Simple Nature - Light and Matter

forces, as if the hand were a pair of pliers.force acting on personrope’s static frictional force onpersonrope’s normal force onpersonrope’s normal force onpersonforce related to it by Newton’sthird lawperson’s static frictional forceon ropeperson’s normal force onropeperson’s normal force onrope(There are presumably other forces acting on the person as well,such as gravity.)If a rope goes over a pulley or around some other object, thenthe tension throughout the rope is approximately equal so long asthe pulley has negligible mass and there is not too much friction. Arod or stick can be treated in much the same way as a string, butit is possible to have either compression or tension.Discussion QuestionA When you step on the gas pedal, is your foot’s force being transmittedin the sense of the word used in this section?3.2.8 WorkEnergy transferred to a particleTo change the kinetic energy, K = (1/2)mv 2 , of a particle movingin one dimension, we must change its velocity. That will entail achange in its momentum, p = mv, as well, and since force is the rateof transfer of momentum, we conclude that the only way to changea particle’s kinetic energy is to apply a force. 6 A force in the samedirection as the motion speeds it up, increasing the kinetic energy,while a force in the opposite direction slows it down.Consider an infinitesimal time interval during which the particlemoves an infinitesimal distance dx, and its kinetic energy changesby dK. In one dimension, we represent the direction of the forceand the direction of the motion with positive and negative signs forF and dx, so the relationship among the signs can be summarizedas follows:6 The converse isn’t true, because kinetic energy doesn’t depend on the directionof motion, but momentum does. We can change a particle’s momentumwithout changing its energy, as when a pool ball bounces off a bumper, reversingthe sign of p.160 Chapter 3 Conservation of Momentum