Fiducial mark

An object placed in the field of view for an observer to use as a reference point.

Anomalous value

An anomalous value in a set of results is one that does not fit the overall trend in the data. it is therefore discounted from any analysis.

Random error

Random errors give values that are scattered randomly above and below the true value when the measurement is repeated. Taking a mean of repeated values obtains a better result.

Accuracy

Accuracy is a measure of the closeness of agreement between an individual test result and the true value. If a test result is accurate, it is in close agreement with the true value.

Error

Error (of measurement) is the difference between an individual measurement and the true value (or accepted reference value) of the quantity being measured.

Precision

Precision is the closeness of agreement between independent measurements obtained under the same conditions. It depends only on the distribution of random errors (i.e. the spread of measurements) and does not relate to the true value.

Repeatability

Repeatability is the precision obtained when measurement results are produced over a short timescale by one person (or the same group) using the same equipment in the same place.

Reproducibility

Reproducibility is the precision obtained when measurement results are produced over a wider timescale by different people using equivalent equipment in different (but equivalent) places.

Resolution

Resolution is the smallest change in the quantity being measured that can be detected by an instrument.

Uncertainty

Uncertainty is an estimate attached to a measurement which characterises the range of values within which the true value is asserted to lie. This is normally expressed as a range of values such as 44.0 ± 0.4.

Valididty

Validity can apply to an individual measurement or a whole investigation. A measurement is valid if it measures what it is supposed to be measuring. An investigative procedure is valid if it is suitable to answer the question being asked.

Systematic error

A systematic error is one that doesn't happen by chance, but as a result of an inaccuracy in the apparatus or its use by the person using it. This type of error tends to shift all the results in the same direction.

Zero error

A zero error is a type of systematic error caused by an instrument not being properly calibrated or adjusted. It then gives a non-zero value when the value should be zero.

Percentage uncertainty

Percentage uncertainty is the uncertainty divided by the measured value and expressed as a percentage.

Percentage difference

Percentage difference is the difference between two values divided by the mean and expressed as a percentage.

Scalar

A scalar quantity is one which has magnitude but not direction.

Vector

A vector quantity is one which has both magnitude and direction.

Vector triangle

A vector triangle is a type of scale diagram with two vectors drawn tip-to-tail, to show how they can be added together.

Resultant vector

The vector sum of two or more vectors.

The components of a vector

The components of a vector are the parts of a vector in two perpendicular directions.

One Newton

One Newton is the force needed to give a mass of 1 kg an acceleration of 1 metre per second squared.

Equilibrium

Objects are in equilibrium when all the forces acting on them in the same plane (coplanar forces) are balanced - there is zero net or resultant force.

The moment of a force

The moment of a force (or turning moment) is the product of the force and the perpendicular distance of its line of action from the pivot or axis.

The principle of moments

The principle of moments states that for an object to be in rotational equilibrium, the sum of the clockwise moments must equal the sum of the anti-clockwise moments.

Torque

Torque is the moment of a couple. The torque of a couple is the product of one of the forces and the perpendicular distance between them.

Centre of mass

The centre of mass of an object is the single point at which all of the mass of the object can be assumed to be situated.

Density

The density of an object is its mass per unit volume.

Archimedes' principle

Archimedes' principle states that the upward buoyant force (upthrust) exerted on an object immersed in a fluid is equal to the weight of the fluid displaced.

Work done

The work done, or energy transferred, is the product of the force and the distance moved by the force in the direction of movement.

A closed system.

A closed system is any system in which all the energy transfers are accounted for.

Energy or matter cannot enter or leave a closed system.