SACE Stage 2 Physics key terms

The straight-line distance and direction from an object's starting point to its current position. It is a vector quantity, so moving around a closed loop gives zero total displacement.

The rate at which displacement changes with time, including direction. Average velocity divides total displacement by total time, while instantaneous velocity describes the motion at one particular moment.

The rate at which velocity changes with time. Because velocity is a vector, acceleration occurs whenever speed changes, direction changes, or both change, and it is measured in metres per second squared.

The curved path followed by an object launched into the air and acted on only by gravity. Horizontal and vertical motions are treated separately, with constant horizontal velocity and constant downward acceleration.

A quantity that has both magnitude and direction, such as force or velocity. Vectors are added head-to-tail or resolved into perpendicular components, unlike scalars which have magnitude only.

A quantity described completely by its magnitude, with no direction needed, such as mass, time, or speed. Scalars add arithmetically, while vectors require direction-aware addition.

The net force on an object equals its mass multiplied by its acceleration. It links cause and effect in motion and can also be written as force equals the rate of change of momentum.

The product of an object's mass and velocity, giving a vector measure of how hard it is to stop. In any isolated system the total momentum before an interaction equals the total afterwards.

The change in momentum produced by a force acting over a time interval. Increasing the time over which a force acts reduces the force needed for the same momentum change, as in airbags.

Motion along a circular path at constant speed. The velocity direction constantly changes, so a centripetal acceleration points towards the centre and requires a net inward force.

The net force directed towards the centre of a circular path that keeps an object moving in that circle. It is supplied by tension, gravity, friction, or a normal force, not a separate kind of force.

A coordinate system with a chosen observer from which positions and motions are measured. Measurements of velocity depend on the frame, which is central to understanding relative motion.

A frame of reference in which an object with no net force remains at rest or moves at constant velocity. Non-inertial frames accelerate and introduce apparent forces such as centrifugal effects.

Two assumptions: the laws of physics are the same in all inertial frames, and the speed of light in a vacuum is the same for all observers regardless of their motion relative to the source.

The effect whereby a clock moving relative to an observer ticks more slowly than a clock at rest with that observer. The size of the effect grows as relative speed approaches the speed of light.

The time interval measured by a clock that is at rest relative to the events being timed, so both events occur at the same place for that observer. It is the shortest measured interval between those events.

The shortening of a moving object's measured length along its direction of motion, as seen by an observer relative to whom it moves. The proper length is measured in the object's rest frame.

The energy equivalent of an object's mass when it is at rest, given by mass times the speed of light squared. It shows that mass itself is a form of energy.

A region around a charge where another charge experiences a force. Field strength is force per unit positive charge, and field lines point away from positive charges and towards negative ones.

The electrostatic force between two point charges is proportional to the product of the charges and inversely proportional to the square of the distance between them. The force acts along the line joining them.

The work done per unit positive charge in bringing a charge from infinity to a point in an electric field. It is a scalar measured in volts and equals potential energy per unit charge.

The difference in electric potential between two points, equal to the work done per unit charge moved between them. It is commonly called voltage and is measured in volts.

The energy per unit charge supplied by a source such as a battery or generator to drive current around a circuit. Despite the name it is a voltage, not a force, measured in volts.

The resistance within a source of electromotive force that causes some energy to be dissipated as heat inside the source itself. It reduces the terminal voltage below the emf when current flows.

For an ohmic conductor at constant temperature, the current through it is directly proportional to the potential difference across it. The constant of proportionality is the resistance.

A measure of how strongly a component opposes current, equal to potential difference divided by current. It depends on material, length, cross-sectional area, and temperature, and is measured in ohms.

A measure of the number of magnetic field lines passing through a given area, equal to field strength times area times the cosine of the angle between field and area normal. It is measured in webers.

The strength of a magnetic field, defined as the force per unit current per unit length on a conductor placed perpendicular to the field. It is measured in tesla.

The force on a moving charge in electric and magnetic fields. The magnetic part acts perpendicular to both the velocity and the field, so it changes direction but not speed.

The induced emf in a circuit equals the negative rate of change of magnetic flux through it. Changing field strength, area, or orientation can all induce an emf.

An induced current flows in a direction such that its own magnetic field opposes the change in flux that produced it. This is a consequence of conservation of energy and explains the negative sign in Faraday's law.

A disturbance that transfers energy from one place to another without transferring matter. Waves may be mechanical, requiring a medium, or electromagnetic, which can travel through a vacuum.

The distance between consecutive corresponding points on a wave, such as crest to crest, measured along the direction of travel. It is related to speed and frequency by the wave equation.

The number of complete wave cycles passing a point each second, measured in hertz. Frequency is set by the source and does not change when a wave enters a different medium.

When two or more waves meet at a point, the resultant displacement equals the vector sum of their individual displacements. This principle explains interference, standing waves, and beats.

The reinforcement or cancellation that occurs when waves overlap. Constructive interference gives a larger amplitude when waves arrive in phase, while destructive interference gives a smaller amplitude when out of phase.

A property of two wave sources that maintain a constant phase relationship, producing a stable interference pattern. Lasers are highly coherent, whereas ordinary lamps are not.

The spreading of waves as they pass through a gap or around an obstacle. The effect becomes more pronounced when the gap width is comparable to the wavelength.

The change in direction of a wave as it passes from one medium into another where its speed differs. The frequency stays constant while wavelength and speed change.

The ratio of the speed of light in a vacuum to its speed in a given medium. It determines how much light bends at a boundary and is always at least one.

When light travelling in a denser medium strikes a boundary beyond the critical angle, it is entirely reflected back rather than refracted out. This effect underpins optical fibres.

A wave pattern formed when two identical waves travelling in opposite directions superpose, producing fixed nodes and antinodes. Energy is stored rather than transported along the wave.

A large amplitude response that occurs when a system is driven at one of its natural frequencies. Standing wave patterns in strings and pipes occur at these resonant frequencies.

The emission of electrons from a metal surface when light of sufficiently high frequency strikes it. It provides evidence that light delivers energy in discrete quanta called photons.

A discrete packet of electromagnetic energy whose energy equals Planck's constant multiplied by frequency. Higher-frequency light carries more energetic photons, which explains threshold behaviour.

The minimum energy needed to remove an electron from the surface of a particular metal. It sets the threshold frequency below which no photoelectrons are emitted, whatever the intensity.

One of a set of discrete energies that an electron in an atom may possess. Transitions between levels involve absorption or emission of a photon with energy equal to the difference.

The minimum energy required to remove an electron completely from an atom in its ground state, leaving a positive ion. It corresponds to the energy needed to reach the highest energy level.

The time taken for half of the radioactive nuclei in a sample to decay. It is constant for a given isotope and is unaffected by temperature, pressure, or chemical state.

The spontaneous transformation of an unstable nucleus into a more stable one, emitting alpha, beta, or gamma radiation. It is random for individual nuclei but predictable for large samples.

The number of nuclear decays occurring per second in a sample, measured in becquerels. It decreases exponentially over time and depends on the number of undecayed nuclei present.

The difference between the total mass of a nucleus's separate nucleons and the mass of the assembled nucleus. This missing mass appears as binding energy holding the nucleus together.

The energy required to separate a nucleus completely into its individual protons and neutrons. It equals the mass defect multiplied by the speed of light squared and indicates nuclear stability.