AS Physics Definitions
Cambridge AS Physics (9702) | Key Terms by Topic
Topics 1-11 | Syllabus 2025-2027
Why Definitions Matter
In the exam, the command word "define" means give a precise meaning. Saying "velocity is speed with direction" misses the key idea; "velocity is the rate of change of displacement" states it. Learn the key idea in each definition below.
Jump to Topic:
Subscribe for the Definitions List
Subscribe with your email to unlock the definitions list and the printable PDF.
We email about once a month. No spam. Unsubscribe anytime.
1. Physical Quantities and Units
Subscribe above to view the definitions
1. Physical Quantities and Units
A quantity that has magnitude only.
Examples: mass, speed, energy, temperature, distance
A quantity that has both magnitude and direction.
Examples: displacement, velocity, force, momentum, acceleration
The units of the SI base quantities, from which all derived units are built. At AS you must recall: kilogram (kg), metre (m), second (s), ampere (A), kelvin (K).
The mole (mol) is also an SI base unit; it is A Level content (Topic 15)
An error that causes readings to be shifted consistently in one direction (all too high or all too low) from the true value.
Cannot be reduced by averaging; must be removed by correcting the technique or the reading (e.g. a zero error)
An error that causes readings to scatter around the true value with no pattern.
Can be reduced by averaging multiple readings
A measure of how close repeated measurements are to each other.
High precision = low scatter in readings
A measure of how close a measurement is to the true value.
Can be accurate but not precise, or precise but not accurate
2. Kinematics
The distance moved in a specified direction from a reference point.
Vector quantity; can be positive or negative
The rate of change of distance travelled.
Scalar quantity
The rate of change of displacement.
Vector quantity; includes direction
The rate of change of velocity.
Vector quantity; can be negative (deceleration)
Constant acceleration, where velocity changes by equal amounts in equal time intervals.
Motion under gravity with no other forces acting (no air resistance).
Acceleration = g = 9.81 m s⁻² near the Earth's surface
3. Dynamics
How strongly an object resists any change to its motion (its inertia).
Scalar quantity; measured in kg
The product of mass and velocity.
Vector quantity; unit: kg m s⁻¹
The rate of change of momentum.
For constant mass: F = ma
An object remains at rest or continues with constant velocity unless acted upon by a resultant force.
The resultant force on an object is proportional to its rate of change of momentum (and is in the same direction).
When body A exerts a force on body B, body B exerts an equal and opposite force on body A.
Forces are same type, act on different bodies
The gravitational force on an object: W = mg, where g is the acceleration of free fall.
The constant (maximum) velocity reached when the resistive force has grown until the resultant force on the object is zero, so there is no acceleration. For a falling object, drag plus any upthrust then equals the weight.
The total momentum of a system remains constant provided no external resultant force acts on the system.
A collision in which both momentum and kinetic energy are conserved.
Relative speed of approach = relative speed of separation
A collision in which momentum is conserved but kinetic energy is not conserved.
Some KE is converted to other forms (heat, sound, deformation)
4. Forces, Density and Pressure
The single point where the entire weight of an object may be considered to act.
The product of the force and the perpendicular distance from the pivot to the line of action of the force.
Unit: N m
A pair of equal and opposite forces whose lines of action are parallel but do not coincide, so they produce rotation only (no resultant force, so no translation).
The product of one of the forces and the perpendicular distance between the lines of action of the forces.
For a body in rotational equilibrium, the sum of clockwise moments about any point equals the sum of anticlockwise moments about that point.
When the resultant force and the resultant torque on a body are both zero.
Object is either at rest or moving with constant velocity
Mass per unit volume.
Unit: kg m⁻³
The force acting normal (perpendicular) to a surface per unit area of the surface.
Unit: Pa (pascal) = N m⁻²
The pressure at a point in a fluid due to the weight of fluid above it.
The upward force on an object wholly or partly immersed in a fluid, caused by the difference in hydrostatic pressure between its lower and upper surfaces. It equals the weight of fluid displaced.
Archimedes' principle
5. Work, Energy and Power
The product of force and displacement in the direction of the force.
Unit: joule (J) = N m
Energy cannot be created or destroyed; it can only be transferred from one form to another.
The energy an object has due to its motion.
The energy an object has due to its position in a gravitational field.
In a uniform field near Earth's surface
The rate of doing work, or the rate of energy transfer.
Unit: watt (W) = J s⁻¹
The ratio of useful energy output from a system to the total energy input, often expressed as a percentage.
6. Deformation of Solids
The extension of a spring is directly proportional to the applied force, provided the limit of proportionality is not exceeded.
The force per unit extension of a spring.
Unit: N m⁻¹
The point beyond which the extension is no longer proportional to the applied force.
The maximum stress that can be applied to a material before it becomes permanently deformed.
Deformation where the material returns to its original shape when the force is removed.
Permanent deformation where the material does not return to its original shape when the force is removed.
Force per unit cross-sectional area.
Unit: Pa (pascal)
Extension per unit original length.
No unit (ratio)
The ratio of stress to strain for a material, within its limit of proportionality.
Unit: Pa; measure of stiffness
The energy stored in a material because it has been deformed (stretched or compressed).
The formula applies only within the limit of proportionality; otherwise use the area under the force-extension graph
7. Waves
A wave that transfers energy from one place to another.
