Simple Nature - Light and Matter

3.2.4 Forces between solidsConservation laws are more fundamental than Newton’s laws,and they apply where Newton’s laws don’t, e.g., to light and to theinternal structure of atoms. However, there are certain problemsthat are much easier to solve using Newton’s laws. As a trivialexample, if you drop a rock, it could conserve momentum and energyby levitating, or by falling in the usual manner. 5 With Newton’slaws, however, we can reason that a = F/m, so the rock mustrespond to the gravitational force by accelerating.Less trivially, suppose a person is hanging onto a rope, and wewant to know if she will slip. Unlike the case of the levitating rock,here the no-motion solution could be perfectly reasonable if her gripis strong enough. We know that her hand’s interaction with the ropeis fundamentally an electrical interaction between the atoms in thesurface of her palm and the nearby atoms in the surface of the rope.For practical problem-solving, however, this is a case where we’rebetter off forgetting the fundamental classification of interactionsat the atomic level and working with a more practical, everydayclassification of forces. In this practical scheme, we have three typesof forces that can occur between solid objects in contact:A normal force, F n ,Static friction, F s ,Kinetic friction, F k ,is perpendicular to the surface of contact,and prevents objects from passingthrough each other by becoming asstrong as necessary (up to the pointwhere the objects break). “Normal”means perpendicular.is parallel to the surface of contact, andprevents the surfaces from starting toslip by becoming as strong as necessary,up to a maximum value of F s,max .“Static” means not moving, i.e., notslipping.is parallel to the surface of contact, andtends to slow down any slippage onceit starts. “Kinetic” means moving, i.e.,slipping.self-check CCan a frictionless surface exert a normal force? Can a frictional forceexist without a normal force? ⊲ Answer, p. 922If you put a coin on this page, which is horizontal, gravity pullsdown on the coin, but the atoms in the paper and the coin repel eachother electrically, and the atoms are compressed until the repulsionbecomes strong enough to stop the downward motion of the coin.We describe this complicated and invisible atomic process by saying5 This pathological solution was first noted on page 83, and discussed in moredetail on page 910.152 Chapter 3 Conservation of Momentum