Physics
Kinetic Friction
Kinetic friction refers to the force that opposes the relative motion of two surfaces in contact. It arises when an object slides or moves across another surface. The magnitude of kinetic friction depends on the nature of the surfaces and the normal force pressing them together.
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eBook - PDFEngineering Science N2 Student's Book
TVET FIRST
- MJJ van Rensburg(Author)
- 2016(Publication Date)
- Troupant(Publisher)
150 M O D U L E Friction 6 Setting the scene 1. In a frictionless world, how would you start the fire for your Saturday afternoon braai? 2. Give two advantages and two disadvantages of friction in a motor car. 3. Does a higher speed have any effect on the friction generated in an engine? Figure 6.1: When the match is struck on the striking surface of the box, the friction produces enough heat to start a chemical reaction Objectives and overview When you have completed Module 6, you should be able to: • Define static friction, Kinetic Friction and the coefficient of friction, and state the laws of friction. • Name the advantages and disadvantages of friction as well as applications of friction in practice. • Represent all the forces acting on a body subject to friction, on both horizontal and inclined planes, by means of a diagram. • Define the angle of rest. • Indicate and apply the relationship between the coefficient of friction and the angle of friction, and represent the angle of friction diagrammatically. • Apply and manipulate the applicable formulae in order to calculate the coefficient of friction, angle of friction, weight components and the effective upward and downward pulling forces. Introduction Friction is a force that exists between any two surfaces that are in contact with each other. A frictional force opposes motion when the surface of one object moves across another surface or makes an effort to move across it. A smooth surface will result in less friction, while a rough surface will result in more friction. The force of gravity aggravates (worsens) the effect of friction, as will be seen later in this module. 151 There are two types of friction, Kinetic Friction and static friction : • Kinetic Friction (also called sliding friction) opposes the movement of a body that is already in motion. The air resistance experienced by a runner is a form of Kinetic Friction. • Static friction prevents an object from moving when it is at rest.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Learning Press(Publisher)
Another way to reduce friction between two parts is to superimpose micro-scale vibration to one of the parts. This can be sinusoidal vibration as used in ultrasound-assisted cutting or vibration noise, known as dither. Energy of friction According to the law of conservation of energy, no energy is destroyed due to friction, though it may be lost to the system of concern. Energy is transformed from other forms into heat. A sliding hockey puck comes to rest because friction converts its kinetic energy into heat. Since heat quickly dissipates, many early philosophers, including Aristotle, wrongly concluded that moving objects lose energy without a driving force. When an object is pushed along a surface, the energy converted to heat is given by: where is the normal force, is the coefficient of Kinetic Friction, is the coordinate along which the object transverses. Energy lost to a system as a result of friction is a classic example of thermodynamic irreversibility. Work of friction In the reference frame of the interface between two surfaces, static friction does no work, because there is never displacement between the surfaces. In the same reference frame, ________________________ WORLD TECHNOLOGIES ________________________ Kinetic Friction is always in the direction opposite the motion, and does negative work. However, friction can do positive work in certain frames of reference. One can see this by placing a heavy box on a rug, then pulling on the rug quickly. In this case, the box slides backwards relative to the rug, but moves forward relative to the frame of reference in which the floor is stationary. Thus, the Kinetic Friction between the box and rug accelerates the box in the same direction that the box moves, doing positive work. The work done by friction can translate into deformation, wear, and heat that can affect the contact surface properties (even the coefficient of friction between the surfaces).
