Physics
Static Friction
Static friction is the force that prevents an object from moving when a force is applied to it. It arises between two surfaces in contact and acts in the opposite direction of the applied force. The magnitude of static friction depends on the nature of the surfaces and the force pressing them together.
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9 Key excerpts on "Static Friction"
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Learning Press(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Static Friction Static Friction is friction between two solid objects that are not moving relative to each other. For example, Static Friction can prevent an object from sliding down a sloped surface. The coefficient of Static Friction, typically denoted as μ s , is usually higher than the coefficient of kinetic friction. The Static Friction force must be overcome by an applied force before an object can move. The maximum possible friction force between two surfaces before sliding begins is the product of the coefficient of Static Friction and the normal force: . When there is no sliding occurring, the friction force can have any value from zero up to . Any force smaller than attempting to slide one surface over the other is opposed by a frictional force of equal magnitude and opposite direction. Any force larger than overcomes the force of Static Friction and causes sliding to occur. The instant sliding occurs, Static Friction is no longer applicable—the friction between the two surfaces is then called kinetic friction. An example of Static Friction is the force that prevents a car wheel from slipping as it rolls on the ground. Even though the wheel is in motion, the patch of the tire in contact with the ground is stationary relative to the ground, so it is static rather than kinetic friction. The maximum value of Static Friction, when motion is impending, is sometimes referred to as limiting friction , although this term is not used universally. It is also known as traction. Kinetic friction Kinetic (or dynamic) friction occurs when two objects are moving relative to each other and rub together (like a sled on the ground). The coefficient of kinetic friction is typically denoted as μ k , and is usually less than the coefficient of Static Friction for the same materials. In fact, Richard Feynman reports that with dry metals it is very hard to show any difference.
eBook - PDF- Raymond Serway, Chris Vuille(Authors)
- 2017(Publication Date)
- Cengage Learning EMEA(Publisher)
As with the kinetic friction force, the Static Friction force arises from microscopic interactions between the object and the surface it rests upon. Figure 4.19c illustrates graphically how Static Friction works. As the applied force gradually increases, so does the Static Friction force, acting in the opposite direc- tion. When the applied force exceeds the maximum Static Friction force, the object begins to move, and kinetic friction takes over. In general, the kinetic friction coef- f- f ficient is less than the Static Friction coefficient: m k , m s . The magnitude of the Static Friction force f s s acting on an object at rest satisfies acting on an object at rest satisfies the inequality 0 # f s s # f s s , max 5 m s n [4.15] where n is the normal force and is the normal force and m s is the coefficient of the s is the coefficient of the s maximum static fric- tion force between the object and the surface. The force f s s acts in the direc s acts in the direc s - tion opposite that of the impending motion (the motion that results if Static Friction is not sufficient to prevent it). Unlike the kinetic friction force, the Static Friction force takes on any value between zero and its maximum value of m s n, depending on the magnitude of the applied force. A common error is to substitute the maximum value routinely, when in fact that special situation usually doesn’t hold. The Static Friction force self- adjusts, depending on the net force acting on an object. Such self-adjustment is illustrated in Figure 4.20. Static Friction force c Figure 4.19 (a) and (b) When pulling on a trash can, the direction of the force of friction ( f S s in part (a) and in part (a) and f S S k in part (b)) between the can and a rough surface is opposite the direction of the applied force F S S . (c) A graph of the magnitude of the friction force versus applied force.
- 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 - 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 .
