CHAPTER 1 ENERGY BASICs TI rock falling off a cliff is different from the same rock lying on the ground below A rubber band pulled taut is different from the same rubber band left slack A glowing lightbulb is different from the same bulb when the electricity is switched off It is the same rock the same rubber band the same lightbulb The differ ence is one of energy Energy is one of the most basic ideas of science All activity in the universe can be explained in terms of energy and matter But the definition of energy is not at all simple since energy occurs in many different forms and it is not always easy to tell how these forms are related to one another and what they have in common One of the best known defini tions of energy is the classical definition used in physics Energy is the ability to do work The many forms of energy include kinetic potential chemical nuclear electrical light heat and sound energy All these types of energy can do work IO
ENERGY BASICs ICE Energy is the difference between the glowing bulb in the center and the dark ones that surround it Shutterstock com ENERGY SSTEMs Energy is readily transferred from object to object especially in the form of heat For this reason it is often necessary to study an entire group of objects that may be transferring energy back and forth among themselves Such a group is called a system The energy of a system is the ability of the entire system II
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