eBook - PDF- Kenneth G. Budinski(Author)
- 2013(Publication Date)
- CRC Press(Publisher)
133 © 2008 Taylor & Francis Group, LLC 11 Sliding Friction The ASTM G 2 Committee on Wear and Erosion maintains a list of definitions for friction, wear, and erosion terms: ASTM G 40. Unfortunately, this list of terms does not include friction . It defines friction-related terms: friction force, coefficient of friction, kinetic coeffi-cient of friction, static coefficient of friction, and stiction. The dictionary definitions of fric-tion range from “rubbing” to “the resistance that a body encounters when movement on another body is attempted or sustained.” The terms rubbing and resistance are included in the majority of the dictionary definitions. However, friction is encountered in fluids even between molecules rubbing in a solid. For example, plastics can locally heat when they are repeatedly stressed as in a fatigue test. Also, it is well known that friction of ship hulls versus water is a significant factor for fuel usage. A proper definition of friction should include all of these types of friction, all of those shown in Figure 11.1: Sliding—solids rubbing Rolling—a solid rolling on a solid surface Lubricated—sliding or rolling with lubricant in the contact Internal—within a solid Atomic—between the atoms of contacting surfaces An inclusive definition might be the force resisting the start of motion or continued motion of one body or substance on or within another body or substance. The problem with this definition is that friction is also inextricably connected with energy dissipation. As mentioned, some materials like automobile tires get hot from flexing. This heat must be dissipated. A significant amount of energy, heat, must be dissipated in braking a vehicle or moving part on a machine. Thus, rubbing solids generate heat. Forces resisting motion generate heat. And this is logical since the forces occur in sliding and sliding involves rela-tive motion or distance, and we know that a force times a distance yields an energy term.
- Fridrun Podczeck(Author)
- 1998(Publication Date)
- ICP(Publisher)
CHAPTER 2 BRIEF OVERVIEW OF THE THEORY OF FRICTION 2.1 Definition and Importance of Friction Forces Surfaces in contact to each other are held by forces directed normally to the interface (adhesion) and tangentially to the interface (friction). The latter be-come manifested if a relative displacement of the contacting surfaces is forced (Deryaguin et al. 1978a, pp. 380-381). This makes the friction phenomenon important for all powder handling processes, where particles move along a sur-face (for example powder flow through a hopper) or change their position in a powder bed (for example during mixing). It has been estimated, that about 5 % of the gross national product in the developed countries is wasted due to uncontrolled friction and wear (Persson 1995). However, friction can be advantageous, for example, when driving a car, where the design and the sur-face profile can influence the friction properties and hence the safety of driving (Tabor 1994). Friction should therefore not always be considered negatively. The friction force expresses itself as a force directed to oppose the velocity vector of the moving solid body. Friction forces can be divided into a static and dynamic type depending upon the absence or type of relative motion between the surfaces in contact. A finite force i. e. yield stress must be applied to initiate motion, and friction then arises from a transfer of energy between the surfaces in contact (Yoshizawa et al. 1993). The static friction force is very often larger than dynamic friction forces due to sliding or rolling (Bowden and Tabor 1954, p. 105; Heslot et al. 1994). Sliding friction is characterized by a slippage of two surfaces over each other. Rolling friction, however, does not involve any slippage, but can be described as a series of adhesion and detachment processes (Deryaguin et al. 1978a, p. 280), which causes one of two bodies in contact to roll over the surface of the second body.
eBook - PDFHandbook of Lubrication and Tribology, Volume II
Theory and Design, Second Edition
- Robert W. Bruce(Author)
- 2012(Publication Date)
- CRC Press(Publisher)
5 -1 5.1 Introduction Friction, the resistance to relative motion between a solid body and another solid body or substance, is so common an everyday experience that it has been the source of philosophical contemplation and experimental investigation throughout history. Friction broadly encompasses not only fluid friction in pipes, aerodynamic friction, and the friction between dry or wet sliding surfaces in mechanical parts like seals, brakes, bearings, gears, but also friction in natural phenomena, ranging from human joints to tectonic plates. Because friction takes so many forms, no universal, overarching theory or model suc-cessfully explains it. A common quantity used to describe the magnitude of friction between sliding solid bodies is called the friction coefficient (alternatively, the “coefficient of friction”). It is the dimensionless ratio of two mutually perpendicular forces: a force that resists relative motion and a normal force that acts per-pendicular to the interface. A popular notion persists that the friction coefficient is a single-valued, fundamental property of a given material pair. That presumption fails in the face of experimental evidence. The degree to which variables like normal force, relative velocity, directional changes in rela-tive motion, contact surface roughness, temperature, humidity, magnetic fields, and material proper-ties affect frictional response is tribosystem dependent. This chapter considers the causes of friction, the concepts that researchers have used to explain friction, and the control of friction in engineering systems. 5 Friction 5.1 Introduction ...................................................................................... 5 -1 5.2 Historical Studies of Friction .......................................................... 5 -2 5.3 Static and Kinetic Friction Coefficients ........................................ 5 -2 5.4 Frictional Transients and Instabilities ..........................................
