Force
Friction
Dry friction acts between surfaces of rigid objects or parts of human body that are not covered with any connective tissue. Its orientation is parallel with surfaces that are in contact. Friction is a result of interaction between the molecules of the surfaces in contact. If the surfaces are not moving relative to each other, we talk of static friction. Static friction is also called limit friction because it represents the biggest possible friction between given surfaces under given conditions. If dry friction acts between surfaces that are moving relative to each other, we talk of dynamic friction.
Friction force is directly proportional to normal contact force Fn.
This means that the bigger the vertical force with which a walker acts on the ground, the bigger the friction force between the soles of his/her shoes and the surface.
Friction force is also influenced by properties of the surfaces that are in contact.
There is more friction between soft and rough surfaces than between hard and smooth surfaces. Soles of shoes will have different friction on forest soil compared to icy pavement.
Dry friction is not influenced by the area of the surfaces that are in contact.
The size of the shoes (sole area) has no influence on friction force during walking because normal force is resolved over larger area and the interactions among individual molecules of both surfaces are weaker.
Static friction is bigger than dynamic friction.
Mathematically friction force can be expressed as:
Ft = μ Fn12,
Where Ft is static or dynamic friction force (N), μ is coefficient of static or dynamic friction, i.e. the number expressing the influence of individual materials on friction force, and Fn is normal contact force (N).
The already mentioned walker, through his/her muscle activity, acts with his/her soles backward on the ground (action) and the ground acts with the same force of opposite direction on soles (reaction), which has a direct influence on motion. It gives the walker acceleration and is by definition a static friction force, which, under given circumstances, reaches only such extent of its maximum so as to compensate for the intensity of action force.
Friction in sport and human locomotion
Friction in sport and physical exercise is sometimes an advantage and sometimes a disadvantage. For example in skiing we demand the lowest possible kinetic friction coefficient so that we can glide along the snow as fast as possible without being slowed down by friction (that is why we wax the runners). In other activities, such as dancing or bowling, we demand a relatively low kinetic friction coefficient of shoe soles so that we can make use of sliding across the surface. In most sport shoes we demand high friction forces, so the soles have a high kinetic friction coefficient. For example footballers have screw-in studs on the soles of football boots to increase the friction force. Handles of hockey sticks and tennis rockets are covered with special tapes to increase friction, and magnesium powder is used on palms in gymnastics and javelin throw. In everyday life friction between shoes and floor surfaces is important in accident prevention.
12 This formula represents the highest possible intensity of static friction force, which needs not to be fully used under real circumstances of sport and physical exercise.Zpět