Introduction to Sports Biomechanics: Analysing Human Movement ...

The law of reaction
For every torque that is exerted by one object on another, an equal and opposite torque is exerted
by the second object on the first. In Figure 5.14(a), the forward and upward swing of the long
jumper’s legs (thigh flexion) evoke a reaction causing the forward and downward motion of the
trunk (trunk flexion). As the two torques are both within the jumper’s body, there is no change in
her angular momentum; the two torques involved produce equal but opposite impulses. In Figure
5.14(b), the ground provides the reaction to the action torque generated in the racket arm and
Figure 5.14 Action and reaction: (a) airborne; (b) with ground contact.
CAUSES OF MOVEMENT – FORCES AND TORQUES
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