Introduction to Sports Biomechanics: Analysing Human Movement ...

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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
INTRODUCTION TO SPORTS BIOMECHANICS Figure 5.8 Generation of lift: (a) inclination of an axis of symmetry; (b) body asymmetry; (c) Magnus effect. 178 This increases the velocity difference across the boundary layer and separation still occurs. The resulting wake has, therefore, been deflected downwards, as can be seen in Figure 5.8(c). Newton’s laws of motion imply that the wake deflection is due to a force provided by the ball acting downwards on the air and that a reaction moves the ball away from the wake. This phenomenon is known as the Magnus effect. For a ball with backspin, the force acts perpendicular to the motion of the ball – it is a lift force. Golf clubs are lofted so that the ball is undercut, producing backspin. This varies from approximately 50 Hz for a wood to 160 Hz for a nine iron. The lift force generated can even be sufficient to cause the initial ball trajectory to be curved slightly upwards. The lift force increases with the spin and substantially increases the length of drive compared to that with no spin. The main function of golf ball dimples is to assist the transfer of the rotational motion of the ball to the boundary layer of air to increase the Magnus force and give optimum lift. Backspin and topspin are used in games such
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Introduction to Sports Biomechanics
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First edition published 1997 This e
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Contents List of figures x List of
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Study tasks 216 Glossary of importa
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FIGURES 1.26 Underarm throw - femal
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FIGURES 5.9 Typical path of a swimm
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Tables 2.1 Examples of slowest sati
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Preface Why have I changed the cove
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Introduction MISSING TEXT The first
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INTRODUCTION principal movements in
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INTRODUCTION TO SPORTS BIOMECHANICS
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Figure 2.2 ‘Principles’ approac
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Figure 2.13 Identifying critical fe
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Figure 6.8 Main skeletal muscles: (
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INDEX 282 balls air flow past 173 b
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INDEX 284 energy 219 kinetic 219 mi
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INDEX 286 isokinetic contraction 24
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INDEX 288 muscle length-tension rel
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INDEX 290 three-dimensional 146-7,
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INDEX 292 validity 220 vantage poin
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