MATSEC SEC Physics key terms

A physical quantity that has magnitude only and no direction, such as mass, time, speed or energy. Scalars are added using ordinary arithmetic.

A physical quantity that has both magnitude and direction, such as displacement, velocity, acceleration or force. Vectors must be combined using a scale diagram or by resolving into components.

The distance travelled by an object per unit time, a scalar measured in metres per second. Average speed equals total distance divided by total time taken.

The rate of change of displacement with time, a vector measured in metres per second. Its direction matters: a car going round a bend at steady speed still changes velocity.

The rate at which velocity changes with time, measured in metres per second squared. It occurs when speed changes, direction changes, or both, and can be negative when slowing down.

The single force that has the same effect as all the forces acting on a body combined. A non-zero resultant causes acceleration in its direction; zero resultant means balanced forces.

The resultant force on a body equals its mass multiplied by its acceleration (F = ma). The acceleration is always in the same direction as the resultant force.

A contact force that opposes relative motion between two surfaces. It transfers kinetic energy to thermal energy, and without it vehicles could not start, steer or stop.

The turning effect of a force about a pivot, calculated as force multiplied by the perpendicular distance from the pivot. Measured in newton metres.

For a body in rotational equilibrium, the total clockwise moment about a point equals the total anticlockwise moment about the same point. Used to solve balance-beam and lever problems.

The point at which the whole weight of a body may be considered to act. A suspended object hangs with this point directly below the suspension point.

The energy transferred when a force moves an object through a distance in the direction of the force, calculated as force times distance. Measured in joules.

The rate at which energy is transferred or work is done, equal to energy transferred divided by time taken. Measured in watts, where one watt is one joule per second.

The energy a body possesses because of its motion, calculated as one half of mass times speed squared. Doubling the speed quadruples the kinetic energy.

The energy stored by an object because of its height above a chosen reference level, calculated as mass times gravitational field strength times height. It increases as the object is raised.

Energy cannot be created or destroyed, only transferred between stores or transformed from one form to another. The total energy of a closed system stays constant.

The proportion of energy supplied to a device that is transferred usefully, often expressed as a percentage. Calculated as useful output energy divided by total input energy.

A wave in which the oscillations are perpendicular to the direction of energy travel, as in light and water waves. It can show polarisation.

A wave in which the oscillations are parallel to the direction of energy travel, producing compressions and rarefactions. Sound in air is the standard example.

The number of complete oscillations or waves passing a point each second, measured in hertz. It equals one divided by the period and determines the pitch of a sound.

The distance between two consecutive corresponding points on a wave, such as crest to crest, measured in metres. It is related to frequency and wave speed by the wave equation.

The distance travelled by a wave crest per second, equal to frequency multiplied by wavelength. In a given medium it stays constant even if frequency changes.

The change in direction of a wave as it crosses a boundary between media where its speed changes. Light bending on entering glass or water is a familiar example.

The ratio of the speed of light in a vacuum to its speed in a given medium. It also equals the sine of the angle of incidence divided by the sine of the angle of refraction.

The complete reflection of light inside a denser medium when the angle of incidence exceeds the critical angle. It is the principle behind optical fibres and some prisms.

The spreading of waves as they pass through a gap or around an obstacle. The effect is most noticeable when the gap width is similar to the wavelength.

The energy needed to raise the temperature of one kilogram of a substance by one degree Celsius. Water's high value explains its use as a coolant.

The energy required to change the state of one kilogram of a substance without changing its temperature. Latent heat of fusion applies to melting; vaporisation applies to boiling.

Energy transferred between bodies because of a temperature difference, always flowing from hotter to cooler regions. It can occur by conduction, convection or radiation.

A measure of the average kinetic energy of the particles in a substance. It determines the direction of thermal energy transfer and is measured in degrees Celsius or kelvin.

The current through a metallic conductor is directly proportional to the potential difference across it, provided temperature stays constant. Expressed as V = IR.

The rate of flow of electric charge past a point in a circuit, measured in amperes. One ampere is one coulomb of charge passing per second.

The energy transferred per unit charge between two points in a circuit, measured in volts. One volt is one joule of energy per coulomb of charge.

A measure of how strongly a component opposes current, calculated as potential difference divided by current. Measured in ohms; it increases with length and temperature for a wire.

The generation of a potential difference in a conductor when it experiences a changing magnetic field, or when it moves through a magnetic field. It underpins generators and transformers.

The mass of a substance per unit volume, calculated as mass divided by volume and measured in kilograms per cubic metre. It explains why some objects float and others sink.

The force acting normally per unit area of a surface, measured in pascals. One pascal is one newton per square metre; pressure in fluids increases with depth.

The extension of a spring is directly proportional to the force applied, provided the elastic limit is not exceeded. The constant of proportionality is the spring constant.

The factor the experimenter deliberately changes or selects in an investigation, plotted on the horizontal axis. All other variables should be controlled to give a fair test.

The quantity that is measured in an experiment and expected to change in response to the independent variable. It is plotted on the vertical axis of a graph.

A factor that is kept constant throughout an experiment so that it cannot affect the dependent variable. Controlling variables makes a comparison fair and the conclusion trustworthy.

The extent to which repeated measurements give consistent results. It is improved by taking repeats, discarding anomalies and calculating a mean, not simply by using better equipment.

How close a measured value is to the true value. It is affected by systematic errors, such as a zero error or an uncalibrated instrument, rather than by random scatter.

How closely repeated measurements agree with each other, reflecting the size of random scatter. A precise set of readings may still be inaccurate if a systematic error is present.