CSET Science Physics key terms

The straight-line distance and direction of a point from its starting position, treated as a vector quantity. It differs from distance travelled, which is a scalar and ignores direction.

The rate of change of displacement with respect to time, a vector quantity with both magnitude and direction. Average velocity divides total displacement by total time taken.

The rate at which velocity changes with time, including changes in speed, direction, or both. It is measured in metres per second squared and is a vector.

The resultant force on a body equals the rate of change of its momentum; for constant mass this reduces to force equals mass times acceleration, measured in newtons.

The product of a body's mass and its velocity, a vector measured in kilogram metre per second. For a chosen system, total momentum is conserved when the net external impulse is zero; closure to matter alone does not establish isolation from force.

The time integral of resultant force, or average resultant force times duration, equal to the change in momentum of the chosen body or system. It is measured in newton seconds and explains why airbags reduce injury.

The inward radial component of the resultant force for circular motion; in nonuniform motion a tangential resultant component can also be present. It has magnitude mv squared divided by radius r.

The rate of change of angular displacement, measured in radians per second. It links linear speed to circular motion through the relation speed equals radius times angular velocity.

The turning effect of a force about a pivot, equal to the force multiplied by the perpendicular distance from the pivot to its line of action. Measured in newton metres.

For a body in rotational equilibrium, the sum of clockwise moments about any point equals the sum of anticlockwise moments about that same point.

The single point through which the entire weight of a body may be considered to act, whatever its orientation. Its position affects stability and toppling.

The ratio of the speed of light in a vacuum to the speed of light in a medium, For light incident from vacuum or approximately air, it also equals sin(incidence)/sin(refraction); for two general media that ratio is their relative refractive index.

The angle of incidence in the denser medium for which the angle of refraction in the less dense medium is exactly ninety degrees. Beyond it total internal reflection occurs.

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

The spreading of waves as they pass through an aperture or around an obstacle, becoming more pronounced when the wavelength is comparable to the gap or obstacle size.

The superposition of two or more waves to produce regions of reinforced and cancelled displacement. Constructive interference gives maxima; destructive interference gives minima.

Two wave sources that maintain a constant phase difference and have the same frequency, so that a stable interference pattern can be observed. Lasers provide a common example.

A wave pattern formed when two waves of equal frequency and amplitude travel in opposite directions, producing fixed nodes and antinodes. Energy is not transported along the medium.

The large-amplitude oscillation that occurs when a system is driven at a frequency matching one of its natural frequencies. It is used in tuning circuits and musical instruments.

The minimum energy needed to remove an electron from the surface of a metal. It is a property of the metal and appears in the photoelectric equation.

The emission of electrons from a metal surface when light of frequency above a threshold falls on it. It demonstrates the particle nature of light.

The lowest frequency of incident radiation that can just cause photoelectric emission from a given metal surface. Below it, the ordinary weak-illumination single-photon model predicts no emitted photoelectrons.

The time taken for half the nuclei in a given radioactive sample to decay, or for the activity to fall to half its initial value. It is constant for a given isotope.

The probability per unit time that an individual nucleus will decay. It is related to half-life by the expression lambda equals natural log of two divided by half-life.

The number of nuclear disintegrations occurring per second in a radioactive sample, measured in becquerels. It decreases exponentially with time according to the decay constant.

The difference between the total mass of the separate nucleons in a nucleus and the mass of the assembled nucleus. It appears as binding energy according to mass-energy equivalence.

The energy required to separate a nucleus completely into its individual protons and neutrons. It equals the mass defect multiplied by the square of the speed of light.

A consistent deviation of measurements from the true value in one direction, caused by faulty apparatus or flawed method. It affects accuracy but not necessarily precision.

Unpredictable variation in measurements about a mean value, caused by fluctuating conditions or limited resolution. It reduces precision and can be lessened by averaging repeated readings.

How close a measured value is to the accepted true value. It is affected mainly by systematic error, whereas precision concerns the spread of repeated readings.