Introduction to Sports Biomechanics: Analysing Human Movement ...

Buoyancy
dry and wet surface friction coefficients of around 1.0 and 0.4, respectively. The treads
allow water to be removed from the contact area between the tyre and road surface.
Likewise, most sports shoes have treaded or cleated soles, and club and racket grips are
rarely perfectly smooth. In some cases, the cleats on the sole of a sports shoe will also
provide some form locking with certain surfaces.
Pulley friction
Passing a rope around the surface of a pulley makes it easier to resist a force of large
magnitude at one end by a much smaller force at the other end because of the friction
between the rope and the pulley. This principle is used, for example, in abseiling
techniques in rock climbing and mountaineering. It also explains the need for synovial
membranes to prevent large friction forces when tendons pass over bony prominences.
Buoyancy is the force experienced by an object immersed, or partly immersed, in a
fluid. It always acts vertically upwards at the centre of buoyancy (CB in Figure 5.6).
The magnitude of the buoyancy force (B) is expressed by Archimedes’ principle, ‘the
upthrust is equal to the weight of fluid displaced’, and is given by B = V ρ g, where
V is the volume of fluid displaced, ρ is the density of the fluid and g is gravitational
acceleration. The buoyancy force is large in water – pure fresh water has a density of
1000 kg/m 3 – and much smaller, but not entirely negligible, in air, which has a density
of around 1.23 kg/m 3 . For a person or an object to float and not sink, the magnitudes of
the buoyancy force and the weight of the object must be equal, so that B = G.
The swimmer in Figure 5.6 will only float if her average body density is less than or
equal to the density of water. How much of her body is submerged will depend on the
ratio of the two densities. If the density of the swimmer is greater than that of the water,
Figure 5.6 Buoyancy force: (a) forces acting; (b) forces in equilibrium.
CAUSES OF MOVEMENT – FORCES AND TORQUES
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