Contact Forces Introduced12 of 22Constants | Periodic TableLearning Goal:To introduce contact forces (normal and friction forces) and to understand that, except for friction forcesunder certain circumstances, these forces must be determined from: net Force = ma.Normal and friction forcesTwo solid objects cannot occupy the same space at the same time. Indeed, when the objects touch, theyexert repulsive normal forces on each other, as well as frictional forces that resist their slipping relative toeach other. These contact forces arise from a complex interplay between the electrostatic forces betweenthe electrons and ions in the objects and the laws of quantum mechanics. As two surfaces are pushedtogether these forces increase exponentially over an atomic distance scale, easily becoming strongenough to distort the bulk material in the objects if they approach too close. In everyday experience,contact forces are limited by the deformation or acceleration of the objects, rather than by thefundamental interatomic forces. Hence, we can conclude the following:Two types of contact forces operate in typical mechanics problems, the normal and frictional forces, usually designated by n and f (or Firic, or something similar) respectively. These are the components of the overall contact force: n perpendicular toand f parallel to the plane of contact.Kinetic friction when surfaces slideWhen one surface is sliding past the other, experiments show three things about the friction force (denoted fk):The magnitude of contact forces is determined by EF=må, that is, by the other forces on, andacceleration of, the contacting bodies. The only exception is that the frictional forces cannot exceed un(although they can be smaller than this or even zero).1. The frictional force opposes the relative motion at the point of contact,2. fr is proportional to the normal force, and3. the ratio of the magnitude of the frictional force to that of the normal force is fairly constant over a wide range of speeds.The constant of proportionality is calledcoefficient of kinetic friction, often designated lz. As long as the sliding continues,frictional force is thenfk = Hkn (valid when the surfaces slide by each other).Static friction when surfaces don't slideWhen there is no relative motion of the surfaces, the frictional force can assume any value from zero up to a maximum gn, where ls is the coefficient of static friction. Invariably, ls is larger than k, in agreement with the observation that when a forceis large enough that something breaks loose and starts to slide, it often accelerates.The frictional force for surfaces with no relative motion is thereforefs < Hn (valid when the contacting surfaces have no relative motion).The actual magnitude and direction of the static friction force are such that it (together with other forces on the object) causes the object to remain motionless with respect to the contacting surface as long as the static friction force required does notexceed l, n. The equation fs = Hsn is valid only when the surfaces are on the verge of sliding.Part AWhen two objects slide by one another, which of the following statements about the force of friction between them, is true?O The frictional force is always equal to n.O The frictional force is always less than n.O The frictional force is determined by other forces on the objects so it can be either equal to or less than l n.SubmitRequest AnswerPart BWhen two objects are in contact with no relative motion, which of the following statements about the frictional force between them, is true?O The frictional force is always equal to l,n.O The frictional force is always less than l, n.O The frictional force is determined by other forces on the objects so it can be either equal to or less than gn.SubmitRequest AnswerPart CWhen a board with a box on it is slowly tilted to larger and larger angle, common experience shows that the box will at some point "break loose" and start to accelerate down the board.The box begins to slide once the component of gravity acting parallel to the board F. just begins to exceeds the maximum force of static friction. Which of the following is the most general explanation for why the box accelerates down the board?

Question A Answer a) option When two objects are in motion, static friction opposes the motion of…

/designe mal forces on each other, as well as frictional forces that resist their slipping relative to contact forces arise from a complex interplay between the electrostatic forces between lons in the objects and the laws of quantum mechanics. As two surfaces are pushed ces increase exponentially over an atomic distance scale, easily becoming strong the buik material in the objects ifthey approach too close. In everyday experience, limited by the deformation or acceleration of the objects, rather than by the atomic forces. Hence, we can conclude the following contact forces is determined by EF = mä, that is, by the other forces on, and e contacting bodies. The only exception is that the trictional forces cannot exceed jun n be smaler than this or even zero) and f paralel to the plane of contact. Kinetic friction when surfaces slide When one surface is siding past the other experiments show three things about the friction force (denoted f a 1. The frictional force opposes the relative motion at the point of contact. 2 f is proportional to the nomal force, and 3. the ratio of the magnitude of the frictional force to that of the normal force is fairty constant over a wide range of speeds The constant of proportionality is called the coefficient of kinetic triction, often designated a As long as the sliding continues, the frictional force is then = Pan (valid when the surfaces side by each other). Static friction when surfaces don't slide When there is no relative motion of the surfaces, the trictional force can assume any value from zero up to a maximum n where is the coeficient of static triction. Invariably. is larger than a in agreen is large enough that something breaks loose and starts to slide, it offen accelerates. The frictional force for surfaces with no relative motion is therefore f

11th Edition

ISBN:9781305952300

Author:Raymond A. Serway, Chris Vuille

Publisher:Raymond A. Serway, Chris Vuille

Chapter1: Units, Trigonometry. And Vectors

Section: Chapter Questions

Problem 1CQ: Estimate the order of magnitude of the length, in meters, of each of the following; (a) a mouse, (b)...