Pancyprian Technical School Physics (4-hour) key terms

The straight-line vector from an object's initial position to its final position, including direction. It differs from distance travelled, which is a scalar and depends on the actual path taken.

The rate at which velocity changes with time, a vector quantity measured in metres per second squared. It arises whenever speed changes, direction changes, or both change simultaneously.

The resultant force on a body equals the product of its mass and acceleration, written F = ma. The acceleration points in the same direction as the resultant force acting on the body.

A sketch showing one chosen object isolated from its surroundings, with all external forces drawn as arrows acting at their points of application. Used to identify the resultant force before applying Newton's laws.

A contact force opposing relative sliding or the tendency to slide between surfaces. Limiting static friction is proportional to the normal reaction, the constant of proportionality being the coefficient of friction.

The perpendicular contact force a surface exerts on a body resting against it. On a horizontal surface it equals the weight only when no other vertical forces act.

In a system with no net external force, the total vector momentum before an interaction equals the total vector momentum after it. This holds for collisions and explosions in any number of dimensions.

The product of a force and the time interval over which it acts, equal to the change in momentum produced. Its SI unit is the newton second, equivalent to the kilogram metre per second.

A collision in which both total momentum and total kinetic energy are conserved. In a perfectly inelastic collision momentum is still conserved but the bodies stick together and kinetic energy is lost.

The product of the force component along the displacement and the displacement magnitude, measured in joules. Work is zero when force and displacement are perpendicular and negative when they oppose each other.

The energy a body possesses because of its motion, equal to one half of its mass times the square of its speed. It is a scalar and always non-negative.

Energy stored in a body because of its position in a gravitational field, near Earth equal to mass times gravitational field strength times height above a chosen reference level. Only changes in this quantity are physically meaningful.

When only conservative forces act on a system, the sum of kinetic and potential energy stays constant. Where friction or air resistance acts, mechanical energy decreases and heating occurs instead.

The rate of transferring energy or doing work, measured in watts, where one watt is one joule per second. For a body moving at constant velocity it equals force times speed.

The ratio of useful energy output to total energy input for a device, usually expressed as a percentage. It is always less than one hundred per cent because some energy is dissipated, often as heat.

The acceleration of a body moving along a circular path, directed towards the centre and equal to speed squared divided by radius. It changes the direction of velocity but not its magnitude.

The resultant force directed towards the centre of a circular path that keeps a body moving in that path, equal to mass times centripetal acceleration. It is provided by tension, gravity, friction or another real force.

The rate of change of angular displacement, measured in radians per second. For uniform circular motion it equals two pi divided by the period of one revolution.

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

The large-amplitude oscillation that occurs when a system is driven at a frequency close to its natural frequency. Damping reduces the peak amplitude and broadens the response curve.

The progressive loss of oscillation energy from a vibrating system, usually through friction or air resistance, so amplitudes decrease over time. Critical damping returns a system to equilibrium fastest without overshooting.

The square of a planet's orbital period is proportional to the cube of the semi-major axis of its orbit. The constant of proportionality depends on the mass of the central body.

The gravitational force per unit mass experienced by a small test mass at a point, measured in newtons per kilogram. Near Earth's surface it is about 9.8 N/kg, directed towards Earth's centre.

The normal force per unit area exerted by a fluid or solid surface, measured in pascals. In a static fluid pressure increases with depth according to density, gravitational field strength and depth.

The upward force a fluid exerts on an immersed or floating body, equal to the weight of fluid displaced. A body floats when the buoyant force balances its weight.

The mass of a substance per unit volume, measured in kilograms per cubic metre. Relative density compares a substance's density with that of water and has no units.

Temperature measured on the Kelvin scale, starting at absolute zero, the lowest temperature theoretically possible. Kelvin values are obtained by adding 273 to Celsius readings.

The equation pV = nRT relating pressure, volume, amount of gas and absolute temperature, where R is the molar gas constant. It describes gases well at low pressure and moderate temperature.

The energy needed to raise the temperature of one kilogram of a substance by one kelvin, measured in joules per kilogram kelvin. Water's unusually high value explains its use as a coolant.

The energy required to change the state of one kilogram of a substance at constant temperature, without changing its temperature. It is released again when the substance changes back.

The electrostatic force per unit positive charge at a point, a vector measured in newtons per coulomb. Between parallel plates it equals voltage divided by plate separation.

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

The rate of flow of electric charge past a point, measured in amperes, where one ampere is one coulomb per second. Conventional current direction is opposite to electron drift in metals.

For a conductor at constant temperature, current is directly proportional to potential difference across it, so resistance equals voltage divided by current. Components such as filament lamps do not obey it.

The energy supplied by a source per unit charge driven around a complete circuit, measured in volts. It differs from terminal potential difference because of energy lost across internal resistance.

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

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.

The generation of an electromotive force in a conductor whenever the magnetic flux linked with it changes. The induced current opposes the change producing it, as stated by Lenz's law.

The ratio of the speed of light in vacuum to its speed in a medium, always at least one. It also equals the sine of the angle of incidence divided by the sine of the angle of refraction.

The complete reflection of light at a boundary when it travels from a denser to a less dense medium and the angle of incidence exceeds the critical angle. It is the basis of optical fibres.

The spreading of waves as they pass through an aperture or around an obstacle, most pronounced when the wavelength is comparable with the obstacle size. It causes interference patterns behind narrow slits.

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

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

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

The interval within which the true value of a measurement is expected to lie, reflecting limitations of the instrument and technique. It is often quoted as plus or minus half the smallest scale division.

The uncertainty of a measurement expressed as a percentage of the measured value. When quantities are multiplied or divided, their percentage errors add to give the overall percentage error.

The digits in a measured value that carry meaning about its precision, excluding leading zeros. A final answer should be quoted to the same precision as the least precise measurement used.

The change in the vertical quantity divided by the corresponding change in the horizontal quantity, found from a straight line of best fit. It often represents a physical constant such as acceleration or resistance.