Quantities & Units in Mechanics

There are a number of quantities that you will use regularly in Mechanics. Each quantity has its own unit of measurement and this depends on what it is measuring. They can also be scalar quantities (single numbers) or vector quantities (see vectors). The following provides a list of most common quantities seen in Mechanics and their corresponding SI units of measurement. SI units is short for ‘International System of Units’ (the SI are round the other way as it was originally a French translation).

Quantities in Mechanics

Length

Length is a scalar quantity that measures a size, distance or displacement. The SI unit of measurement is the metre (m).

Mass

Mass is a scalar quantity that measures how much of a particular material is present. The SI unit of measurement is the kilogram (kg).

Velocity

Velocity is a vector quantity that measures the speed of an object in a given direction – both of which are required to define a velocity. The SI unit of velocity is metres per second (ms^{-1} or m/s). Note that the magnitude of velocity is speed – see magnitude of a vector.

Acceleration

Acceleration is a vector quantity that measures change in velocity. The SI unit of velocity is metres per second per second (ms^{-2} or m/s/s).

Force

Force is a vector quantity that measures the effect of various influences such as friction, tension, gravity, etc. The SI unit of force is the Newton (N). 1 Newton is equivalent to 1 kilogram metre per second squared, i.e. 1N=1kg ms^{-2}.

Weight

Weight is a vector quantity that measures the force associated with an object’s mass: this is dependent on the gravitational field, for example, our weight on Earth is larger than that on the moon despite our mass being consistent. Although we tend to think we are measuring our weight in kilograms, in actual fact, we are measuring our mass and the SI unit of weight in actually Newtons (N).

Moments (A2 Maths)

Moments measure the turning effect of a force acting on a rigid body. Moments at a point are calculated by multiplying the force by its perpendicular distance and so the SI unit for moments is Newton metres (Nm)

AS Mechanics Quantities & Units in Mechanics