AS level physics definations (First 6 Topc

--- TOPIC 1: PHYSICAL QUANTITIES AND UNITS ---

Physical quantity

A quantity that can be measured and consists of a numerical magnitude and a unit.

SI base units

The 7 fundamental units: metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), candela (cd).

Scalar quantity

A quantity that has magnitude only. Examples: mass, speed, temperature, energy.

Vector quantity

A quantity that has both magnitude and direction. Examples: force, velocity, acceleration, displacement.

Homogeneous equation

An equation where all terms have the same base units on both sides; used to check validity of equations.

Uncertainty (absolute)

The margin of error in a measurement. Expressed as ± value. Formula: absolute uncertainty = half the range of repeated readings.

Uncertainty (percentage)

Formula: % uncertainty = (absolute uncertainty / measured value) × 100%

Combining uncertainties (add/subtract)

Add absolute uncertainties: Δ(A ± B) = ΔA + ΔB

Combining uncertainties (multiply/divide)

Add percentage uncertainties: %Δ(A × B) = %ΔA + %ΔB

Combining uncertainties (power)

Multiply percentage uncertainty by the power: %Δ(Aⁿ) = n × %ΔA

Random error

An error that causes readings to scatter around the true value; reduced by repeating measurements and averaging.

Systematic error

An error that causes all readings to be consistently higher or lower than the true value; cannot be reduced by averaging.

Precision

How close repeated measurements are to each other (small random error).

Accuracy

How close a measured value is to the true value (small systematic error).

Order of magnitude

A power of 10 used to give a rough estimate of a quantity's size.

--- TOPIC 2: KINEMATICS ---

Displacement

The distance moved in a specified direction. Unit: metre (m). It is a vector.

Speed

The rate of change of distance. Formula: v = d/t. Unit: m s⁻¹. Scalar.

Velocity

The rate of change of displacement. Formula: v = Δs/Δt. Unit: m s⁻¹. Vector.

Acceleration

The rate of change of velocity. Formula: a = Δv/Δt. Unit: m s⁻². Vector.

Uniform acceleration

Constant acceleration; the velocity changes by equal amounts in equal time intervals.

SUVAT equation 1

v = u + at (relates velocity, initial velocity, acceleration, and time)

SUVAT equation 2

s = ut + ½at² (relates displacement, initial velocity, acceleration, and time)

SUVAT equation 3

v² = u² + 2as (relates velocity, initial velocity, acceleration, and displacement)

SUVAT equation 4

s = ½(u + v)t (relates displacement, average velocity, and time)

Free fall

Motion under gravity alone, with no air resistance. Acceleration = g ≈ 9.81 m s⁻².

Projectile motion

Motion of an object under gravity where horizontal velocity is constant and vertical motion has constant acceleration g downward.

Gradient of displacement–time graph

Equals velocity.

Gradient of velocity–time graph

Equals acceleration.

Area under velocity–time graph

Equals displacement.

Terminal velocity

The constant velocity reached when drag force equals the driving/gravitational force, so net force = 0 and acceleration = 0.

--- TOPIC 3: DYNAMICS ---

Newton's First Law

An object remains at rest or moves with constant velocity unless acted upon by a resultant force.

Newton's Second Law

The resultant force on an object equals its rate of change of momentum. Formula: F = ma (for constant mass). Unit: newton (N).

Newton's Third Law

If object A exerts a force on object B, then object B exerts an equal and opposite force on object A. (Action–reaction pairs act on different objects.)

Momentum

The product of mass and velocity. Formula: p = mv. Unit: kg m s⁻¹. Vector.

Impulse

The change in momentum of an object. Formula: Impulse = FΔt = Δp. Unit: N s or kg m s⁻¹.

Principle of conservation of momentum

The total momentum of a system remains constant provided no external force acts on it.

Elastic collision

A collision in which both momentum and kinetic energy are conserved.

Inelastic collision

A collision in which momentum is conserved but kinetic energy is not (some is converted to other forms).

Force (Newton's 2nd Law form)

F = Δp/Δt — resultant force equals rate of change of momentum.

Newton (unit)

1 N = 1 kg m s⁻² — the force that gives a 1 kg mass an acceleration of 1 m s⁻².

--- TOPIC 4: FORCES, DENSITY AND PRESSURE ---

Weight

The gravitational force on an object. Formula: W = mg. Unit: N. It is a vector.