- H. D. Ram, A. K. Chauhan(Authors)
- 2015(Publication Date)
- Cambridge University Press(Publisher)
2 FRICTION 2.1 Introduction Friction is a kind of natural force. It develops between two surfaces in contact when either there is a tendency of relative motion or a relative motion works between them. Friction is not a reaction force. It has got numerous applications in day to day life. It is the friction, which makes walking possible on the earth. There are situations where friction is undesirable. For example, bearings, fluid transmission pipes, power screws, etc. There are many engineering applications where friction is very much desirable like brakes, clutches, power transmission devices, etc. Frictional forces can be classified in following categories: (i) Dry friction (ii) Wet friction Dry Friction: The friction between two solid surfaces is called dry friction. Wet friction: The friction in thin film of liquid layer sandwiched between two solid surfaces is called wet friction. Dry friction is generally large compared to wet friction. Lubricants are introduced to reduce the friction between two solid surfaces. In this chapter, we shall confine the discussion to dry friction only. Dry friction can be further classified as: • Sliding friction: The friction between two plane surfaces, which slip or slide over each other, is called sliding friction. • Rolling friction: The friction between rolling surfaces is called rolling friction. Sliding friction occurs when both the surfaces in contact are plane, whereas rolling friction occurs when at least one of the surfaces is curved. 2.2 Nature of sliding friction (i) Frictional force always tries to retard the relative motions between the surfaces in contact. Friction | 83 Suppose bodies A and B are in contact and are moving relative to each other (Figure 2.1a). Body B is moving to the right of A. The relative motion between two surfaces can be retarded by applying a force on A so that it moves towards left as shown in Figure 2.1b.
eBook - PDF- John Williams(Author)
- 2005(Publication Date)
- Cambridge University Press(Publisher)
4.2 Static and kinetic friction: stick-slip effects If we carry out in practice the simple demonstrations of the laws of friction illustrated in Fig. 4.1 then another characteristic of virtually all dry sliding frictional contacts becomes apparent: the frictional force required to initiate motion is more than that needed to maintain the surfaces in the subsequent relative sliding. In other words the coefficient of Static Friction n s is greater than that of kinetic friction ju k ; sometimes this is known as the third of Amontons' laws of friction. This feature, together with the inevitable natural elasticity of any mechanical system, can often lead to the troublesome phenomenon of stick-slip motion. Figure 4.6 represents, in its simplest form, the essential mechanical features of the sliding system: the spring of stiffness k allows for the natural elasticity or compliance of the loading mechanism, while A models its 146 The friction of solids Fig. 4.6 A model sliding system consisting of a mass m, spring of stiffness m, and dashpot with rate A. inherent damping. The idealized damper or dashpot in the figure generates a resistive force which is proportional to the relative velocity of its end fixings. Mechanical systems in which A is small are especially prone to stick-slip oscillation. When the lower specimen moves from left to right the block will be carried with it, so stretching the 'spring'; this continues until, when the spring force equals the Static Friction, the block will begin to slip. The system will then start to execute damped simple harmonic motion until the forward velocity of the block again matches that of the lower surface. When this happens the two surfaces will 'stick' so that the displacement time curve once again becomes linear; this continues until the spring force is again large enough to dislodge the block and the cycle recommences. The resultant displacement-time characteristic is illustrated in Fig.
eBook - PDFApplied Mathematics
Made Simple
- Patrick Murphy(Author)
- 2014(Publication Date)
- Butterworth-Heinemann(Publisher)
(2) Experimental Investigation of Static Friction A small block of wood of mass M is placed on a rough horizontal board, and a spring-balance is attached to the block as shown in Fig. 160. The spring is gradually pulled to one side at right-angles to the edge of the block to which it is attached until the block begins to move. Readings are taken from the spring both at the time of just moving and immediately after-wards when the block is being pulled along very slowly. The experiment is 184 Applied Mathematics Made Simple repeated about ten times and averages taken of the different readings T x for start of motion and T 2 during motion. In order to ensure that the same sur-faces are in contact, the block must be returned to its original position each time a reading is taken. The block is now turned over so as to present a different area of the same material in contact with the board, and the experi-ment is repeated. Fig. 160 We now vary the normal reaction (but not the surfaces) by placing extra masses on top of the block. For convenience we increase TV to l i , 2, 2 and 3 times its original value, and repeat the experiment. Results: (i) Throughout the experiment we have F = Γ, the tension in the spring. When the block is just starting to move, the force T x measures the limiting force of friction Fi which is opposing motion; just afterwards, as the block is gently moved forward, the force T 2 measures the friction force F 2 which is opposing sliding. The first observation is that F 2 < F x . (ii) The same results will be obtained for either face of the block, i.e. the force of limiting friction is independent of the area of contact. (This is only true provided the normal reaction is not too great, otherwise the surfaces are crushed at points of contact and they begin to adhere to each other.) (iii) For the two surfaces in question there is always a fixed relation between Fi, F 2 , and N.
