Pancyprian Physics key terms

The vector change in position of an object, measured as a straight line from the starting point to the finishing point with a stated direction. It differs from distance travelled, which is a scalar path length.

The rate at which velocity changes with time, including changes in speed, direction, or both. It is a vector measured in metres per second squared and is produced by a net force.

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

The product of a body's mass and its velocity, a vector quantity measured in kilogram metres per second. In an isolated system the total momentum stays constant during any interaction.

The product of a force and the time for which it acts, equal to the change in momentum it produces. It explains why airbags and crumple zones reduce injury by extending collision time.

The principle that the total momentum of a system stays constant when no external resultant force acts on it. It applies to collisions and explosions, including elastic and inelastic cases.

The resultant force directed towards the centre of a circular path that keeps a body moving in that circle. It has magnitude mv squared over r and is provided by tension, gravity, friction or similar.

The rate of change of angular displacement, measured in radians per second. It relates linear speed to radius by v equals omega times r for motion on a circular path.

Oscillation in which acceleration is proportional to displacement from equilibrium and directed towards it. The motion is sinusoidal with a fixed period independent of amplitude, described by a equals minus omega squared x.

The large-amplitude response that occurs when a system is driven at a frequency close to its natural frequency. Damping reduces the peak amplitude and shifts the maximum to a slightly lower driving frequency.

A wave pattern formed when two waves of equal frequency and amplitude travel in opposite directions and superpose. It has fixed nodes and antinodes and transfers no net energy along the medium.

The principle that when two or more waves meet at a point, the resultant displacement is the vector sum of the individual displacements. It underlies interference, beats and standing waves.

The change in observed frequency of a wave when the source and observer move relative to each other. Approaching motion raises the observed frequency; receding motion lowers it for sound and light alike.

The force per unit positive charge experienced at a point in an electric field, a vector measured in newtons per coulomb or volts per metre. For a point charge it falls as one over r squared.

The work done per unit positive charge in bringing a small test charge from infinity to a point in an electric field, measured in volts. It is a scalar and equals zero at infinity.

The energy converted from non-electrical forms into electrical energy per unit charge passing through a source, measured in volts. It equals the terminal potential difference plus the lost volts across internal resistance.

The resistance within a source of electromotive force that causes its terminal potential difference to fall as current increases. Lost volts equal current multiplied by internal resistance.

Two circuit rules: the sum of currents entering a junction equals the sum leaving it, and the sum of potential differences around any closed loop is zero. They follow from conservation of charge and energy.

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

The product of magnetic flux density and the area perpendicular to the field, measured in webers. When flux through a circuit changes, an electromotive force is induced in it.

The induced electromotive force in a circuit equals the negative rate of change of magnetic flux linkage through it. Faster flux changes and more turns give larger induced voltages.

The direction of an induced current is such that it opposes the change in magnetic flux that produced it. The minus sign in Faraday's law expresses this consequence of energy conservation.

The energy needed to raise the temperature of one kilogram of a substance by one kelvin, measured in joules per kilogram kelvin. It is used in energy balance calculations for heating and cooling.

The energy transferred per unit mass when a substance changes phase at constant temperature, such as melting or boiling. Specific latent heat values differ for fusion and vaporisation.

The sum of the randomly distributed kinetic and potential energies of the particles within a system. For an ideal gas it depends only on temperature and equals the total kinetic energy of its molecules.

The change in internal energy of a system equals the heat added to it minus the work done by it. It is the principle of conservation of energy applied to thermal processes.

The equation of state pV equals nRT, relating pressure, volume, amount of substance and absolute temperature for an ideal gas. It combines the gas laws and requires temperature in kelvin.

The lowest attainable temperature, zero kelvin, at which a substance has minimum internal energy. It equals minus 273.15 degrees Celsius and cannot be reached in practice.

The ratio of the speed of light in a vacuum to its speed in a medium, always at least one. It determines how much light bends at a boundary through Snell's law.

The complete reflection of light at a boundary when it travels from a denser to a less dense medium at an angle above the critical angle. It carries signals along optical fibres.

The superposition of two or more coherent waves producing regions of reinforcement and cancellation. Constructive interference needs a path difference of a whole number of wavelengths for light.

A property of two waves having a constant phase difference and the same frequency, so their interference pattern is stable. Lasers provide coherent light for demonstrating interference.

The spreading of waves when they pass through an aperture or around an obstacle, most noticeable when the gap is comparable to the wavelength. It limits the resolution of optical instruments.

The emission of electrons from a metal surface when light of frequency above a threshold falls on it. It shows light behaves as photons with energy equal to Planck's constant times frequency.

The minimum energy required to remove an electron from the surface of a metal. It sets the threshold frequency below which no photoelectric emission occurs, whatever the intensity.

The lowest frequency of incident light that can cause photoelectric emission from a given metal. It equals the work function divided by Planck's constant.

The time taken for half the nuclei in a radioactive sample to decay, a constant for each isotope. Activity and undecayed nuclei fall exponentially with time.

The difference between the total mass of a nucleus and the sum of the masses of its separate nucleons. It is equivalent to the binding energy that holds the nucleus together.

The upthrust on a body immersed in a fluid equals the weight of fluid displaced by the body. It explains floating, sinking and apparent weight loss in liquids and gases.

A measure of a fluid's resistance to flow, arising from internal friction between layers moving at different speeds. It decreases with rising temperature in liquids but increases in gases.

The constant speed reached by a falling body when the drag force, including upthrust, balances its weight so the resultant force is zero. It is higher for larger or denser objects.

The ratio of tensile stress to tensile strain in the linear region of a stress-strain graph, a property of the material. It is measured in pascals and indicates stiffness.

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

Stress is force per unit cross-sectional area, measured in pascals; strain is extension divided by original length and is dimensionless. Their ratio in the linear region gives the Young modulus.

The range within which the true value of a measurement is expected to lie, often stated as plus or minus a value. It is estimated from instrument resolution and random variation.

Scatter in repeated measurements caused by unpredictable variations, which reduces the precision of a result. It is reduced by taking more readings and averaging them.

A consistent shift in readings in one direction, caused by a faulty instrument or flawed method, which reduces accuracy. It is not reduced by averaging and requires calibration or correction.

The absolute uncertainty of a measurement expressed as a percentage of the measured value. When quantities are multiplied or divided, their percentage uncertainties are added.