Contact Forces

3 Key excerpts on "Contact Forces"

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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)

  As we know, biomechanics is concerned with the study of forces and the effects of these forces on living things. Most of the forces with which we are concerned in biomechanics tend to be external forces that are acting on the body or object of interest (the forces that cause the body to move). External forces are outside of the body (external) and these can be both contact and non-contact type of forces (gravity could be described as a non-contact external force). Internal forces are forces that are within the body (internally) and these are usually forces that result from the net effect of the external forces. The net force on the player’s foot as he/she kicks a soccer ball would be an external force whereas the force on the anterior cruciate ligament in the knee caused by the kicking action would be an internal force. In mechanics (and biomechanics) it is important to distinguish between these types of forces. For example, a force applied at part D in a body or object will tend to distort some other part of the body (i.e., part E). The forces between the two parts of the body (D and E) are called internal forces. If the body is in equilibrium (when the algebraic sum of the all the forces or moments acting is zero) under the action of external forces both the external and internal force systems are separately in equilibrium.

  Forces can be resolved into individual component parts, such as vertical and horizontal forces.

  Fig. B6.1

  shows the Contact Forces that exist between the foot and the ground at heel strike during running (sagittal plane only).

  The ground reaction force (GRF) that exists as a result of the foot contacting the ground at heel strike in walking is the result of all the reaction forces acting between the foot and the ground during this contact (i.e., in three dimensions). This GRF, which is only shown in the sagittal plane (two dimensions) in

  Fig. B6.1

  , can be resolved into two components which are shown as a vertical and horizontal component. In this case it is again important to point out that we are only considering this (

  Fig. B6.1

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  Engineering Principles for Electrical Technicians

  The Commonwealth and International Library: Electrical Engineering Division

  • K. M. Smith, P. Holroyd, N. Hiller(Authors)
  • 2013(Publication Date)
  • Pergamon

    (Publisher)

  CHAPTER 1

  Forces and Equilibrium

  Publisher Summary

  When we think of forces we usually consider the action of lifting a weight, bending a piece of wire or setting a vehicle in motion. Whenever we move, or move other objects around us, then forces are brought into play. One of the most important forces is the one that causes objects to fall downwards, the Force of Gravity. This chapter discusses the types of force, effects of forces, and the equilibrium of forces. The effect of a number of forces acting on a body depends not only on the magnitude of the forces and their directions, but also on the points at which the forces are applied to the body. This action of the force turning about an axis or fulcrum is called the turning moment or moment of the force, or torque. The chapter also presents the experiment to find the coefficient of friction between two surfaces in contact.

  1.1 Types of force

  When we think of forces we usually consider the action of lifting a weight, bending a piece of wire or setting a vehicle in motion. Whenever we move, or move other objects around us, then forces are brought into play. In fact, approximately three centuries ago Sir Isaac Newton gave us a definition of a force which includes all the forces that we can imagine. “Force changes or tends to change the state of a body’s rest or uniform motion in a straight line.” One of the most important forces we meet is the one which causes objects to fall downwards, the Force of Gravity .

  It has been found that between all bodies there is a force of attraction defined as the gravitational force. The larger the bodies the greater the attraction between them, but as the distance between the bodies increases the attraction decreases. Normally bodies on the surface of the earth do not show this attraction between themselves because the force is very small, but if we consider two masses, one of which is the earth, then the force of attraction between these masses can be considerable. The earth being the larger body will not move perceptibly and therefore the smaller body, if free to do so, will move towards the centre of the earth. This force of gravity acting on the body is called its Weight

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  Principles and Applications of Tribology

  • Bharat Bhushan(Author)
  • 2013(Publication Date)
  • Wiley

    (Publisher)

  In a frictionless contact, the contact stresses and deformations are unaffected by the sliding motion. Sliding motion or any tendency to slide introduces a tangential force (or traction) referred to as friction force F, active on both surfaces in a direction opposite to the sliding direction. During steady sliding motion, the friction force F represents the “kinetic” or “dynamic” friction between the surfaces. In the case of two bodies with no relative velocity but tending to slide (incipient sliding), the friction force arises from “static” friction (Chapter 5). The static friction force is greater than or equal to the kinetic friction force. From Amontons' law the friction force is proportional to the normal force (Chapter 5), F = μW, where μ is a constant known as the coefficient of friction. The tangential force at the contact surface affects the stress distributions and size and shape of the contact area. If the two solids sliding past each other are homogeneous and have the same elastic constants, any tangential force transmitted between them gives rise to equal and opposite normal displacements of any point on the interface. Thus, the warping of one surface conforms exactly with that of the other and does not disturb the distribution of normal pressure. The shape and size of the contact area are then fixed by the profiles of the two surfaces and the normal load, and are independent of the tangential force. With solids of different elastic properties (E, ν), this is no longer the case and the tangential forces do interact with the normal pressure. The influence of a difference in elastic constants has been analyzed by Bufler (1959). He has shown that the contact area and contact pressure distribution are no longer symmetrically placed; their center is displaced from the axis of symmetry and the contact pressure no longer has circular distribution. These differences are the function of differences in the elastic constants and coefficient of friction

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