No longer available |Learn more- Ping YI, Jun LIU, Feng JIANG(Authors)
- 2022(Publication Date)
- EDP Sciences(Publisher)
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. Static equilibrium. When force F is very small, the block would be still in static equilibrium. Two surfaces tend to, but do not, move relatively. To hold the block in equilibrium, there should be a normal reactive force N = P and a tangent reactive force F s = F from the ground, figure 5.3b. This tangent reactive force, which pre- vents or retards sliding of the block relative to the ground, is the friction force or Static Friction force. It should be noted that normal force N is located at distance d to the right of the line of action of P. This distance can be determined by writing the moment equilibrium equation about point O, i.e., Pd – Fh = 0 or d = Fh/P . FIG. 5.2 – Sliding friction and rolling resistance. FIG. 5.3 – Three states of the block and the corresponding friction forces. 172 Engineering Mechanics To fully understand these two reactive forces, it is necessary to consider that the contacting surfaces are nonrigid and have irregularities. Then the actual reactions include nonuniform distributed normal reaction q N and nonuniform distributed frictional reaction q Fs along the contacting surface, as shown in figure 5.3c. N and F s are actually the resultants of the distributed normal and frictional forces. Impending Motion. If the magnitude of F is increased gradually, fictional force F s increases correspondingly, continuing to oppose F, until it approaches a certain maximum value F smax and the block is on the verge of sliding, figure 5.3d. If F is further increased, the friction force could not balance it anymore and the block would start sliding. Besides this impending sliding, there is another type of impending motion: impending tipping.
- A. P. Roberts(Author)
- 2003(Publication Date)
- Cambridge University Press(Publisher)
We shall refer to this maximum value as limiting friction . The laws of friction for dry surfaces in contact are as follows. 1. Limiting friction is proportional to the normal component (perpendicular to the surface) of reaction between the surfaces in contact. 2. Limiting friction is independent of the area in contact. 3. The first two laws still apply for friction when there is sliding. Referring to friction when there is sliding as kinetic friction and to friction when there is no sliding as Static Friction , for any two dry surfaces in contact, the magnitude of kinetic friction is less than that of limiting Static Friction. For a low velocity of slide, the kinetic friction is independent of velocity. 98 99 7.2 Sliding or toppling? P N R W F l Figure 7.1. A block on the point of slipping. The first law can be expressed simply by the equation F l = µ N , where F l is limiting friction, N is the normal component of reaction and µ is the proportionality constant called the coefficient of friction . The value of µ depends on the nature and materials of the surfaces of contact. The value of µ is often about 0 . 5, as in the case of leather on wood, metal on wood and masonry on dry clay. It is less for metal on metal, more like 0 . 2, and with some materials it can be much smaller, e.g. for steel on ice it is about 0 . 03. The value of µ for a motor car’s tyres on dry concrete is about 1 . 0, so the maximum braking force available is likely to be about the same as the weight of the vehicle. Since kinetic friction is less than limiting Static Friction, a driver will not bring a vehicle to a halt in the shortest possible distance if the brakes are applied so hard that the tyres skid. It is for this reason that many cars are equipped with ABS (antilock braking system). Let a block of weight W rest on a horizontal surface, as shown in Figure 7.1. Apply a pull P which is just sufficient for the block to be on the point of slipping.
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