Centre of gravity

The single point where the entire weight of an object appears to act.

Upthrust

The upward force exerted by a fluid on a submerged or floating object, equal to the weight of fluid displaced (Archimedes' Principle). Formula: U = ρVg.

Density

Mass per unit volume. Formula: ρ = m/V. Unit: kg m⁻³.

Pressure

Force per unit area (perpendicular). Formula: P = F/A. Unit: pascal (Pa) = N m⁻².

Pressure in a fluid

Formula: P = ρgh, where ρ = fluid density, g = gravitational field strength, h = depth below surface.

Moment of a force

The turning effect of a force about a point. Formula: Moment = F × d (where d is perpendicular distance from pivot). Unit: N m.

Principle of moments

For a body in rotational equilibrium, the sum of clockwise moments about any point equals the sum of anticlockwise moments about the same point.

Torque of a couple

Formula: Torque = F × d (where F is one of the equal and opposite forces and d is the perpendicular distance between them). Unit: N m.

Couple

A pair of equal, parallel, opposite forces that produce a turning effect (torque) but no resultant force.

Equilibrium

An object is in equilibrium when the resultant force is zero AND the resultant torque/moment about any point is zero.

Resolving forces

Splitting a force into two perpendicular components. Fₓ = F cosθ, Fᵧ = F sinθ.

Friction

A force that opposes relative motion between surfaces in contact.

--- TOPIC 5: WORK, ENERGY AND POWER ---

Work done

Energy transferred when a force moves an object. Formula: W = Fs cosθ (θ is angle between force and displacement). Unit: joule (J).

Joule (unit)

1 J = 1 N m = 1 kg m² s⁻². The work done when a force of 1 N moves through 1 m in the direction of the force.

Kinetic energy

Energy due to motion. Formula: Eₖ = ½mv². Unit: J.

Gravitational potential energy

Energy due to position in a gravitational field. Formula: Eₚ = mgh. Unit: J.

Elastic potential energy

Energy stored in a deformed (stretched/compressed) elastic material. Formula: E = ½Fx = ½kx². Unit: J.

Principle of conservation of energy

Energy cannot be created or destroyed; it can only be converted from one form to another. The total energy of a closed system is constant.

Power

The rate of doing work or transferring energy. Formula: P = W/t = Fv. Unit: watt (W) = J s⁻¹.

Efficiency

The ratio of useful output energy (or power) to total input energy (or power). Formula: Efficiency = (useful output / total input) × 100%.

Work–energy theorem

The net work done on an object equals its change in kinetic energy. Formula: W_net = ΔEₖ = ½mv² − ½mu².

--- TOPIC 6: DEFORMATION OF SOLIDS ---

Hooke's Law

The extension of a spring is directly proportional to the applied force, provided the elastic limit is not exceeded. Formula: F = kx.

Spring constant (k)

The force per unit extension. Formula: k = F/x. Unit: N m⁻¹. A measure of stiffness.

Elastic limit

The maximum force beyond which a material does not return to its original shape when the force is removed.

Elastic deformation

Deformation that is fully recovered when the load is removed (below the elastic limit).

Plastic deformation

Permanent deformation that remains after the load is removed (beyond the elastic limit).

Tensile stress

Force per unit cross-sectional area. Formula: σ = F/A. Unit: Pa (N m⁻²).

Tensile strain

The fractional extension (extension per unit original length). Formula: ε = x/L. No unit (dimensionless).

Young modulus (E)

The ratio of tensile stress to tensile strain for a material within the elastic region. Formula: E = σ/ε = (FL)/(Ax). Unit: Pa (N m⁻²).

Elastic strain energy

Energy stored per unit volume in a stretched material. Formula: Energy = ½Fx = ½kx² (or area under F–x graph). Unit: J.

Limit of proportionality

The point beyond which stress is no longer proportional to strain (Hooke's Law no longer applies).

Yield point

The stress at which a large increase in strain occurs with little or no increase in stress (material begins to flow plastically).

Ultimate tensile stress (UTS)

The maximum stress a material can withstand before fracture. Also called tensile strength.

Ductile material

A material that can be drawn into wires; shows significant plastic deformation before breaking (e.g. copper).

Brittle material

A material that fractures with little or no plastic deformation (e.g. glass, ceramics).

Stiff material

A material with a high Young modulus; little deformation under stress.