NSEP key terms

The straight-line vector from an object's starting point to its current point, with both size and direction. It differs from distance travelled, which counts the whole path length and has no direction.

The rate at which displacement changes with time. Average velocity divides total displacement by total time; instantaneous velocity is the limit of this ratio as the time interval shrinks toward zero.

The rate of change of velocity with time. Because velocity is a vector, acceleration occurs whenever speed changes, direction changes, or both change, such as a car turning at constant speed.

Motion of an object launched into the air that then moves only under gravity, with air resistance ignored. Horizontal and vertical motions are treated separately: horizontal velocity stays constant while vertical motion accelerates downward.

The velocity of one body as measured by an observer moving with another body. It equals the vector difference of their velocities, which is why rain seems to slant when you run through it.

The net force on a body equals the rate of change of its momentum, which reduces to force equals mass times acceleration when mass is constant. It links cause (force) with effect (acceleration).

The tendency of a body to resist changes in its state of rest or uniform motion. Mass is the quantitative measure of inertia, so a heavier object needs a larger force for the same acceleration.

The product of a force and the time interval over which it acts. It equals the change in momentum, which explains why airbags and bending your knees on landing soften the effect of a sudden stop.

The product of a body's mass and its velocity, a vector pointing along the velocity. In an isolated system the total momentum stays constant, forming the basis of collision analysis.

The net inward force that keeps a body moving in a circle at constant speed. It changes only the direction of velocity, not its magnitude, and equals mass times speed squared divided by radius.

The turning effect of a force about an axis, equal to force times perpendicular distance from the axis to the line of action. It changes rotational motion in the way force changes linear motion.

A body's resistance to change in its rotational motion, depending on mass and how that mass is distributed about the axis. Mass far from the axis raises it sharply, as with a long rod spun at its end.

The rate of change of angular displacement with time, usually measured in radians per second. It is related to linear speed along a circular path by speed equals radius times angular velocity.

The net work done on a body by all forces equals the change in its kinetic energy. It provides a shortcut for problems where forces vary or act over a known displacement.

The energy a body possesses because of its motion, equal to half its mass times the square of its speed. Doubling speed quadruples this energy, which is why stopping distances grow steeply.

Energy stored in a system because of the relative positions of its parts or its configuration. For gravity near Earth it equals mass times gravitational acceleration times height above a chosen reference level.

A force for which the work done moving between two points is independent of the path taken, so work around any closed loop is zero. Gravity and spring forces qualify; friction does not.

The minimum speed an object needs at a planet's surface to escape its gravity without further propulsion. It equals the square root of twice gravitational constant times planet mass divided by radius.

The speed a satellite needs to stay in a circular orbit at a given radius, equal to the square root of gravitational constant times central mass divided by that radius. It falls as the orbit grows larger.

The gravitational potential energy per unit mass at a point in a field, taken as zero at infinite distance. It is always negative near a mass and increases toward zero as you move away.

Oscillation in which the restoring force is directly proportional to displacement from equilibrium and directed opposite to it. Its displacement follows a sine or cosine curve with a fixed period independent of amplitude.

The large-amplitude response that occurs when a system is driven at a frequency close to its natural frequency. It underlies tuning circuits, musical instruments, and the collapse of bridges under rhythmic loading.

A wave pattern formed when two identical waves travel in opposite directions and superpose, producing fixed nodes and antinodes. It transfers no net energy and is set up in strings and pipes.

A relation stating that wave speed equals frequency multiplied by wavelength. It applies to sound, light, and waves on strings, and lets you find one quantity when the other two are known.

The apparent change in a wave's frequency when source and observer move relative to each other. Approaching motion raises the observed frequency; receding motion lowers it, as with a passing siren.

The periodic variation in loudness heard when two sound waves of slightly different frequencies superpose. Its rate equals the absolute difference between the two frequencies.

The heat energy absorbed or released by a substance during a change of state at constant temperature, such as melting or boiling. It breaks or forms bonds rather than changing molecular speed.

The heat needed to raise the temperature of one kilogram of a substance by one kelvin. Water's high value makes it a good coolant and moderates coastal climates.

The statement that the heat added to a system equals the increase in its internal energy plus the work it does on its surroundings. It is energy conservation applied to thermal processes.

A change in a system with no heat exchange with its surroundings, usually achieved by fast change or good insulation. In an ideal gas, compression raises temperature without any heat being added.

The electric 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 force per unit positive charge at a point in space, a vector pointing away from positive charge and toward negative charge. Field lines show its direction and relative strength.

The work done per unit positive charge in bringing a test charge from infinity to a point, measured in volts. Unlike the vector field, it is a scalar and can be added algebraically.

The ability of a conductor arrangement to store charge for a given potential difference, equal to charge divided by voltage. It depends on geometry and the dielectric between the plates, not on the charge itself.

For many conductors at constant temperature, current is directly proportional to potential difference across them. The constant of proportionality is resistance, measured in ohms.

An intrinsic property of a material describing how strongly it opposes current flow, equal to resistance times cross-sectional area divided by length. It depends on material and temperature, not on shape.

Two rules for circuits: the sum of currents entering a junction equals the sum leaving, and the sum of potential differences around any closed loop is zero. They rest on charge and energy conservation.

The production of an electromotive force in a conductor when the magnetic flux through it changes. The induced emf equals the negative rate of change of flux, a result known as Faraday's law.

A measure of the magnetic field passing through a surface, equal to field strength times area times the cosine of the angle between field and surface normal. It is measured in webers.

An induced current flows in a direction that opposes the change in magnetic flux producing it. This is the minus sign in Faraday's law and a direct consequence of energy conservation.

The ratio of light speed in vacuum to its speed in a medium, always at least one. It governs how much a ray bends at an interface according to Snell's law.

The angle of incidence beyond which light travelling from a denser to a rarer medium is totally internally reflected instead of refracted. Its sine equals the ratio of the two refractive indices.

The complete reflection of light back into a denser medium when it strikes the boundary at an angle greater than the critical angle. It carries light along optical fibres.

The superposition of two coherent waves producing regions of reinforced and cancelled intensity. Constructive interference occurs when path difference is a whole number of wavelengths.

The spreading of waves around obstacles and through narrow openings, becoming noticeable when the opening is comparable in size to the wavelength. It sets the resolution limit of optical instruments.

A fixed phase relationship between two waves that allows them to produce a stable interference pattern. Ordinary light sources are incoherent; lasers provide highly coherent light.

The emission of electrons from a metal surface when light of sufficiently high frequency falls on it. It shows light delivering energy in discrete packets, and below a threshold frequency nothing is emitted.

The minimum energy needed to remove an electron from a metal surface. In the photoelectric effect, it equals the photon energy minus the maximum kinetic energy of the emitted electron.

A discrete packet of electromagnetic energy whose energy equals Planck's constant times frequency. Photons have zero rest mass and travel at light speed in vacuum.

The time in which half the nuclei in a radioactive sample decay. It is fixed for each isotope and independent of the starting amount or of temperature and pressure.

The spontaneous transformation of an unstable nucleus into a more stable one, emitting alpha, beta, or gamma radiation. The rate depends only on the number of undecayed nuclei present.

The difference between the total mass of a nucleus's separate nucleons and the mass of the bound nucleus. It appears as binding energy according to mass-energy equivalence, and accounts for nuclear energy release.

A scalar is a quantity with magnitude only, such as mass or temperature. A vector has both magnitude and direction, such as force or displacement, and vectors add by the triangle rule.