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38827_Ward's World+MGH Friction

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1 Friction Article by: Vernon D. Barger, Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin. Access to this content is available to Ward's World readers for free from McGraw Hill's AccessScience, an award-winning, digital STEM resource that provides immediate, authoritative answers to students' thirst for scientific knowledge on topics such as climate change, virology, pollution, and more. Ward's World and McGraw Hill have partnered to offer educators a no-obligation, free trial subscription to this product. Request your free trial today and discover how valuable AccessScience can be for you and your students. Resistance to sliding, a property of the interface between two solid bodies in contact. Many everyday activities like walking or gripping objects are carried out through friction, and most people have experienced the problems that arise when there is too little friction and conditions are slippery. However, friction is a serious nuisance in devices that move continuously, like electric motors or railroad trains, since it constitutes a dissipation of energy, and a considerable proportion of all the energy generated by humans is wasted in this way. Most of this energy loss appears as heat, while a small proportion induces loss of material from the sliding surfaces, and this eventually leads to further waste, namely, to the wearing out of the whole mechanism. Almost all the frictional energy appears as heat at the interface between the sliding surfaces. This frictional heat was used by humans in prehistoric times to light fires, and this use survives today in the striking of matches, with chemical combustion initiated by the temperature rise produced by sliding (Fig. 1). However, frictional heat usually is a nuisance, and sliding surfaces must often be cooled to prevent heat damage. Friction is stud- ied in the field of tribology — the science of interactive surfaces in relative motion — which also studies lubrication and wear. + ward ' s science Content • Mechanism • Friction coefficient values • Frictional oscillations • Lubrication Key Concepts • Friction is a force that opposes the sliding of two parts or objects in contact, resulting in both energy loss as heat and material loss from the sliding surfaces. • The friction coefficient, f, is a value describing the relationship between the force of friction and the force that is normal to the surface of contact between the two sliding parts or objects. • It is a misconception that the main cause of friction is surface roughness, as friction coefficients are largely independent of surface texture. • If friction decreases when the sliding speed increases, then frictional vibrations, or oscillations, may be produced. • Engineers use lubricants, such as oils, to reduce the amount of friction and any associated oscillations to acceptable levels. Fig. 1: Ignition of a match. (Credit: KConstantine/Shutterstock.com)

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