Energy can change from one form to another without a net loss or gain. Energy Notes • Definition of energy: • Energy can be stored, transferred and used. • Definition of work: • Work is done when a force causes something to move, and, as a result, energy moves from place to place or changes from one form to another. • SI Unit for Work and Energy: • Difference between kinetic energy and potential energy Kinetic energy The energy of motion The ability to cause change. Every change involves energy. W = Fd (Work equals force times distance) Joules (J) Potential energy The energy of position or stored energy Energy Notes • What is the equation for KE? • How does KE increase or decrease? KE = 1/2mv2 Kinetic energy = ½ mass x velocity2 Increase or decrease the velocity of the object (change speed). Double your speed, Quadruple the KE Prove it: Calculate the KE of a 2500 kg car traveling at 20 m/s and at 40 m/s • • KE at 20 m/s (500,000 J) KE at 40 m/s (2,000,000 J) More mass, same speed, more KE. Double the mass, double the KE Prove it: Calculate the KE of a 100 kg cart and a 200 kg cart, each traveling at 15 m/s • • 100 kg cart at 15 m/s 200 kg cart at 15 m/s • (11,250 J) (22,500 J) Definitions • • • • Energy (the ability to cause change) can be transferred, stored or used Work (net force x distance moved) method of energy use, transfer and storage • Power (work per unit time) • the rate at which work is done or energy is transferred Chapter 9.1 Work 9.1 Work Energy is the ability to cause change. Work is done when a net force acts on an object and the object moves in the direction of the net force (force causes displacement). Work involves a transfer of energy between something and its surroundings. 9.1 Work Two categories of work: 1) Work done against another force a) Transfer of energy to change position or store energy—work against another force Ex: Pulling back the string on a bow stores energy. b) Transfer of energy (one form to another) Ex: Rubbing your hands together transfers one form (kinetic) of energy to another (thermal). 2) Work done to change the speed of an object Ex: Bringing an automobile up to speed increases its (kinetic) energy. 9.1 Work = force × distance Did the weightlifter do work on the barbell and weights? •Yes, when he first lifted them above his head. Is the weightlifter currently doing work on the barbell and weights? No, the barbell and weights are not moving. •Explain two ways that the work done by the weightlifter be increased. 1) Increase the weight on the ends of the barbell 2) Increase the distance over which the weightlifter pushes the barbell and weights. 9.1 Work Work has the same units as energy Joules Newton x meter J Nxm •One joule (J) of work is done when a force of 1 N is exerted over a distance of 1 m (lifting an apple over your head). 9.1 Work If we lift two loads, we do twice as much work as lifting one load the same distance, because the force needed is twice as great. If we lift one load twice as far, we do twice as much work because the distance is twice as great. 9.1 Work While the weight lifter is holding a barbell over his head, he may get really tired, but he does no work on the barbell. Work may be done on the muscles by stretching and squeezing them, but this work is not done on the barbell. When the weight lifter raises the barbell, he is doing work on it. 9.1 Work think! Suppose that you apply a 60-N horizontal force to a 32-kg package, which pushes it 4 meters across a mailroom floor. How much work do you do on the package? 9.1 Work think! Suppose that you apply a 60-N horizontal force to a 32-kg package, which pushes it 4 meters across a mailroom floor. How much work do you do on the package? Answer: W = Fd = 60 N × 4 m = 240 J 9.1 Work When is work done on an object? When is work not done on an object?