AAPT NCPT (Level 1) key terms

A vector quantity giving the straight-line change in position of an object from its starting point to its ending point, including both distance and direction, regardless of the path actually traveled.

The rate at which an object's displacement changes with respect to time. It is a vector, so it carries both a magnitude (speed) and a direction of motion.

The rate at which velocity changes with time, whether the speed is increasing, decreasing, or the direction of motion is turning. It is a vector measured in meters per second squared.

The net force acting on an object equals the product of its mass and its acceleration. It links cause (force) to effect (acceleration) and shows that heavier objects need larger forces for the same acceleration.

The tendency of an object to resist changes in its state of motion. An object at rest stays at rest and a moving object keeps its velocity unless a net external force acts on it.

The product of an object's mass and its velocity, describing how hard it is to stop the object. In an isolated system the total momentum stays constant during collisions.

The product of an average force and the time interval over which it acts. It equals the change in an object's momentum, so a longer contact time reduces the required force.

The energy transferred to or from an object when a force acts on it while it moves. It equals force times displacement times the cosine of the angle between them.

The energy an object possesses because of its motion, equal to one-half the mass times the square of its speed. It is always non-negative and doubles when speed doubles squared.

The principle that energy can change form but cannot be created or destroyed in an isolated system. The total mechanical energy stays constant when only conservative forces act.

The net force directed toward the center of a circular path that keeps an object moving in that circle. It changes the direction of velocity, not the speed, and equals mass times speed squared over radius.

A measure of how effectively a force causes an object to rotate about an axis. It equals the force times the perpendicular distance from the axis to the line of action of the force.

A measure of rotational motion equal to the product of an object's moment of inertia and its angular velocity. It remains constant when no net external torque acts on the system.

A rotational analog of mass that measures how strongly an object resists changes in its rotation. It depends on the object's mass and on how that mass is distributed about the axis.

A periodic back-and-forth oscillation in which the restoring force is directly proportional to displacement from equilibrium and points toward it. Its period is independent of amplitude for an ideal spring.

The condition in which two or more objects in thermal contact have reached the same temperature, so no net heat flows between them. It is the basis for reading temperature with a thermometer.

A measure of the average random kinetic energy of the particles in a substance. It determines the direction in which heat flows: from higher temperature toward lower temperature.

Energy transferred between objects because of a temperature difference. It is not the energy stored in an object but the energy in transit from hot to cold.

The amount of heat needed to raise the temperature of one kilogram of a substance by one kelvin. Materials with high values warm up slowly because they absorb more energy per degree.

The energy absorbed or released when a substance changes phase at constant temperature, such as melting or boiling. It breaks or forms bonds between particles rather than changing their average speed.

An equation relating pressure, volume, absolute temperature, and amount of gas: pressure times volume equals the number of moles times the gas constant times temperature. It describes gases in the dilute limit.

A model explaining gas properties by treating particles as tiny, randomly moving bodies that collide elastically with each other and the container walls. It connects pressure and temperature to molecular motion.

A statement of energy conservation for thermodynamic systems: the heat added to a system equals the change in its internal energy plus the work it does on its surroundings.

The total kinetic and potential energy of all particles making up a system, excluding overall motion and position of the system itself. Its change depends on heat added and work done.

The principle that the total entropy of an isolated system never decreases and that heat spontaneously flows only from hot to cold. It sets limits on the efficiency of heat engines.

A measure of the number of microscopic arrangements consistent with a system's macroscopic state, often described as disorder. It increases in any spontaneous process occurring in an isolated system.

The distance between two consecutive corresponding points on a wave, such as crest to crest. It relates to frequency and wave speed by the wave equation.

The number of complete oscillations or wave cycles passing a point each second, measured in hertz. It is the inverse of the period and stays fixed when light changes medium.

The time required for one complete cycle of a repeating motion or wave to pass a fixed point. It equals the reciprocal of frequency and is measured in seconds.

The bending of a wave as it passes from one medium into another where its speed differs. The change in direction follows Snell's law and arises from the change in wave speed.

The spreading of waves around obstacles and through openings, most noticeable when the opening is comparable in size to the wavelength. It shows that light behaves as a wave.

The combination of two or more waves overlapping to produce a resultant wave. Constructive interference adds amplitudes, while destructive interference cancels them when waves meet out of phase.

A condition in which a system is driven at one of its natural frequencies, producing a large-amplitude response. It occurs because each driving push adds energy in step with the motion.

A vector field that describes the force per unit charge experienced by a test charge placed at a point in space. It points away from positive charge and toward negative charge.

The rule that the electric force between two point charges is proportional to the product of the charges and inversely proportional to the square of the distance between them, acting along the joining line.

The electric potential energy per unit charge at a point in an electric field, measured in volts. It is a scalar quantity, so potentials add as ordinary numbers rather than vectors.

The ability of a device to store electric charge for a given potential difference, equal to charge divided by voltage. It depends on the geometry of the conductors and the material between them.

A vector field produced by moving charges and magnets that exerts forces on other moving charges and on magnetic materials. Its direction is the direction a compass north pole points.

The generation of an electromotive force in a conductor when the magnetic flux through it changes. It underlies generators and transformers and is described by Faraday's law.

The relationship stating that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance, valid for ohmic materials at constant temperature.

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

A discrete packet of electromagnetic energy whose energy equals Planck's constant times the frequency of the light. Photons have no rest mass and travel at the speed of light.

The minimum energy needed to remove an electron from the surface of a particular metal. In the photoelectric effect, light below the corresponding threshold frequency ejects no electrons.

The time required for half of the radioactive nuclei in a sample to decay. After each half-life the remaining amount is halved, giving exponential decay independent of the sample size.

The spontaneous transformation of an unstable nucleus into a more stable one by emitting particles or radiation. It is random for individual nuclei but predictable for large samples.

The principle that mass and energy are interchangeable, expressed as energy equals mass times the speed of light squared. It explains the energy released in nuclear reactions and particle annihilation.

The rule that certain paired quantities, such as position and momentum, cannot both be known with unlimited precision at the same time. The product of their uncertainties is bounded below by Planck's constant over four pi.

The digits in a measured value that carry meaningful information about its precision, including all certain digits plus one estimated digit. They determine how precisely a final answer should be reported.

A repeatable inaccuracy that shifts measurements consistently in one direction, caused by calibration faults, faulty equipment, or flawed procedure. It does not average out with repeated trials.

Unpredictable variation in measurements that scatters results above and below the true value, caused by fluctuations in conditions or reading. Averaging many trials reduces its effect.

The absolute difference between an experimental value and an accepted value, divided by the accepted value and multiplied by one hundred. It expresses accuracy as a percentage.

A statistical method that fits the straight line best describing the relationship between two measured variables. The slope and intercept of that line often reveal the physical constant being tested.