ENERGY DEFINING WoRK Physicists define work in a way that does not always agree with the average person s idea of work In physics work is done when a force applied to an object moves it some distance in the direction of the force Mathematically W Fs where W is the work done F is the force applied and s is the distance moved If either Fors is equal to zero W is also equal to zero so no work is done DISTANCE FORCE MASS LTS IDF The horse is exerting force to pull the cart The amount of work done by the horse is equal to the force exerted times the distance the cart is pulled Copyright Encyclopaedia Britannica rendering for this edition by Rosen Educational Services
ENERGY BASICs Holding weights steady at shoulder level might qualify as a workout but it does not meet the physics definition of work Jupiterimages Comstock Thinkstock If a person walks up a flight of stairs he may regard it as work he exerts effort to move his body to a higher level In this instance he also does work according to the definition accepted by physicists because he exerts a force to lift himself over a distance the distance from the bottom to the top of the stairs However if a person stands without moving with a 100 pound 45 kilogram weight in her outstretched arms she is not doing any work as physicists define work She is exerting a force that keeps the weight from falling to the floor the position of the but weight remains unchanged It is not moved any distance by the force The person is of course exerting considerable muscular effort to avoid dropping the weight and the average person would say that she is working very hard indeed But she is not doing any work according to the definition accepted in
ENERGY to do work If the parts of a system do work on one another but do not change anything outside the system then the total amount of energy in the system stays the same However the amount of energy in one part of the sys tem may decrease and the amount of energy of another part may increase Consider a system consisting of a rainforest with many trees a vine hanging from the cen tral tree the ground supporting the trees and a monkey standing beneath the tree from which the vine is hanging The monkey holding the free end of the vine climbs up the central tree It then moves several treetops away main taining the same altitude Finally the monkey grasps the vine that is still tied to the central tree and swings down past the central tree and up again until it lands in a third tree An observer watching the monkey swinging from one tree to the other will conclude that the sys tem possesses energy and can do work The necessary elements in this sys tem are the monkey the trees Earth and the vine The monkey provides the ini tial energy by climbing the trees while the trees support the monkey against the force of gravity which pulls downward Earth's gravitational attraction is the force 14
ENERGY BASICs that draws the monkey downward once it begins to swing on the vine The vine sup ports the monkey so that it remains free to swing upward against the force of gravity and into another tree When all these ele ments occur together the system is capable of doing work it has energy KINETIC AND POTENTIAL ENERGY As the monkey swings on the vine it is act ing like a pendulum Like any pendulum it is exhibiting the difference between two kinds of energy kinetic energy and potential energy Kinetic energy is the energy of motion While the monkey is swinging from one tree to the other it has kinetic energy So too does a speeding bus a falling raindrop and a spinning top Any moving object has this type of energy Potential energy is the energy an object or system has because of the position of its parts It is often thought of as stored energy though it is important to remember that energy is not a substance For example a stretched spring has potential energy Force has been applied to stretch the spring I 15
