Introduction to Sports Biomechanics: Analysing Human Movement ...

Figure 5.3 (a) Ground reaction force and (b) its components.
CAUSES OF MOVEMENT – FORCES AND TORQUES
or traction, force. Traction is the term used when the force is generated by interlocking
of the contacting objects, such as spikes penetrating a Tartan track; this interaction
between objects is known as form locking. In friction, the force is generated by
force locking, in which no surface penetration occurs. Without friction or traction,
movement in sport would be very difficult.
If an object, such as a training shoe (Figure 5.4(a)), is placed on a sports surface
material such as Tartan, it is possible to investigate how the friction force changes. The
forces acting on the plane are shown in the ‘free body diagram’ of the shoe removed
from its surroundings, but showing the forces that the surroundings exert on the
shoe (Figure 5.4(b)). Because the shoe is not moving, these forces are in equilibrium.
Resolving the weight of the shoe (G) along (F t) and normal (F n) to the plane, the
magnitudes of the components are, respectively: F t = G sinθ; F n = G cosθ. Dividing F t
by F n, we get F t /F n = tanθ. If the angle of inclination of the plane (θ) is increased, the
friction force will eventually be unable to resist the component of the shoe’s weight
down the slope and the shoe will begin to slide. The ratio of F t/F n (= tanθ) at which this
occurs is called the coefficient of (limiting) static friction (µ s). The maximum sliding
friction force that can be transmitted between two bodies is: F t max = µ s F n. This is
known as Newton’s law of friction and refers to static friction, just before there is any
relative movement between the two surfaces. It also relates to conditions in which only
the friction force prevents relative movement. For such conditions, the maximum
friction force depends only on the magnitude of the normal force pressing the surfaces
together and the coefficient of static friction (µ s). This coefficient depends only on the
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