Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
Reaction forces
Friction
is little difference between the positions of the centre of mass (see later) and the centre
of gravity. The former is the term preferred in most modern sports biomechanics
literature and will be used in the rest of this book. One reason for this preference is that
the centre of gravity is a meaningless concept in weightless environments, such as space
shuttles. An athlete with a mass of 50 kg has a weight (G) of about 490 N at sea level, at
which the standard value of gravitational acceleration, g, is assumed to be 9.81 m/s 2 .
Reaction forces are the forces that the ground or other external surface exerts on the
sports performer as a reaction to the force that the performer exerts on the ground or
surface. This principle is known as Newton’s third law of linear motion or the law of
action–reaction. The vertical component of force acting on a person performing
a standing vertical jump – with no arm action – is shown as a function of time in
Figure 5.2. This movement pattern shows the period during which the jumper is on the
ground (A), then in the air (B), and finally during landing (C). The component of
the reaction force tangential to the surface, known as friction or traction, is crucially
important in sport and is considered in the next section.
The ground, or other, contact force acting on an athlete (Figure 5.3(a)) can be resolved
into two components, one (F n) normal and one (F t) tangential to the contact surface
(Figure 5.3(b)). The former component is the normal force and the latter is the friction,
Figure 5.2 Vertical component of ground reaction force in a standing vertical jump with no arm action: (A) on the ground
before take-off for the jump; (B) in the air; (C) landing.
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