Introduction to Sports Biomechanics: Analysing Human Movement ...

Figure 5.1 Directional quality of force.
Weight
CAUSES OF MOVEMENT – FORCES AND TORQUES
moment of force or a turning effect, is then introduced; this is an effect tending to
rotate the object (see below). A quantitative analyst should exercise care when solving
systems of forces graphically and would usually adopt a vector approach (see Appendix
4.1).
The SI unit of force is the newton (N) and the symbol for a force vector is F. One
newton is the force that when applied to a mass of one kilogram (1 kg), causes that
mass to accelerate at 1 m/s 2 in the direction of the force application. A sports performer
experiences forces both internal to and external to the body. Internal forces are
generated by the muscles and transmitted by tendons, bones, ligaments and cartilage;
these will be considered in Chapter 6. The main external forces, the combined effect of
which determines the overall motion of the body, are as follows.
Weight is a familiar force (Figure 5.1) attributable to the gravitational pull of the Earth.
It acts vertically downwards through the centre of gravity of an object towards the
centre of the Earth. The centre of gravity (G in Figure 5.1) is an imaginary point at
which the weight of an object can be considered to act. For the human performer, there
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