MATSEC IM 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 reversing direction reduces it, unlike the scalar distance travelled.

The rate at which velocity changes with time, including changes in direction. It is measured in metres per second squared and is caused by an unbalanced force acting on a body.

The rate of change of displacement with time, expressed as a vector with both speed and direction. Average velocity divides total displacement by total time, not total distance.

The resultant force on a body equals the rate of change of its momentum; for constant mass this is force equals mass times acceleration. Force and acceleration point in the same direction.

The product of a body's mass and its velocity, measured in kilogram metres per second. It is a vector, and in any closed system the total momentum stays constant during collisions.

The product of a force and the time for which it acts; it equals the change in momentum produced. Extending the contact time reduces the force needed for a given momentum change.

For a body in rotational equilibrium, the sum of clockwise moments about any point equals the sum of anticlockwise moments about that same point. A moment is force multiplied by perpendicular distance from the pivot.

The single point through which the entire weight of a body may be considered to act, whatever its orientation. For a uniform regular solid it coincides with the geometric centre.

A pair of equal, opposite and parallel forces whose lines of action are separated, so they produce rotation without any net translational force. Its moment equals one force times the perpendicular separation.

The turning effect of a force about an axis, equal to force times perpendicular distance from the axis. It changes angular momentum and is measured in newton metres.

The ability of an object to return to equilibrium after being slightly displaced. It increases when the centre of gravity is low and the base area is wide, giving a large angle before toppling.

The state of a body when the resultant force and resultant moment acting on it are both zero. It may be stable, unstable or neutral depending on how forces change after a small displacement.

The extension of a spring or wire is directly proportional to the applied force, provided the elastic limit is not exceeded. The constant of proportionality is the stiffness of the object.

The greatest stress a material can withstand and still return to its original shape when the load is removed. Beyond it, permanent plastic deformation occurs.

The ratio of tensile stress to tensile strain in the linear region of a stress-strain graph. It measures a material's stiffness and has units of pascals.

The normal force acting per unit area on a surface, measured in pascals. In a fluid at rest it increases with depth and acts equally in all directions at a given point.

The upward buoyant force a fluid exerts on an immersed object, equal to the weight of fluid displaced. An object floats when upthrust balances its weight.

The mass of a substance per unit volume, measured in kilograms per cubic metre. It determines whether an object floats and how pressure varies with depth in a fluid.

The constant speed reached by a falling object when the drag force equals its weight, so the resultant force is zero. It depends on shape, mass and the surrounding fluid.

Oscillation in which acceleration is proportional to displacement from equilibrium and directed towards it. The period is independent of amplitude, and displacement follows a sinusoidal variation with time.

The large-amplitude response that occurs when a periodic driving frequency matches an object's natural frequency. Energy transfer is maximum and damping broadens but lowers the peak.

The progressive loss of energy from an oscillating system, usually through friction or air resistance, so amplitude decreases over time. Critical damping returns a system to equilibrium fastest without overshoot.

The distance between two consecutive points in phase on a wave, such as adjacent crests. It equals wave speed divided by frequency.

The number of complete oscillations or wave cycles passing a point each second, measured in hertz. It is fixed by the source and does not change when a wave enters a new medium.

The distance a wave profile advances per second, equal to frequency multiplied by wavelength. It depends on the properties of the medium, not on the amplitude.

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

The reinforcement or cancellation that occurs when coherent waves overlap. Constructive interference gives a path difference of a whole number of wavelengths; destructive gives an odd number of half wavelengths.

A property of two wave sources that maintain a constant phase difference and equal frequency. Coherent sources are required to produce a stable, observable interference pattern.

The spreading of waves as they pass through an aperture or around an obstacle. It is most pronounced when the gap width is comparable to the wavelength.

A pattern formed by two identical waves travelling in opposite directions, with fixed nodes of zero displacement and antinodes of maximum displacement. Adjacent nodes are half a wavelength apart.

For a medium, the ratio of the speed of light in a vacuum to its speed in that medium. It also equals the ratio of the sine of the angle of incidence to the sine of the angle of refraction.

The complete reflection of light at a boundary when travelling from a denser to a less dense medium at an angle greater than the critical angle. It is used in optical fibres.

The angle of incidence in the denser medium for which the refracted ray travels along the boundary at ninety degrees. Beyond it, total internal reflection occurs.

The rate of flow of electric charge past a point, measured in amperes. One ampere is one coulomb of charge passing per second.

The energy transferred per unit charge between two points in a circuit, measured in volts. One volt is one joule per coulomb.

The energy supplied per unit charge by a source such as a cell or generator. It equals the terminal potential difference plus the potential difference lost across internal resistance.

The opposition to current flow in a component, equal to potential difference divided by current. It is measured in ohms and depends on material, length, area and temperature.

The resistance within a source of electromotive force that causes its terminal voltage to fall as current increases. It dissipates energy as heat inside the source.

Two conservation rules for circuits: charge is conserved at any junction, and energy is conserved around any closed loop. They are used to solve multi-loop circuits.

The charge stored per unit potential difference across a capacitor, measured in farads. Energy stored equals half the charge multiplied by the potential difference.

For a capacitor-resistor circuit, the product of resistance and capacitance; it is the time for charge to fall to about thirty-seven per cent of its initial value during discharge.

The magnetic force per unit current per unit length on a conductor placed at right angles to the field, measured in teslas. It describes the strength of a magnetic field.

The product of magnetic flux density and the area perpendicular to the field, measured in webers. It quantifies the field passing through a surface.

The generation of an electromotive force in a conductor when the magnetic flux through it changes. The induced emf is proportional to the rate of change of flux linkage.

The direction of an induced current is such that it opposes the change in magnetic flux that produced it. This is a consequence of conservation of energy.

The energy needed to raise the temperature of one kilogram of a substance by one kelvin, measured in joules per kilogram per kelvin. Water has an unusually high value.

The energy required to change the state of one kilogram of a substance at constant temperature, without changing its temperature. It is measured in joules per kilogram.

The sum of the random kinetic and potential energies of all the particles in a system. It increases when temperature rises or when a substance melts or boils.

The lowest possible temperature, zero kelvin, at which particles have minimum kinetic energy. It equals minus two hundred and seventy-three degrees Celsius.

A model gas whose particles have negligible volume and exert no forces except during elastic collisions. Its pressure, volume and absolute temperature obey a simple relationship.

The emission of electrons from a metal surface when light of sufficiently high frequency falls on it. It provides evidence that light behaves as discrete photons.

The minimum energy needed to remove an electron from the surface of a metal. It is characteristic of the metal and relates to the threshold frequency of the photoelectric effect.

A quantum of electromagnetic radiation carrying energy equal to Planck's constant multiplied by frequency. Its energy is proportional to frequency and inversely proportional to wavelength.

The time taken for half the nuclei in a radioactive sample to decay. It is constant for a given isotope and independent of the sample size.

The spontaneous disintegration of unstable nuclei, emitting alpha, beta or gamma radiation. It is random and unaffected by temperature, pressure or chemical state.

The number of nuclear decays occurring per second in a radioactive sample, measured in becquerels. It decreases exponentially with time according to the half-life.

The gravitational force per unit mass experienced at a point in a field, measured in newtons per kilogram. Near Earth's surface it is approximately nine point eight.

The force per unit positive charge at a point in an electric field, measured in volts per metre or newtons per coulomb. Field lines point away from positive charge.

The work done per unit mass or per unit positive charge in bringing it from infinity to a point in a field. It is a scalar quantity measured in joules per kilogram or volts.