Introduction to Sports Biomechanics: Analysing Human Movement ...

CAUSES OF MOVEMENT – FORCES AND TORQUES
Figure 5.9 Typical path of a swimmer’s hand relative to the water: (a) side view; (b) view from in front of the swimmer; (c)
view from below the swimmer.
as tennis and table tennis both to vary the flight of the ball and to alter its bounce. In
cricket, a spin bowler usually spins the ball so that it is rotating about the axis along
which it is moving (its velocity vector), and the ball only deviates when it contacts
the ground. However, if the ball’s spin axis does not coincide with its velocity vector, the
ball will also move laterally through the air. A baseball pitcher also uses the Magnus
effect to ‘curve’ the ball; at a pitching speed of 30 m/s the spin imparted can be as high
as 30 Hz, which gives a lateral deflection of 0.45 m in 18 m. A soccer ball can be made
to swerve, or ‘bend’, in flight by moving the foot across the ball as it is kicked. This
causes rotation of the ball about the vertical axis. If the foot is moved from right to left
as the ball is kicked, the ball will swerve to the right. Slicing and hooking of a golf ball
are caused, inadvertently, by sidespin imparted at impact. In crosswinds, the relative
direction of motion between the air and the ball is changed; a small amount of sidespin,
imparted by ‘drawing’ the ball with a slightly open club face, or ‘fading’ the ball with a
slightly closed club face depending on the wind direction, can then increase the length
of the drive.
A negative Magnus effect can also occur for a ball travelling below the critical
Reynolds number. This happens when the boundary layer flow remains laminar on the
side of the ball moving in the direction of the relative air flow, as the Reynolds number
here remains below the critical value. On the other side of the ball, the rotation
increases the relative speed between the air and the ball so that the boundary layer
becomes turbulent. If this happens on a back-spinning ball, laminar boundary layer
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