Forces & Their InteractionsOxford AQA IGCSE Physics: Revision notes
Section 1
What is the difference between contact and non-contact forces?
Forces can be classified into two types:
- Non-contact (field) forces act between objects that are not physically touching — gravity, electrostatic forces, and magnetism
- Contact forces act only between objects that are physically touching — friction, air resistance, tension, and the normal contact force
Friction is a force between two surfaces that impedes (resists) relative motion, and may result in heating of the surfaces. Air resistance is a form of friction acting on an object moving through air.
Section 2
How do forces act in pairs?
Forces always arise from an interaction between a pair of objects: each object exerts a force on the other. These forces are represented as vectors — they have both magnitude and direction, usually drawn as arrows.
Section 3
What is the difference between scalars and vectors?
| Scalar (magnitude only) | Vector (magnitude and direction) |
|---|---|
| distance | displacement |
| speed | velocity |
| time | acceleration |
| force | |
| momentum |
A scalar quantity is fully described by a number and a unit; a vector quantity also needs a direction to be fully described.
Section 4
How do we calculate weight?
Weight is the force of gravity acting on an object's mass:
W = m x g
where W is weight (N), m is mass (kg), and g is gravitational field strength (N/kg). Weight is a non-contact force, always acting towards the centre of the gravitational body (e.g. downwards on Earth). Mass is a fixed property of an object, but weight changes depending on the gravitational field strength it is in.
Mass and weight are not the same thing: mass (kg) does not change with location, but weight (N) does, because it depends on gravitational field strength.
Section 5
What happens when an elastic object is distorted, and how does Hooke's Law work?
When a force is applied to an object it may change shape:
- Elastic distortion — the object returns to its original shape once the force is removed
- Inelastic distortion — the object does not return to its original shape
A force applied to an elastic object stores elastic potential energy in it.
Hooke's Law describes the relationship between force and extension for a spring within its limit of proportionality:
F = k x e
where F is force (N), k is the spring constant (N/m), and e is extension (m).
Required practical: investigate the relationship between force and extension for a spring — add known masses (weights) one at a time, measure the resulting extension with a ruler, and plot force against extension; the graph is a straight line through the origin while Hooke's Law is obeyed.
Always distinguish 'extension' (increase in length beyond the natural length) from the spring's total length in a Hooke's Law calculation.
Must Know
- Non-contact forces: gravity, electrostatics, magnetism. Contact forces: friction, air resistance, tension, normal contact force
- Friction impedes motion and may cause heating; air resistance is a form of friction
- Forces act in interacting pairs and are represented as vectors
- Scalars (distance, speed, time) have magnitude only; vectors (displacement, velocity, acceleration, force, momentum) have magnitude and direction
- W = m x g (weight)
- Elastic distortion: object returns to original shape. Inelastic: it does not
- Hooke's Law: F = k x e, within the limit of proportionality
That's the notes covered.
Carry on to the next subtopic.