Physics key terms

A vector giving the straight-line change in position from a starting point to an ending point, including direction. Unlike distance travelled, it depends only on the two endpoints, not on the path taken.

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 can be negative when opposing motion.

The net force on a body equals its mass multiplied by its acceleration. Written as F = ma, it links cause (net force) to effect (acceleration) and underlies nearly all dynamics problems.

A body's resistance to any change in its state of motion, quantified by its mass. An object at rest stays at rest and a moving object continues at constant velocity unless a net external force acts.

A sketch showing one object in isolation with every external force drawn as an arrow acting on it, labelled by source. Resolving these arrows into components is the standard first step in force problems.

The energy transferred to or from a body when a force acts through a displacement, calculated as force times displacement times the cosine of the angle between them. It is a scalar measured in joules.

A force for which the work done moving between two points is independent of the path taken, so a potential energy can be defined. Gravity and spring forces are conservative; friction is not.

The product of a body's mass and velocity, a vector measured in kilogram metres per second. In an isolated system the total momentum before and after any interaction is unchanged.

The product of a net force and the time interval over which it acts, equal to the change in momentum it produces. Extending the contact time reduces the peak force for a given momentum change.

A collision in which both total momentum and total kinetic energy are conserved. Billiard-ball impacts approximate this; most real collisions are partly inelastic and lose kinetic energy to heat or deformation.

The acceleration of a body moving on a curved path, directed toward the centre of curvature, with magnitude v squared divided by radius. It changes direction of velocity, not speed.

The turning effect of a force about an axis, equal to force times perpendicular distance from the axis to the line of action. It is the rotational analogue of force and is measured in newton metres.

A body's resistance to angular acceleration about a chosen axis, equal to the sum of each mass element times its squared distance from the axis. It depends on mass distribution, not just total mass.

The rate of change of angular displacement with time, usually in radians per second. It is a vector along the rotation axis and relates linear speed to radius by v equals omega times r.

The gravitational force per unit mass experienced at a point in space, measured in newtons per kilogram. Near Earth's surface it is about 9.8 N/kg, numerically equal to the free-fall acceleration.

Energy stored in a system because of the relative positions of its parts or because of its configuration, such as height in a gravitational field or extension of a spring. It converts to kinetic energy as the system relaxes.

The energy a body possesses because of its motion, equal to one half its mass times the square of its speed. Doubling speed quadruples kinetic energy, which is a scalar in joules.

The electrostatic force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation. Like charges repel, opposite charges attract, and the force acts along the joining line.

A vector region around a charge where another charge experiences a force, defined as force per unit positive charge at a point. Field lines point away from positive charge and toward negative charge.

The electric potential energy per unit positive charge at a point in an electric field, measured in volts. Potential difference between two points drives current through a conductor.

The charge stored per unit potential difference across a capacitor, measured in farads. It depends on plate area, plate separation, and the dielectric between the plates, not on the applied voltage.

For a conductor at constant temperature, current is directly proportional to potential difference across it, with resistance as the constant of proportionality. It defines resistance but does not hold for all devices.

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

The product of magnetic flux density and the area perpendicular to the field through a surface, measured in webers. Changing flux through a loop is what induces an electromotive force.

The induced electromotive force in a circuit equals the negative rate of change of magnetic flux through it. The minus sign, Lenz's law, means induced effects oppose the change that produced them.

The distance between successive points in phase on a wave, such as crest to crest, measured in metres. It equals wave speed divided by frequency and determines interference patterns.

The number of complete oscillations or wave cycles passing a point per second, measured in hertz. For a wave it is set by the source and stays constant when the wave enters a new medium.

The ratio of the speed of light in vacuum to its speed in a medium, a dimensionless number always at least one. It governs how much a ray bends at a boundary by Snell's law.

The complete reflection of a ray inside a medium when it strikes a boundary beyond the critical angle, so no light refracts out. It requires travel from a denser to a less dense medium.

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

The superposition of two or more waves to give a resultant displacement. Waves arriving in phase reinforce (constructive), while those arriving out of phase cancel (destructive), producing fringes or beats.

Temperature measured on the Kelvin scale, starting at absolute zero where particle thermal motion is minimal. It must be used in gas laws and thermodynamic efficiency formulas because ratios require a true zero.

The total kinetic and potential energy of the particles making up a system, excluding bulk motion and external position. For an ideal gas it depends only on temperature.

The equation of state relating pressure, volume, amount, and absolute temperature of a gas: pressure times volume equals moles times the gas constant times temperature. It assumes negligible particle volume and no intermolecular forces.

The change in internal energy of a system equals heat added to it minus work done by it. It is conservation of energy applied to thermal processes, with careful sign conventions for heat and work.

A change in a system with no heat exchange with its surroundings, often achieved by fast change or good insulation. Work done comes entirely from internal energy, so the gas cools on expansion.

The emission of electrons from a metal surface when light above a threshold frequency strikes it. It shows light delivers energy in discrete quanta, since intensity alone cannot free electrons below threshold.

A quantum of electromagnetic radiation carrying energy equal to Planck's constant times frequency. Photons have no rest mass, travel at light speed, and their energy depends only on frequency.

The minimum energy needed to remove an electron from the surface of a particular metal. In the photoelectric effect, photon energy must exceed it for emission to occur.

The time for half the nuclei in a radioactive sample to decay, independent of the starting amount. After n half-lives a fraction one over two to the n remains.

The difference between the total mass of a nucleus's separate nucleons and the mass of the bound nucleus. It equals the binding energy divided by the square of the speed of light.

The digits in a measured value that carry meaningful information about precision, including one estimated digit. Results of calculations should be rounded to match the least precise input.

An estimate of the range within which a measured value probably lies, often quoted as plus or minus a value. It may be absolute in the same units or fractional as a percentage of the reading.

A consistent offset in measurements caused by faulty instruments, zero errors, or flawed method, shifting all readings in the same direction. It is not reduced by averaging repeated readings.