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Monday January 11 Tuesday January 12 Unit Question What is energy? Guiding Question / Learning Target Identify and calculate kinetic and potential energy. Warm-up What type of energy is responsible for this? Agenda 1. Energy Math Notes & practice 2. Newton’s Cradle & Conservation of Energy Quiz Friday Homework Rest of Semester: Monday Tuesday Wednesday Thursday Friday 11 12 13 14 15 18 NO SCHOOL 19 20 21 COCOA AND CRAM 6–8:30 IN GH (EXTRA CREDIT) 22 LAST DAY FOR MISSING WORK 25 26 2 & 6 FINAL 27 1 & 5 FINAL 28 HALF DAY 3 & 4 FINAL 29 NO SCHOOL There will be no A.T. the first few weeks of second semester, same as there was no A.T. the first few weeks of first semester. You WILL have a final • Study guide next week • Open note Newton’s POE The Law of Conservation of Energy Energy cannot be created or destroyed; it can only change forms. How does the picture describe the Law of Conservation of Energy? Will the girl swing forever? Why or why not? Following Energy’s Trail • If energy is conserved, why do objects that are moving eventually stop (seem to lose energy)? • The energy is converted to other forms • This is why perpetual motion is impossible Calculating GPE and Motion energy • The unit for all energy is Joules (J) Kinetic Energy • The kinetic (motion) energy of any moving object depends on both its mass and velocity. • Velocity = distance(in meters) ÷ time(in seconds) If you know an object’s mass and velocity, you can calculate its kinetic energy. Calculations with KE= ½ mv2 • m = mass in kilograms • v = velocity in m/s Calculations with KE= ½ mv2 • • • • PEMDAS! Start by squaring the velocity Divide your mass by 2 Multiply the answers KE= ½ mv 2 Steps for ½ 2 mv • What is the kinetic energy of a 1500kg car moving at a speed of 29 m/s? 1. Find your velocity 29*29 = 841 2. Square it 3. Find your mass 1500/2 = 750 4. Divide it by 2 5. Multiply your two answers 841*750 = 630750 J Unit! Kinetic Energy (KE) • An object has a lot of KE for two reasons • It has a lot of mass (a lot of weight) • And/or it has lots of velocity (speed) **Note: if you double the mass, it doubles the KE. If you double the velocity, it will QUADRUPLE the KE KE= ½ mv 2 Practice Problems • Calculate the kinetic energy in joules of a 1500kg car moving at the following speeds: i. 29m/s ii. 18m/s iii. 42km/s Answers • Calculate the kinetic energy in joules of a 1500kg car moving at the following speeds: • i. 29m/s KE=1/2mv2=(1/2)(1500kg)(29m/s)2= 630000 J • ii. 18m/s KE=(1/2)(1500kg)(18m/s)2= 240000 J • iii. 42m/s KE=(1/2)(1500kg)(42m/s)2= 1323000 J Potential Energy of Position (gravitational potential) • GPE depends on an objects mass (kg), height (m), and the acceleration due to gravity. PE= mgh More height = more potential energy. More mass = more potential energy. If you know an object’s height and mass, you can calculate its potential energy. Calculations with PE= mgh • m = mass in kilograms • g = acceleration due to gravity 9.8 m/s2 • h = height in meters PGE=weight x height • Weight = mass * gravity • The units for weight in the metric system are Newton (N) • A Newton is a kg-m/s2 Which Equation to use? • Look at your UNITS! • If the mass is given, the unit is kg. • Use mgh • If the weight is given the unit is N. • Use weight x height Practice Problems • Calculate the gravitational potential energy in the following systems: • A car with a mass of 120 kg at the top of a 42 m hill • A 637N climber on top of Mt Everest (8800 m high) • A 0.52 kg bird flying at an altitude of 550 m A car with a mass of 1200 kg at the top of a 42 m hill Which equation? mgh or weight*height PE=mgh=(1200kg)(9.8m/s2)(42m)= 493,920 Joules A 673 N climber on top of Mount Everest (8800m high) Which equation? mgh or weight*height PE=weight x height (673)(8800m)= 5,922,400 J A 0.52kg bird flying at an altitude of 550m Which equation? mgh or weight*height PE=(0.52kg)(9.8m/s2)(550m)= 2802.8 J Example