Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
Figure 5.16 Generation of rotation: (a) force couple; (b) the torque (or moment) of the couple.
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Eccentric force
external axis of rotation, the body will rotate about an axis through its centre of mass.
The swimmer in Figure 5.2(a) is acted upon by a force couple of her weight and the
equal, but opposite, buoyancy force.
An eccentric force (‘eccentric’ means ‘off-centre’) is effectively any force, or resultant of
a force system, that is not zero and that does not act through the centre of mass of an
object. This constitutes the commonest way of generating rotational motion, as in
Figure 5.17(a).
The eccentric force here can be transformed by adding two equal and opposite forces
at the centre of mass, as in Figure 5.17(b), which will have no net effect on the object.
The two forces indicated in Figure 5.17(b) with an asterisk can then be considered
together and constitute an anticlockwise force couple, which can be replaced by a
torque M as in Figure 5.17(c). This leaves a ‘pure’ force (F) acting through the centre
of mass, which causes only linear motion, F = d(m v)/dt; the torque M causes only
rotation, M = dL/dt. The magnitude of the torque, M, is F r.
This example could be held to justify the use of the term ‘torque’ for the turning
effect of an eccentric force although, strictly, ‘torque’ is defined as the moment of a
force couple. The two terms, torque and moment, are often used interchangeably.
There is a strong case for abandoning the use of the term moment of a force or couple
entirely in favour of torque, given the various other uses of the term moment in
biomechanics.
Checking of linear motion
Checking of linear motion occurs when an already moving body is suddenly stopped
at one point. An example is the foot plant of a javelin thrower in the delivery stride,
although the representation of such a system as a quasi-rigid body is of limited use.