Introduction

We experience the effects of force in everyday life and have an intuitive notion of force. For example, we exert a force on our body when we lift or push an object, while we continuously (fortunately) feel the effect of gravitational forces, for instance while sitting, walking, etc. All parts of the human body in one way or the other are loaded by forces. Our bones provide rigidity to the body and can sustain high loads. The skin is resistant to force: simply pull on the skin to witness this. The cardiovascular system is continuously loaded dynamically owing to the pulsating blood pressure. The bladder is loaded and stretched when it fills up. The intervertebral discs serve as flexible force-transmitting media that give the spine its flexibility. Beside force, we are using levers all the time in our daily life to increase the ‘force’ that we want to apply to some object, for example by opening doors with the latch, opening a bottle with a bottle-opener. We feel the effect of a lever arm when holding a weight close to our body instead of using a stretched arm. These experiences are the result of the moment that can be exerted by a force. Understanding the impact of force and moment on the human body requires us to formalize the intuitive notion of force and moment. That is the objective of this chapter.

Definition of a Force Vector

Imagine pulling on a thin wire that is attached to a wall. The pulling force exerted on the point of application is a vector with a physical meaning: it has

The graphical representation of a force vector, denoted by, is given in Fig. 2.1. The ‘shaft’ of the arrow indicates the orientation in space of the force vector. The point of application of the force vector is denoted by the point P.