6 Key excerpts on "Contact Forces"

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  eBook - ePub

  Instant Notes in Sport and Exercise Biomechanics

  Second Edition

  • Paul Grimshaw, Michael Cole, Adrian Burden, Neil Fowler(Authors)
  • 2019(Publication Date)
  • Garland Science

    (Publisher)

  Figure B3.1 ). It is worth remembering that in reality the forces acting will be in three dimensions and there will also be a medial–lateral (side to side) GRF component.

  The force that is perpendicular to the surface (vertical) is called the normal force and this always acts at 90 degrees to the contact surface. The force acting parallel to the surface of contact (horizontal) is termed the friction force . Friction forces act between any two surfaces that are in contact and oppose motion or sliding between two objects.

  The friction force is essential to human movement and locomotion, and without frictional forces between two objects, it would be very difficult to initiate and maintain movement. For example, imagine trying to run across an ice rink in normal shoes. The frictional force between the ice and the shoe is very small and the result is a slipping of the foot during locomotion. The relationship between the two surfaces in contact that gives rise to friction can be described by what is termed the coefficient of friction. This is represented by the Greek letter mu (µ ).

  Figure B3.1   Normal and frictional forces at heel strike during walking (sagittal plane components only are shown (two-dimensional))

  To illustrate the coefficient of friction, Figure B3.2 describes the friction force that develops as a stationary bobsleigh on a horizontal surface is brought to motion. In Figure B3.2a the stationary bobsleigh has only two forces acting on it, its weight (W) and the ground reaction force (GRF). Because the surface is flat, the GRF is perpendicular, or normal (N) to the surface. A small horizontal force (Fa) is applied to the bobsleigh in Figure B3.2b and, in opposition to this force, a small friction force exists between the bobsleigh and the ice. Because these two forces are equal in magnitude, but opposite in direction, the bobsleigh remains stationary. In Figure B3.2c the bobsleigh driver increases the applied horizontal force and the frictional force increases accordingly. If the driver applies any more force the bobsleigh will slide forward and the frictional force will decrease slightly. Thus, the friction force in Figure B3.2c is termed the maximal frictional force (Fmax) . The point immediately before motion occurs is the coefficient of static friction ( µ s )

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  eBook - ePub

  BIOS Instant Notes in Sport and Exercise Biomechanics

  • Paul Grimshaw, Neil Fowler, Adrian Lees, Adrian Burden(Authors)
  • 2007(Publication Date)
  • Routledge

    (Publisher)

  dynamic dry friction (when one or both of the objects in contact are in motion). The friction force, whether in the static or dynamic situation, depends on the type and nature of each surface in contact. For example, different surfaces in contact will have different coefficients of friction. Similarly, different roughness of surfaces in contact will also have different frictional properties: steel and plastic (as used in artificial hip joint replacements) have very low coefficients of friction and move easily over each other; a rough surface acting on another rough surface will have frictional properties different from two smooth surfaces acting together and it should be easier to slide or move the smooth surfaces across each other. Many of these examples can be seen throughout sport and human movement, for example the type of grip on the javelin; the chalk used by weightlifters or gymnasts for better grip; the table tennis bats with rough and smooth surfaces; and even soccer boots with modified uppers for better contact and control of the ball.

  The frictional force that is created between the contact of two objects is independent (not connected with) of the surface area of contact. For example, place a book on a table and try to push it. Now open the outside covers of the book place it flat on the table and try to push it again. The book with its covers closed will create the same frictional force as the book open with both its outside covers in contact (effectively doubling its contact area). The reason for this is that although you have increased the surface area of contact (i.e., when you opened the book) you have also distributed the same mass over a larger area of contact, and have thus created a smaller average force because it is spread over a larger area (the net result of both conditions is the same because you have not changed the mass of the book). In other words, you have maintained the mass of the book but spread it over a larger area thus making each small contact force less – because you have spread the initial load over double the surface area.

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  eBook - ePub

  Foundations of Mechanical Engineering

  • A. D. Johnson(Author)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  Dry friction, sometimes called Coulomb friction after the engineer and physicist C. A. Coulomb, occurs between contacting surfaces in the absence of any lubricating fluid.

  An appreciation of the effects of friction may be gained by considering a block resting on a horizontal surface, Figure 6.2 . The block applies a normal load N to the surface due to gravity acting on its mass. The force P is necessary to move the block along the surface and increases until the block is on the point of moving. At this point

  applied force

  = resisting force (friction)

                                                  P

  =

  F r

  Fig. 6.2 Block resting on a horizontal surface

  Any addition to the applied force P will merely accelerate the block. There are four important concepts which relate to friction and these are considered below.

  1. The friction force F r is proportional to the normal load N . This concept is true only within the limits indicated in Figure 6.3 , which shows the relationship between F r and N . The useful portion is limited to the straight line section of the curve. Since F r and the normal force
     Fig. 6.3 The limits of the linear relationship between the friction force and the normal load.
     N are proportional, their relationship is constant and can be written:
     F r

     N

     = μ

     ⁢ ( 6.1 )
     where μ is the coefficient of friction. This can be arranged to give the basic equation:
     F r

     = μ N

     ⁢ ( 6.2 )
     When the block is on the point of moving the applied force is
     P =

     F r

     = μ N

     ⁢ ( 6.3 )
  2. The friction force is independent of area of contact between the surfaces. In order to understand how the friction force can be independent of area, consideration must be given to the source of friction. It is generally understood that dry friction results from point contact between the microscopically rough surfaces, as shown in Figure 6.1 . When on the point of moving, the applied force P is the same as the friction force

     Fτ

     and it is this force which is needed to move the block over the contact points.
     Increasing the area creates more contact points and proportionally distributes the normal force N . Hence if N remains unchanged, then, no matter what the area, the friction force will also remain unchanged. Since the friction force remains unchanged it follows that the coefficient of friction, μ

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  eBook - ePub

  Basic Engineering Mechanics Explained, Volume 1

  Principles and Static Forces

  • Gregory Pastoll, Gregory Pastoll(Authors)
  • 2019(Publication Date)
  • Gregory Pastoll

    (Publisher)

  Chapter 4 Forces

  • The nature and origins of forces

  • Mass, weight and gravitation

  • Scalars and vectors

  • Vector addition of forces: resultants and equilibrants

  • Components of a force: general and rectangular components

  • Determining a resultant by summation of rectangular components

  • The principle of transmissibility of a force

  • Equilibrium conditions for a particle in 2-D

  The nature and origins of forces What is a force?

