Introduction to Sports Biomechanics: Analysing Human Movement ...

Lift forces
CAUSES OF MOVEMENT – FORCES AND TORQUES
dimension of the object perpendicular to the flow direction to the dimension along the
flow direction. Long, thin wings on gliders minimise the induced drag whereas a javelin
has entirely the wrong shape for this purpose.
If an asymmetry exists in the fluid flow around a body, the fluid dynamic force will act
at some angle to the direction of motion and can be resolved into two component
forces. These are a drag force opposite to the flow direction and a lift force perpendicular
to the flow direction. Such asymmetry may be caused in three ways. For a discus
and javelin, for example, it arises from an inclination of an axis of symmetry of the
body to the direction of flow (Figure 5.8(a)). Another cause is asymmetry of the body
(Figure 5.8(b)); this is the case for sails, which act similarly to the aerofoil-shaped wings
of an aircraft, and the hands of a swimmer, which function as hydrofoils. The Magnus
effect (Figure 5.8(c)) occurs when rotation of a symmetrical body, such as a ball,
produces asymmetry in the fluid flow.
Swimmers use a mixture of lift and drag forces for propulsion, with their hands
acting as rudimentary hydrofoils. A side view of a typical path of a front crawl
swimmer’s hand relative to the water is shown in Figure 5.9(a). In the initial and final
portions of the pull phase of this stroke (marked ‘i’ and ‘f’ in Figure 5.9(a)) only the lift
force (L) can make a significant contribution to propulsion. In the middle region of the
pull phase (marked ‘m’), the side view would suggest that drag (D) is the dominant
contributor. However, a view from in front of the swimmer (Figure 5.9(b)) or below
(Figure 5.9(c)) shows an S-shaped pull pattern in the sideways plane. These sideways
movements of the hands generate significant propulsion through lift forces perpendicular
to the path of travel of the hand (Figure 5.9(c)). Swimmers need to develop a
‘feel’ for the water flow over their hands, and to vary the hand ‘pitch’ angle with respect
to the flow of water to optimise the propulsive forces throughout the stroke. The shape
of both oars and paddles suggests that they also can behave as hydrofoils. In rowing, for
example, the propulsive forces generated are a combination of both lift and drag
components and not just drag. The velocity of the oar or paddle relative to the water is
the crucial factor. If this is forwards at any time when the oar is submerged, the drag is
in the wrong direction to provide propulsion. Only a lift force can fulfil this function
(see Figure 5.9). The wing paddle in kayaking was developed to exploit this propulsive
lift effect.
Many ball sports involve a spinning ball. Consider, for example, a ball moving
through a fluid, and having backspin as in Figure 5.8(c). The top of the ball is moving
in the same direction as the air relative to the ball, while the bottom of the ball is
moving against the air stream. The rotational motion of the ball is transferred to the
thin boundary layer adjacent to the surface of the ball. On the upper surface of the ball
this ‘circulation’ imparted to the boundary layer reduces the difference in velocity
across the boundary layer and delays separation. On the lower surface of the ball, the
boundary layer is moving against the rest of the fluid flow, known as the free stream.
177