Most forces are the result of contact between objects and bodies. Contact forces can be any one of six types:
1 In most sports,
athletes are in contact with the ground,
and the force or resistance that the ground applies
to the athlete is called the ground reaction force.
2 The force that
two bones apply to each other across a joint is called the joint reaction force.
3 Friction is the force created
when one surface is moved across another.
4 In many sports, motion is affected by the fl uid (such as air and water)
in which it is performed.
This is called fl uid resistance.
5 Inertial force can also affect movement.
For example, the ankle swings through when running because
of the force of inertia placed on it by the leg.
6 Elastic force is that where a material changes its length
when a force is applied to it. Examples of elastic
forces are those provided by diving boards, muscles,
sprung fl oors, trampolines and some running shoes.