The distance moved by a particle from its equilibrium position.
The maximum displacement of a particle from its equilibrium position.
The time taken for one complete oscillation.
Unit: second (s)
The number of complete oscillations per unit time.
Unit: hertz (Hz)
The minimum distance between two points oscillating in phase.
Symbol: λ; Unit: metre (m)
The fraction of a cycle by which one wave leads or lags another, expressed in degrees or radians.
The distance travelled by a wave per unit time.
Power per unit area perpendicular to the direction of wave travel.
Unit: W m⁻²; intensity ∝ (amplitude)²
A wave in which the oscillations are perpendicular to the direction of energy transfer.
Examples: light, water waves, waves on a string
A wave in which the oscillations are parallel to the direction of energy transfer.
Examples: sound waves
Restricting the oscillations of a transverse wave to one direction only, at right angles to the direction of travel, so they lie in a single plane that contains the direction of travel.
Only transverse waves can be polarised
The change in observed frequency when there is relative motion between the source and observer.
At AS you only need a moving source of sound and a stationary observer
8. Superposition
When two or more waves meet at a point, the resultant displacement is the vector sum of the individual displacements.
The pattern of reinforcement (maxima) and cancellation (minima) formed when two or more coherent waves overlap.
Superposition where waves are in phase, producing a resultant wave with greater amplitude.
Superposition where waves are in antiphase (180° out of phase), producing a resultant wave with smaller amplitude.
When two sources emit waves with a constant phase difference.
Required for observable interference pattern
The difference in distance travelled by two waves from their sources to a given point.
A wave pattern formed by the superposition of two progressive waves of the same frequency travelling in opposite directions.
A point on a stationary wave where the amplitude is always zero.
A point on a stationary wave where the amplitude is maximum.
The spreading of a wave as it passes through a gap or around an obstacle.
9. Electricity
The rate of flow of electric charge.
Unit: ampere (A)
The charge passing a point equals current × time. One coulomb is the charge that passes a point when a current of one ampere flows for one second.
Unit: coulomb (C); charge is quantised, in whole multiples of e = 1.60 × 10⁻¹⁹ C
The energy transferred per unit charge as charge passes through a component (electrical energy converted to other forms).
Unit: volt (V) = J C⁻¹
The ratio of potential difference across a component to the current through it.
Unit: ohm (Ω)
The current through a conductor is proportional to the potential difference across it, provided physical conditions (e.g., temperature) remain constant.
The resistance of a material of unit length and unit cross-sectional area.
Unit: Ω m
The rate of energy transfer in an electrical component.
Unit: watt (W)
10. DC Circuits
The energy transferred per unit charge by a source in driving charge around a complete circuit.
Unit: volt (V); energy from other forms converted to electrical energy
The resistance within a source of EMF that causes energy to be dissipated inside the source.
The potential difference across the terminals of a source, equal to EMF minus the voltage dropped across internal resistance.
The sum of currents entering a junction equals the sum of currents leaving it.
Based on conservation of charge
The sum of EMFs around any closed loop equals the sum of potential differences around that loop.
Based on conservation of energy
A circuit that uses resistors to divide a voltage into smaller parts.
A device for comparing potential differences using a null method (zero current through galvanometer).
11. Particle Physics
The total number of protons and neutrons in a nucleus.
Also called mass number
The number of protons in a nucleus.
Also called atomic number; defines the element
Atoms of the same element with the same proton number but different nucleon numbers (different number of neutrons).
A unit of mass equal to 1/12 of the mass of a carbon-12 atom.
1 u = 1.66 × 10⁻²⁷ kg (given on the data sheet)
A particle with the same mass as its corresponding particle but opposite charge.
e.g., positron is antiparticle of electron
A helium nucleus (2 protons + 2 neutrons) emitted from an unstable nucleus.
Charge: +2e; Mass: 4u
A high-energy electron emitted from a nucleus when a neutron converts to a proton.
Also emits an electron antineutrino
A positron emitted from a nucleus when a proton converts to a neutron.
Also emits an electron neutrino
High-energy electromagnetic radiation emitted from an excited nucleus.
No charge, no mass
A fundamental particle that combines to form hadrons. Six flavours: up, down, strange, charm, top, bottom.
A fundamental particle that does not experience the strong nuclear force.
Includes electrons and neutrinos
A particle made of quarks that experiences the strong nuclear force.
Includes baryons and mesons
A hadron made of three quarks.
e.g., proton (uud), neutron (udd)
A hadron made of one quark and one antiquark.
e.g. a pion; you need the quark-antiquark make-up (the syllabus names no particular mesons)
More AS Physics Resources
LumiExams.com | Cambridge AS Physics (9702) | Syllabus 2025-2027
Frequently Asked Questions
What definitions do I need to know for AS Physics?
The AS syllabus (Topics 1-11) asks you to define terms such as displacement, speed, velocity, acceleration, linear momentum, force, moment, density, pressure, power, stress, strain, the Young modulus, potential difference, resistance and e.m.f. In the exam, the command word define means give a precise meaning, so a vague answer can miss the mark.
How should I learn physics definitions?
Focus on the key components of each definition. For example, 'potential difference' must include 'energy transferred per unit charge'. Use flashcards, write definitions from memory, and check each one for its key idea. A vague answer, such as calling velocity speed with direction, leaves out the key idea (rate of change of displacement).
Last updated