eBook - PDF- John Williams(Author)
- 2005(Publication Date)
- Cambridge University Press(Publisher)
4 The friction of solids 4.1 The genesis of solid friction 4.1.1 The laws of friction Friction is the resistance encountered when one body moves tangentially over another with which it is in contact. The work expended against friction is often redundant, that is, it makes no useful contribution to the overall operation of the device of which the bodies are part, and ultimately must be dissipated as waste heat. Consequently, in most tribological designs our aim is to keep these frictional forces as small as possible. Of course there are exceptions to this general rule, occasions when sufficient friction is essen-tial to continued progress and there are many practical devices which rely on the frictional transmission of power: automobile tyres on a roadway, vehicle brakes and clutches, as well as several of the variable-speed transmis-sion systems now finding wider application. When two objects are to be held together, the only alternative to methods which rely on friction is the forma-tion of some sort of chemical or metallurgical bond between them. The development of this sort of technique—adhesives and 'superglues', and even welding and brazing—are relatively recent; 'traditional' forms of fixing rely almost exclusively on friction. A nail hammered into a piece of wood is held in place by frictional effects along its length; if the frictional interaction were substantially reduced, the nail would be squeezed out. Similarly, the grip between a nut and a bolt depends on adequate friction between them. Any woven or knitted fabric depends on the friction between adjacent threads — there are no glues or adhesives; it would be impossible to knot together two pieces of very-low-friction string. As children we soon learn some of the qualitative laws of friction. That, for example, it is much easier to slide a light object across a horizontal floor than a very much heavier one.
eBook - PDFFriction Science and Technology
From Concepts to Applications, Second Edition
- Peter J. Blau(Author)
- 2008(Publication Date)
- CRC Press(Publisher)
Subsequent chap-ters show how the details of frictional interactions can complicate the solutions to practical friction problems, especially in dynamic, interfacially contaminated envi-ronments. But it is important to start from a common frame of reference, and to that end this chapter is presented. 2.1 BASIC DEFINITIONS OF FRICTION QUANTITIES When two solid bodies are placed together under a nonzero normal force and acted upon by another force that has a component parallel to the contact surface (a tan-gential force), sliding or slipping may or may not occur, depending on whether the applied force can overcome the friction force opposing it. In some cases, the normal force may be due only to the weight of the upper body resting on the lower, whereas in other cases, it may be due to applied forces other than that due to gravity. The problem in determining whether relative motion will or will not occur is one of balancing the forces involved. The following definitions, from ASTM Standard G-40-93 on Stan-dard Terminology Relating to Erosion and Wear, will serve our present purposes: Friction force —the resisting force tangential to the interface between two bodies when, under the action of an external force, one body moves or tends to move relative to the other. 18 Friction Science and Technology: From Concepts to Applications Coefficient of friction —the ratio of the force resisting tangential motion between two bodies to the normal force pressing those bodies together. Any force field for which the work done in moving an object from one point to another is independent of the path taken is considered to be a conservative force field. Gravitational and electrostatic force fields are examples of conservative force fields. Therefore, gravitational and electrostatic forces are called conservative forces. How-ever, friction forces acting on a body moving from one place to another are noncon-servative forces .
