.

SingaporePhysics

Forces and Dynamics for Singapore N(A)-Level Science (Physics) 5105/5106: scalars and vectors, adding forces in a line and equilibrium, Newton's laws of motion and F = ma, friction, and the turning effect of forces (moments)

A Singapore N(A)-Level Science (Physics) overview of Forces and Dynamics (SEAB 5105/5106). It covers scalars and vectors, adding forces along a line and the conditions for equilibrium, Newton's three laws of motion with F = ma, friction and its effects, and the turning effect of forces using moments and the principle of moments.

Generated by Claude Opus 4.86 min readSEAB-5105Updated 2026-06-13

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this module covers
Scalars, vectors and equilibrium
Forces and Newton's laws
Friction and the turning effect of forces
How this module is examined
Check your knowledge

What this module covers

Forces and Dynamics is the heart of mechanics in N(A)-Level Science (Physics) 5105/5106. It explains why objects start moving, speed up, slow down, stay still or turn. The module separates scalars from vectors, combines forces into a single resultant, links that resultant to acceleration through Newton's laws, deals with friction, and extends force to rotation through moments.

It builds on the kinematics of the previous module, since the acceleration here is the same acceleration measured there, and it underpins energy and pressure later. Each dot point below has full worked answers and practice questions.

Scalars, vectors and equilibrium

Scalars, vectors and equilibrium separates quantities that have size only (scalars, such as mass and time) from those that also have direction (vectors, such as force and velocity). When forces act along a straight line you find the resultant by adding the forces in one direction and subtracting those in the other:

F_{\text{net}} = F_{\text{forward}} - F_{\text{backward}}.

An object is in equilibrium when the resultant force on it is zero, so it stays at rest or moves at constant velocity.

Forces and Newton's laws

Forces and Newton's laws ties force to motion. The first law introduces inertia: with no resultant force, motion does not change. The second law gives the central equation $F = ma$ , and the third law states that forces come in equal and opposite pairs acting on two different objects.

Friction and the turning effect of forces

Friction and turning effect of forces explains friction as the force that opposes relative motion between surfaces in contact. Friction can be useful, as in brakes and grip, or wasteful, as in heating moving parts. The module then extends force to rotation. The moment of a force is

\text{moment} = F \times d,

where

d

is the perpendicular distance from the pivot to the line of action of the force. For an object in equilibrium, the principle of moments says total clockwise moments equal total anticlockwise moments, the key to balancing seesaws and beams.

How this module is examined

Reduce to a resultant. Combine the forces into one resultant before using $F = ma$ .
Quote the laws precisely. State each of Newton's laws in full, and never claim action-reaction pairs cancel on one object.
Balance the moments. For equilibrium set clockwise moments equal to anticlockwise moments about a sensible pivot.

Check your knowledge

Recall and calculation questions across the module. Attempt them, then check the worked solutions.

State the difference between a scalar and a vector. (2 marks)
A resultant force of $20\ \text{N}$ acts on a mass of $5.0\ \text{kg}$ . Calculate the acceleration. (2 marks)
A force of $40\ \text{N}$ acts $0.50\ \text{m}$ from a pivot. Calculate the moment. (2 marks)
State Newton's third law of motion. (2 marks)
State the condition for an object to be in equilibrium. (1 mark)

Sources & how we know this

Singapore-Cambridge GCE N(A)-Level Science (Physics, Chemistry) Syllabus 5105/5106 — Singapore Examinations and Assessment Board (2026)