Forces and moments

Identify H1 Physics contact and field forces, use Hooke’s law, and apply moments and couples to solve equilibrium problems.

GCE A-Level H1 Physics 2027
3 lessons

Start topic

Before you begin

Every force comes from an interaction, and where a force acts decides whether it also turns an object. The lessons start by identifying the field and contact forces on an object, including the force from a spring, then deal with moments, couples and centre of gravity, and finish by combining force balance and moment balance to analyse objects in equilibrium.

Be comfortable with: resolving vectors, because forces must be added and resolved as vectors.

Learning goals

Describe the forces on a mass, a charge and a current-carrying conductor in gravitational, electric and magnetic fields.
Describe normal, frictional and viscous forces, such as air resistance, qualitatively
Apply Hooke's law within the limit of proportionality.
Apply moments, couples and force-and-torque equilibrium using free-body diagrams and vector triangles.
Treat the weight of a body as acting at a single point, its centre of gravity
Apply the principle of moments to new situations and problems
Explain inertia and momentum, then apply Newton's laws using free-body diagrams.

Syllabus statements covered

describe the forces on a mass, charge and current-carrying conductor in gravitational, electric and magnetic fields, as appropriate
show a qualitative understanding of forces including normal force, frictional force and viscous force, e.g. air resistance (knowledge of the concepts of coefficients of friction and viscosity is not required)
recall and apply Hooke’s law (F = kx, where k is the force constant) to new situations or to solve related problems
define and apply the moment of a force and the torque of a couple
show an understanding that a couple is a pair of forces which tends to produce rotation only
show an understanding that the weight of a body may be taken as acting at a single point known as its centre of gravity
apply the principle of moments to new situations or to solve related problems
show an understanding that, when there is no resultant force and no resultant torque, a system is in equilibrium
use free-body diagrams and vector triangles to represent forces on bodies that are in rotational and translational equilibrium

Lessons

Work through them in order.

Practise and check

Or choose

Topic reference

Quick reference

Idea	Relationship	Note
weight	W = mg	force of a gravitational field on a mass
electric force	F = QE	along the field for a positive charge
Hooke’s law	F = kx	up to the limit of proportionality; k in N m⁻¹
moment of a force	Fd_⊥	d_⊥ to the line of action; unit N m
torque of a couple	Fd	d is the separation of the two forces
equilibrium	resultant force = 0; resultant moment = 0	both conditions

Core knowledge to remember

A gravitational field exerts a force on a mass, an electric field on a charge, and a magnetic field on a current-carrying conductor.
The normal contact force acts at right angles to a surface; friction opposes sliding between surfaces; a viscous force, such as air resistance, opposes motion through a fluid and increases with speed.
Hooke’s law: the force needed to stretch a spring is proportional to its extension, F = kx, up to the limit of proportionality.
The moment of a force about a point is the force multiplied by the perpendicular distance from the point to its line of action.
A couple is a pair of equal and opposite forces that tends to produce rotation only. Its torque is one force multiplied by the perpendicular distance between them.
The weight of a body can be taken as acting at a single point, its centre of gravity.
Principle of moments: for a body in equilibrium, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments about that point.
A system is in equilibrium when there is no resultant force and no resultant torque.

Common mistakes

Drawing a force in the direction of motion when nothing exerts it.
Assuming the normal contact force always equals the weight.
Using the distance to where a force acts instead of the perpendicular distance to its line of action.
Doubling Fd for a couple.
Checking only that the forces balance, and not the moments.
Forgetting the weight of a beam, or putting it anywhere but its centre of gravity.