eBook - PDFFriction and Wear
Calculation Methods
- I V Kragelsky, M N Dobychin, V S Kombalov(Authors)
- 2013(Publication Date)
- Pergamon(Publisher)
Chapter 5 DRY A N D BOUNDARY FRICTION BASIC DEFINITIONS Surface friction is an energy dissipative process vdiich takes place with relative tangential displacement of contacting solids in zones of real contact between them formed by the action of an external load. Depending on the type of displacement (Fig. 1), it is possible to distinguish three types of friction, i.e. sliding friction, rotating friction, and rolling friction (this is not discussed in this Chapter). The concept of friction force Τ is introduced for quantitative evaluation of friction. The friction force is the resultant of the tangential resistance forces arising at real contact points when one body slides along the surface of another, and is a non-potential force. Rotating friction is characterized by a friction force moment. In passing from rest to sliding, there is a preliminary displacement region (i.e. section OA in Fig. 2) . Ihe tangential resistance under preliminary displacement conditions is called the inconnplete friction force, although a better name would be the adhesion force, since it is partially potential in character.* The full stationary friction force corresponds to a transition fron preliminary displacement to sliding (point A) , and is conditionally called static friction. After preliminary displacement stable sliding starts, characterized by a sliding friction force (line AiB) . Depending on the problem posed, friction can be evaluated by a force or a moment. Friction on inpact is characterized by a change in the quantity of motion. In engineering practice friction is evaluated by means of three dimensionless quantities: (1) The coefficient of sliding friction, vdiich is the ratio of the friction force to the normal load / = T/N In certain cases such adhesion is called leading or creep friction, but this is undesirable since friction is a dissipative process, and leading friction is partially characterized by processes occurring during elastic deformation. 156
eBook - PDF- Raymond Serway, Chris Vuille(Authors)
- 2017(Publication Date)
- Cengage Learning EMEA(Publisher)
That coefficient depends on the object and the surface and must be determined experimentally. Variations in the surface that are not apparent to the eye can alter the value of the coefficient as the object moves from one point to another. Further, the effect on the object can vary as a function of velocity. Such coefficients are nonetheless useful, although they are best regarded as averages over time and approximations. In this text, we’ll assume a coefficient of friction is always constant for a given surface and object. Some representative values are given in Table 4.2, which also has values for the coefficient of static friction (Section 4.4). A diagram illustrating the effect of a Kinetic Friction force on a block is given in Figure 4.18. In every case, this model of friction requires the calculation of the normal force, followed by multiplying by the Kinetic Friction coefficient, m k , which is a constant derived from experiment. The normal force corresponding to Figure 4.18 can be taken from case 2 and is n 5 mg 2 F app app sin u. The x - component of the second law of motion can now be easily written down: ma x 5 F a a pp,x 2 f k k 5 F a a pp cos u 2 m k n ma x 5 F a a pp cos u 2 m k 1 mg g 2 F a a pp sin u 2 So calculating and using the Kinetic Friction force is no harder than calculating the normal force. In this case, the angle of the applied force is critical in determin- ing the resultant acceleration of the block. As the angle increases from zero, the part of the applied force acting in the x - direction decreases, as do the normal and Kinetic Friction forces. Using calculus, that angle can be chosen so as to optimize the acceleration.
No longer available |Learn more- Ping YI, Jun LIU, Feng JIANG(Authors)
- 2022(Publication Date)
- EDP Sciences(Publisher)
Chapter 5 Friction Objectives Examine characteristics of friction. Solve equilibrium problems involving friction force. Understand the concept of rolling resistance. 5.1 Characteristics of Friction Prior to this chapter, we always assume that the contacting surface is smooth and the friction is neglected. This is because for many engineering problems, the effect of friction is often very small and can be neglected. However, sometimes frictional forces play an important role. For example, a car brake just utilizes the friction force to stop the car; for a gravity dam or a retaining wall, the friction force is an important factor to hold the dam or the slope in equilibrium, figure 5.1. For these cases, the friction cannot be neglected. FIG. 5.1 – Engineering cases in which friction plays an important role. DOI: 10.1051/978-2-7598-2901-9.c005 © Science Press, EDP Sciences, 2022 Friction can be defined broadly as a resistance that occurs between two surfaces in contact when they tend to move relative to each other. Based on the relative movement between the two surfaces, there are two kinds of friction. When the two contacting bodies tend to slide relative to each other, figure 5.2a, there is sliding friction; whereas when they tend to roll relative to each other, figure 5.2b, rolling resistance occurs. This chapter will mainly discuss sliding friction and give a brief introduction to rolling resistance. Friction can also be classified as fluid friction and dry friction. If the contacting surfaces are separated by a thin layer of liquid or gas, it is fluid friction; whereas dry friction occurs when there is no lubricating fluid between the contacting surfaces. In this book only the effects of dry friction will be considered. 5.1.1 Laws of Sliding Friction A simple experiment is used to discuss sliding friction. The block having weight P rests on the rough horizontal ground and horizontal force F is exerted on the block, as shown in figure 5.3a.
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