NSSCO Physics (6118) key terms

One of seven fundamental units (metre, kilogram, second, ampere, kelvin, mole and candela) from which all other SI units are built by multiplication and division, without using prefixes or other units.

A physical quantity that is fully described by its size (magnitude) alone, with no direction needed. Examples include mass, time, distance, speed, energy and temperature.

A physical quantity that needs both a magnitude and a direction to be fully described, such as force, velocity, displacement or acceleration. Vectors can be added using scale diagrams.

An error that varies unpredictably from one reading to the next, often caused by fluctuating conditions or the observer's judgement. Taking repeated readings and averaging reduces its effect on the final result.

A consistent error that shifts every reading in the same direction by about the same amount, such as a zero error on a balance or a mis-calibrated meter. Repeating readings does not remove it; the instrument must be corrected.

The mass of a substance per unit volume, calculated as mass divided by volume. It is measured in kilograms per cubic metre and helps identify materials and predict whether an object floats.

The straight-line distance moved from a starting point to a finishing point, together with the direction of that line. It is a vector, so it can be zero even when the distance travelled is large.

The rate of change of displacement with time, including direction. It is calculated as displacement divided by time taken and is measured in metres per second; it differs from speed because direction matters.

The rate at which velocity changes with time. An object accelerates when it speeds up, slows down or changes direction; the unit is metres per second squared.

Motion of an object under the influence of gravity alone, with air resistance ignored. Near the Earth's surface all free-falling objects accelerate downwards at about 9.8 metres per second squared, whatever their mass.

The tendency of an object to resist any change in its state of rest or uniform motion. It depends only on the object's mass: the greater the mass, the greater the inertia.

The single force that has the same effect as all the forces acting on an object combined. If it is zero the object stays at rest or moves at constant velocity; otherwise the object accelerates in its direction.

The product of an object's mass and its velocity, measured in kilogram metres per second. Momentum is a vector pointing in the direction of motion and measures how hard the object is to stop.

The product of a force and the time for which it acts, equal to the change in momentum it produces. Increasing the time of contact reduces the force needed for the same momentum change, as in crumple zones.

In a closed system with no external resultant force, the total momentum before a collision or explosion equals the total momentum afterwards. Momentum lost by one object is gained by another.

The energy transferred when a force moves an object along the direction of the force, calculated as force times distance moved in that direction. It is measured in joules.

The energy an object possesses because of its motion, calculated as one half times mass times speed squared. Doubling the speed gives four times the kinetic energy.

The energy stored in an object because of its height above a reference level in a gravitational field, calculated as mass times gravitational field strength times height. It is measured in joules.

The rate at which work is done or energy is transferred, calculated as energy transferred divided by time taken. The unit is the watt, where one watt equals one joule per second.

The fraction of energy supplied to a device that is transferred usefully, expressed as a percentage. It equals useful energy output divided by total energy input times one hundred; the rest is usually lost as heat.

The force acting normally per unit area of a surface. It is measured in pascals, where one pascal is one newton per square metre, and it increases when force rises or area falls.

The total kinetic and potential energy of all the particles in a substance. It increases when the substance is heated or when it changes state, even if the temperature stays constant during melting.

The energy needed to raise the temperature of one kilogram of a substance by one degree Celsius (or one kelvin). Water has a high value, about 4200 joules per kilogram per degree.

The energy needed to change the state of one kilogram of a substance at constant temperature, without any change in temperature. It is called latent heat of fusion for melting and vaporisation for boiling.

The transfer of heat through a material by particles passing on energy to neighbouring particles, without the material itself moving. Metals conduct well because free electrons carry energy quickly.

The number of complete waves or oscillations passing a point each second, measured in hertz. It is the reciprocal of the period and determines the pitch of a sound.

The distance between two neighbouring points in phase on a wave, such as from one crest to the next. It is measured in metres and relates to wave speed and frequency by speed equals frequency times wavelength.

The maximum displacement of a point on a wave from its rest position. For sound it determines loudness and for light it determines brightness; it is measured in metres.

The bending of a wave as it passes from one medium into another where its speed changes. Light bends towards the normal when it slows down and away from the normal when it speeds up.

The complete reflection of light inside a denser medium when it strikes the boundary at an angle greater than the critical angle. It only happens when light travels from a denser to a less dense medium.

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

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

The opposition of a component to the flow of electric current, calculated as potential difference divided by current. It is measured in ohms.

The current through a conductor is directly proportional to the potential difference across it, provided its temperature stays constant. The constant of proportionality is the resistance.

A region in which a magnetic force can act on a magnet, a current-carrying conductor or a moving charge. Field lines show its direction and strength, running from north to south outside a magnet.

The production of a potential difference in a conductor when the magnetic field through it changes, for example when a magnet is moved in or out of a coil. The induced current opposes the change that caused it.

Atoms of the same element with the same number of protons but different numbers of neutrons. Isotopes have identical chemical properties but may be stable or radioactive, and their masses affect relative atomic mass.

The time taken for half the nuclei in a sample of a radioactive isotope to decay, or for the activity to halve. It is constant for a given isotope and does not depend on temperature or pressure.

A particle made of two protons and two neutrons, identical to a helium nucleus, emitted during some radioactive decays. It is strongly ionising and has low penetrating power, stopped by paper or a few centimetres of air.