ORT 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 returning to the start gives zero displacement even after travelling a long distance.

The rate of change of displacement with respect to time, including direction. Average velocity divides total displacement by elapsed time; instantaneous velocity is the value at one moment.

The rate at which velocity changes with time, whether in magnitude, direction, or both. Its SI unit is metres per second squared, and it is a vector.

The net force on a body equals the product of its mass and acceleration, F = ma. It links the cause of motion change (force) to the resulting acceleration for a fixed mass.

A sketch showing a single object isolated with arrows representing every external force acting on it. Used to apply Newton's laws correctly by identifying the net force.

A contact force opposing relative motion or the tendency to move between surfaces. Static friction adjusts up to a maximum; kinetic friction acts while surfaces slide and is roughly constant.

The perpendicular contact force a surface exerts on an object resting against it. It balances the component of weight pressing into the surface and can be zero if contact is lost.

The product of an object's mass and velocity, a vector quantity measured in kilogram metres per second. Total momentum in a closed system is conserved in collisions and explosions.

The product of a force and the time for which it acts, equal to the change in momentum. Extending the contact time reduces the peak force for the same momentum change.

Energy cannot be created or destroyed, only transferred between stores or transformed between forms. In mechanics, the total of kinetic and potential energy stays constant when only conservative forces act.

The energy transferred by a force acting through a displacement, calculated as force times displacement times the cosine of the angle between them. Its SI unit is the joule.

The rate at which work is done or energy is transferred, measured in watts. It equals work divided by time, or force multiplied by velocity for a constant force.

Oscillation in which acceleration is proportional to displacement from equilibrium and directed towards it. Period is independent of amplitude, and displacement follows a sinusoidal pattern.

A large-amplitude response occurring when a system is driven at its natural frequency. Damping limits the maximum amplitude and shifts it slightly below the undamped natural frequency.

The total kinetic and potential energy of the particles within a substance, arising from their random motion and interactions. It changes with temperature, phase, and the work done on or by the system.

The change in internal energy of a system equals the heat added to it minus the work it does on its surroundings. It is the energy-conservation principle applied to thermal systems.

The energy needed to raise the temperature of one kilogram of a substance by one kelvin, measured in joules per kilogram per kelvin. It is used in calculations such as Q = mcΔT.

The energy required to change the phase of one kilogram of a substance at constant temperature, without changing its temperature. Melting and boiling each have their own value.

The equation pV = nRT relating pressure, volume, amount in moles, and absolute temperature of an ideal gas. It combines Boyle's, Charles's, and Avogadro's laws into a single relationship.

A model explaining gas pressure and temperature through the random motion of particles colliding with container walls. Pressure arises from the rate of momentum transfer per unit area.

The lowest possible temperature, 0 K or about −273.15 °C, at which particles have minimum kinetic energy. Temperatures must be converted to kelvin for gas-law calculations.

A change in which no heat enters or leaves the system, so any work done changes the internal energy. Rapid compression or expansion of a gas approximates this condition.

The force per unit positive charge experienced at a point in an electric field, a vector measured in newtons per coulomb. Between parallel plates it equals voltage divided by 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. It 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 field strength is its negative gradient.

The energy transferred per unit charge between two points in a circuit, measured in volts. It drives current through components and equals the work done per coulomb.

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

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

The opposition to current flow through a component, equal to voltage across it divided by current through it. It is measured in ohms and depends on material, length, and cross-sectional area.

A measure of the strength and direction of a magnetic field, defined by the force on a current-carrying wire per unit current per unit length. Its SI unit is the tesla.

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

The direction of an induced current is such that its magnetic effect opposes the change producing it. This is a consequence of energy conservation and explains the minus sign in Faraday's law.

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 via Snell's law.

The complete reflection of light at a boundary when it travels from a denser to a less dense medium beyond the critical angle. It underpins optical fibres and prisms in periscopes.

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

The superposition of waves from two or more sources producing regions of reinforcement and cancellation. Constructive interference needs a path difference of whole wavelengths; destructive needs odd half-wavelengths.

A property of waves having a constant phase difference, enabling stable observable interference patterns. Lasers provide coherent light; separate lamps generally do not.

The emission of electrons from a metal surface when light above a threshold frequency strikes it. It demonstrates the particle nature of light, with maximum kinetic energy depending on frequency, not intensity.

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

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

The time for half the nuclei in a radioactive sample to decay. It is constant for a given isotope and allows activity to be predicted over time.

The spontaneous transformation of an unstable nucleus emitting alpha, beta, or gamma radiation to become more stable. It is random and unaffected by temperature or chemical state.

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

The energy required to separate all nucleons in a nucleus completely, equivalent to the mass defect times the speed of light squared. It reflects nuclear stability per nucleon.

The difference between the total mass of separate nucleons and the mass of the assembled nucleus. It appears as binding energy according to E = mc².

The range within which a measured or calculated value is expected to lie, often expressed as a plus-or-minus interval. It is not the same as a mistake and should be quoted with every result.

How close repeated measurements are to each other, reflecting the spread of results. A precise set can still be inaccurate if affected by a systematic error.

How close a measured value is to the true value, limited mainly by systematic errors. It is distinct from precision, which concerns the consistency of repeated readings.

A consistent deviation in one direction caused by a fault in apparatus or method, such as a zero error or calibration offset. Repeating readings does not reduce it; the method must be corrected.

Unpredictable variation in readings in both directions caused by fluctuating conditions or judgement limits. Averaging repeated readings reduces its effect on the result.

The slope of a graph line, calculated as the change in the vertical quantity divided by the change in the horizontal quantity. It often represents a physical constant in linearised plots.

A straight line drawn through data points to minimise the total distance of points from the line. Its gradient and intercept are used to extract physical quantities.