What is force?

Defining Force

Forces arise whenever objects interact with each another. This interaction may be one where the relationship between the objects is one of contact or non-contact. From this simple description it can be seen that forces are present at all times, and an understanding of force is fundamental to an understanding of motion. The essential significance of force may be summarised as follows:

A net force is required to change the state of motion of an object. Such a change occurs when there is:

1. an increase in the speed of the object
2. a decrease in the speed of the object
3. a change in direction of the object independent of, or in association with, the changes in speed.

Note that these three points can be summarised as a 'change of velocity'.

In order to examine this change in more detail it is necessary to consider the nature of the interactions between objects and the associated forces.

Forces can be classified in a number of ways but the classification into contact and non-contact is the one adopted here. Under this classification forces may be considered to be grouped as follows.

 

Non contact	Contact
Gravity	Joint reaction forces
	Muscle forces
	Fluid forces
	Elastic forces
	Ground reaction forces

Of the forces identified above those considered in this section are gravity and ground reaction forces. Whilst this means an analysis of motion may be incomplete, the use of these two categories will enable a good understanding of many sports movements to be achieved.

Before proceeding to a further explanation of these forces, it is necessary to indicate that the examples used in this section to support the explanation of force, involve situations where forces should be considered to be external to the 'system of interest'. Whether a force is regarded as 'external' or 'internal' entirely depends on the object or system that is the focus of interest. For example if the 'object' or 'system of interest' is the sports performer (that is, the whole body) then forces such as muscle forces and joint forces which arise within the body are 'internal' and forces such as gravity and ground reaction forces are 'external'.

However, in another investigation it might be appropriate to consider an individual segment of the body to be the 'system of interest'. In such a case muscle forces and joint forces (as well as gravity and ground reaction forces) will be considered to be 'external' to the system.

Although this might seem a rather complicated set of terms it is essential to note that it is only those forces that are defined as 'external' to the 'system of interest' that can cause a net change in the state of motion of that system. As the system of interest in the vertical jump is the athlete him or herself then if it assumed that air resistance is negligible, then as noted above a good explanation of the vertical jump motion can be gained by considering just two forces - gravity and ground reaction forces.