  It is difficult to define a force, because forces are not visible. You can only see or feel their effect. When a crane lifts a heavy container into the air, you understand that it must be exerting a force in order to overcome the gravitational force that the earth exerts on the container. However, you can’t see the force itself.

  The different effects that forces can have Forces acting on solid objects can have any combination of the following visible effects: They can resist other forces; move an object; rotate an object; or deform an object. If any one of these effects is observed, one can deduce that a force must be acting.

  It is impossible for any agent, human or otherwise, to just ‘exert’ a force, without having something to exert it on. Just try, for example, to push your arms sideways with a force of 200 N when there is nothing to push against. It is impossible. You may have the capability of exerting that amount of force, but unless there is some resistance there to oppose your effort, the actual force you are exerting is limited to that needed to push the air out of the way of your arms. So, a force cannot exist on its own, without pushing or pulling against some resistance.

  This fact is so important that it gives rise to what this author calls The zeroth law of mechanics : a force cannot even exist unless it is opposed

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  eBook - ePub

  Extreme Tribology

  Fundamentals and Challenges

  • Ahmed Abdelbary(Author)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)

  3

  Friction

  3.1    Introduction

  Friction, as a phenomenon, is the force that resists sliding and is described in terms of a coefficient of friction. Two physically different forces of friction should be distinguished: The static friction force and the kinetic friction force. Static friction force is the minimal force needed to initiate sliding. Its value is related to the atomic structure of the sliding surfaces and the adhesion interactions. Kinetic friction force is the force needed to keep the motion. Moreover, it can be considered as the mechanism that converts the energy of motion into dissipated heat. Both forces are highly important. In various tribological applications, either a high or low value of friction may be required.

  The microscopic mechanisms that generate friction are: Adhesion, mechanical interlocking of surface asperities (ploughing by surface asperities) deformation and fracture, plastic deformation by wear particles, and third bodies (Hsu, 1996).

  We mentioned in Chapter One that there are two regimes of friction, namely, static friction between non-moving surfaces and kinetic (or dynamic) friction between moving surfaces. Accordingly, if two solid surfaces are loaded together and a tangential force is applied, then the static friction force is the value of the tangential force which is required to initiate sliding. Likewise, the kinetic friction force is the tangential force which is required to maintain sliding. From an engineering point of view, friction is a major cause of energy waste dissipated as heat and a major cause of failure in machinery components (Bayer, 2002).

  Generally, friction depends very markedly on three main factors: The area of contact between the mating surfaces, the nature of the adhesion or junction at the regions of contact, and the way in which the formed junctions are sheared during sliding (Rothbart and Brown, 2006). For rough surfaces sliding over each other, the friction force is proportional to the real area of contact between both surfaces, which is smaller than the apparent (or nominal) contact area. In mechanical systems, friction may be increased or decreased depending on the contact between sliding surfaces and operating parameters such as roughness, degree of work hardening, and surface cleanliness. Understanding of friction mechanisms requires understanding of surface interaction and the mechanism involved between contacts.

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  eBook - ePub

  Biomechanics of Human Motion

  Applications in the Martial Arts, Second Edition

  • Emeric Arus, Ph.D.(Authors)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  Figure 8.1 using the FBD.

  To calculate the GRF we need to know the weight and the gravity. GRF = w ⋅ g. But this calculation is good only if the total CoM is at the lower abdomen part of the body. If the athlete runs, he has contact with the ground only with one leg; then the CoM is not in the middle of the body and in this case the calculation is different from the aforementioned formula.

  8.3 FRICTION

  The force of friction ( f ) is proportional to the force pressing two surfaces together. The force of friction always opposes any motion. There are different types of friction , such as static and kinetic , which includes the sliding and rolling frictions. An object that does not move with respect to the surface on which it rests is subjected to static friction.

  In martial arts, the dominant force of friction is the sliding force. This is a counterforce of a pushing action against an opponent. For example, a wrestler’s leg force pushes the opponent’s body; it is opposed by a counterforce that acts backwards of the pushing force of the wrestler, and this is the friction force.

  Imagine two judoka ready to grab each other’s kimono. At this time, there is much pushing involved for the action of grabbing (Kumi-kata ) of the kimono. Judoka A on the left side pulls Judoka B on the right side. Let us pretend the magnitude of the limiting friction force is 150 N. If Judoka A exerts a horizontal force of only 110 N against Judoka B, the magnitude of the friction will also be 110 N and Judoka A will not be able to move Judoka B horizontally.

  If Judoka A increases his pulling force to 150 N, he will still not be able to pull Judoka B; this is because the magnitude of the limiting friction force equals Judoka A’s pulling force. If Judoka A is able to pull Judoka B with a force over the limiting friction force of the opponent, for example, pulling with 160 N, then the limiting friction force will not be able to sustain the effect of the friction force. Judoka B will be moved.

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