## Contact Forces

The linear kinetics of the interaction of two objects in contact is also analyzed by resolv ing the forces into right-angle components. These components use a local frame of reference like the two-dimensional muscle angle of pull above, because using horizontal and vertical components are not always convenient (Figure 6.11). The forces between two objects in contact are resolved into the normal reaction and friction. The normal reaction is the force at right angles to the surfaces in contact, while friction is the force acting in parallel to the surfaces. Friction is the force resisting the sliding of the surfaces past each other.

When the two surfaces are dry, the force of friction (F) is equal to the product of the coefficient of friction (|) and the normal reaction (FN), or F = | • FN. The coefficient

Figure 6.11. Forces of contact between objects are usually resolved into the right-angle components of normal reaction (FN) and friction.

Friction

Figure 6.11. Forces of contact between objects are usually resolved into the right-angle components of normal reaction (FN) and friction.

of friction depends on the texture and nature of the two surfaces, and is determined by experimental testing. There are coefficients of static (non-moving) friction (|S) and kinetic (sliding) friction (|K). The coefficients of kinetic friction are typically 25% smaller than the maximum static friction. It is easier to keep an object sliding over a surface than to stop it and start the object sliding again. Conversely, if you want friction to stop motion, preventing sliding (like with anti-lock auto brakes) is a good strategy. Figure 6.12 illustrates the friction force between an athletic shoe and a force platform as a horizontal force is increased. Please note that the friction grows in a linear fashion until the limiting friction is reached (|S • FN), at which point the shoe begins to slide across the force platform. If the weight on the shoe created a normal reaction of 300 N, what would you estimate the |S of this rubber/metal interface?

Typical coefficients of friction in human movement vary widely. Athletic shoes have coefficients of static friction that range from 0.4 to over 1.0 depending on the shoe and sport surface. In tennis, for example, the linear and angular coefficients of friction range from 0.4 to over 2.0, with shoes responding differently to various courts

Figure 6.12. The change in friction force between an athletic shoe and a force platform as a horizontal force is applied to the shoe. The ratio of the friction force (FH) on this graph to the normal force between the shoe and force platform determine the coefficient of friction for these two surfaces.

Time

Figure 6.12. The change in friction force between an athletic shoe and a force platform as a horizontal force is applied to the shoe. The ratio of the friction force (FH) on this graph to the normal force between the shoe and force platform determine the coefficient of friction for these two surfaces.

(Nigg, Luthi, & Bahlsen, 1989). Epidemiological studies have shown that playing on lower-friction courts (clay) had a lower risk of injury (Nigg et al., 1989). Many teams that play on artificial turf use flat shoes rather than spikes because they believe the lower friction decreases the risk of severe injury. The sliding friction between ice and a speed skating blade has been measured, demonstrating coefficients of kinetic friction around 0.005 (van Ingen Schenau, De Boer, & De Groot, 1989).

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