Chapter 1: Introduction
1.2 Physical Quantities and Units
Authors: William Moebs, Samuel Ling, Jeff Sanny
Adapted by: Rob Pryce, Alix Blacklin
Learning Objectives
By the end of this section, you will be able to:
- Describe how SI base units are defined.
- Describe how derived units are created from base units.
- Express quantities given in SI units using metric prefixes
In order to communicate effectively about human movement we need a common framework for describe and measure movement. In biomechanics we’re primarily concerned with three basic properties related to movement. These are:
1) Mass: the amount of matter in an object; used to derive weight
2) Length: used to describe the distance between objects or points
3) Time: the duration between two or more events
These are referred to as physical quantities because they represent properties of an object or system that we can measure. We define physical quantities either by specifying how they are measured or by stating how they are calculated from other measurements. For example, we might define distance and time by specifying methods for measuring them, such as using a meter stick and a stopwatch. Then, we could calculate speed as distance divided by time.
Each of these physical quantities also needs a unit of measure – that is, a standardized way for representing the size or amount of the property. For example, the length of a running race which is a physical quantity, can be expressed in units of meters (for sprinters) or kilometers (for distance runners). Without these standardized units, it would be extremely difficult for people to communicate and compare measurements related to human movement (Figure 1.7)
In biomechanics we use one system to define physical quantities and units of measure: SI units (for the French Système International d’Unités), also known as the metric system. The SI system is also referred to as the kilogram-metres-second system (after the respective base units, see below). Virtually every country in the world now uses SI units as the standard and this is also the system agreed on by scientists and mathematicians in every field. So, being able to use SI units ensures that when we’re describing physical quantities in human movement – such as distance, speed, weight, or force – everyone in the field of biomechanics (across the entire world) knows what we’re talking about.
However, a common source of confusion (and consternation) when dealing with physical quantities is that we often find ourselves using non-SI units when communicating with the general public or other ‘non-biomechanics’ knowledge users. Most commonly, this comes in the form of English units (also known as the customary or imperial system). English units were historically used in nations once ruled by the British Empire and are still widely used in the United States and by many members of the general public (including even ourselves). For example, undoubtedly, all of us have discussed things like speed us