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Have students do lab: using ramp measure speed at 10 cm height, then predict height for double speed. do 20, 30, 40, -- cm and graph. twice as fast = 4 times the energy (can also do impression into clay with marbles twice as fast = 4 times deep impresssion Start bringing books daily (starting tomorrow). We won’t use them everyday…. but pretty frequently Chapter 8 ENERGY POWER WORK WORK Has a different meaning in PHYSICS WORK W=Fd work done on an object force applied to the object distance the object moves Work and impulse are similar but different Impulse = F Dt (time) Work = F d (distance) Impulse = F Dt Work = F d this is important!! = the change in momentum of an object = the change in ENERGY of an object In order to do work on an object a FORCE has to push or pull an object and MOVE IT Work = F d Force Displacement A person pushes a crate horizontally across a floor with a force of 30 N for a distance of 5 m. How much work is done on the crate? Work = F d N Nm m Also called a Joule (J) Or kg m2 If you prefer s2 Two crates are pushed with equal forces. One is pushed twice as far. Compare the amount of work done. 1m 2m Work = F d Two crates are lifted equal distances, one crate is twice as heavy. 20 kg 10 kg Work = F d A crate is twice as heavy and is lifted twice as far. 20 kg 10 kg Work = F d An inmate pushes hard against a solid wall but it doesn’t budge. No work is done, why? Work done on an object falls into 2 basic situations 1.) Pushing an object against another force Lifting weights (at a constant speed) Pushing a crate across a rough floor (at a constant speed) A person lifts a 650 N weight up 0.45 m. How much work is done on the weight? How would you answer change if the object instead had a MASS of 650 kg? A weight lifter holds 560 N mass over his head for 10.0 seconds. How much work is done? W=Fd 0 Work done on an object falls into 2 basic situations 2.) Pushing an object to accelerate it Speeding up or slowing down a car What is the force doing work on the apple and the car? Work = Force x Distance Lastly in order to count as “work”, the force has to be in the parallel with the displacement of the object The force does work on the box displacement Work = Force x Distance Does this force do work on the box? Force displacement rce Work = Force x Distance Work is not done by the force Only Part of this force will do work Force distance How much work is done carrying a 200 N crate across a level floor for 10 m at a constant velocity? How much work is done? 2 meter 1 meter 1 meter 1/2 meter 10 N 10 N 20 N 20 N SHOW WORK WHERE APPLICABLE do first side of work / power ws??? Move on POWER Again a little different in physics Power: is the rate at which work is done What does rate mean here? Speed is the rate at which distance changes Acceleration is the rate at which velocity changes. POWER = Work done (J) Time it took to do it (s) W P= t W P= t Which is more powerful a motor which can do: 100 J of work in 2 seconds or 100 J of work in 1 seconds Work (J) POWER = Time (s) POWER has units of J/s Also called a watt (W) Or kg m2 If you prefer s3 An electric device like a heater or light bulb is rated in watts. This tells you the rate at which it uses electrical energy. W P= t What is the power of a motor which can do 60 J of work in 2 seconds? 60 J P= = 30 W 2s W P= t keep in mind that the variable W is work but the unit W is Watt for power 60 J P= = 30 W 2s Running up a set of stairs Walking up a set of stairs Which requires more work? Which requires more power? Both motors do the same amount of work but one can do it faster because it has more power 1 kg 1 kg What about here? What’s the same & different? 1 kg 10 kg What about here? 1 kg 1 kg Force x distance POWER = Work Time What is the power requirement on a motor which needs to to lift a 1100 kg car a distance of 3.0 meters in 12 seconds? A motor which is 3 times a powerful could do it in…. Other units of power 1 kW = 1000 W 1 hp = 746 W SHOW WORK WHERE APPLICABLE (* questions) Work/Power WS To review (optional) read pages 103-105 in book What is energy How do you get it? Where does it go? What gives an object like you the ability to do work? ENERGY When you do work on a sledgehammer by lifting it up, it then HAS the ability to do work on something else What it has when it is lifted is… ENERGY ENERGY is the ability to do work. ENERGY has many forms MECHANICAL ENERGY (what we will focus on) has 2 TYPES Potential EnergyEnergy due to the POSITION of an object Kinetic EnergyEnergy due to the MOTION of an object Mucho importante concept Work = DEnergy The amount of work done on/by an object is equal to energy gained or lost An object has potential energy due to its position. This energy can be used to do work on something. How does the bow gain PE? The work done pulling the string back is equal to the PE stored in the bow. Work = F d Because a position is relative ( or compared to something else) Potential Energy is always a comparison (not absolute measurement) 10 J of PE What is the amount of PE in the stretched bow compared to? 10 J of PE If the stretched bow has 10 J of energy and the string was pulled back 0.50 m, with what force was the string pulled. Objects (like a bow) which go back to their original shapes after being deformed are said to be ELASTIC. The energy stored by changing the shape of an elastic object is called ELASTIC POTENTIAL ENERGY Can you think of some elastic objects which could store Elastic PE? Remember when work is done on an object, it gains that amount of energy. Does it take work to LIFT an object? It requires WORK to lift an up object up, and it gains that energy as: Gravitational Potential Energy How much work is done to lift a 30 kg rock up 2 meters? How much PE does it gain? Gravitational PE = weight x height PE = m g h h Energy has the same units as work Joules (J) Or Nm Or kg m2 If you prefer s2 A 5.0 kg rock is lifted 2.0 meters how much PE does it gain? PE = 5.0 kg * 9.8 m/s2* 2.0 m 2.0 m Remember that position is relative. What are we comparing the position of the elevated rock to? PE = 98 J 2.0 m Since it is 2 m above the ground, its PE is 98 J more than if it were on the ground. But that’s not the whole picture 2.0 m You can only compare the PE of an object at 2 different locations 2.0 m 3.0 m How much PE does the rock have compared to the lower floor? 2.0 m 3.0 m In which position does the rock have the most PE, given that all are 3 m above the ground? Neglecting friction the work done in each scenario is the same and each gained the same PE Two magnets repel each other how would you increase their potential energy? What if one is switched so that they both attract? In all of our examples of work so far, the Force that we applied (over a distance) was balanced by another force on the object. such as Gravity Friction Springy Things We can still do work on an object even if we are not working against another force acting on the object. It just becomes a different type of energy KINETIC ENERGY Force distance What things would affect how much energy a moving object has? Its speed or velocity & Mass velocity (m/s) Mass (kg) KE = 2 mv 2 Kinetic Energy = ½ mass x velocity2 Which has a bigger impact on KE, Mass or velocity? KE = 2 mv 2 If one object has twice the mass but same speed its KE is TWICE AS MUCH KE = 2 mv 2 If one object has the same mass but twice the speed its KE is FOUR TIMES AS MUCH KE = 2 mv 2 How much kinetic energy does a 2.0 kg object If it is moving at 3 m/s? If it is moving at 6 m/s? KE = 2 mv 2 How much kinetic energy does a 23 kg object at rest have? Don’t confuse Kinetic Energy with momentum KE = 2 mv 2 p = mv How can a mouse and an elephant have the same kinetic energy? If a car has twice the KE of another equal mass car, is it moving twice as fast? Whatmuch if youkinetic (and the ball) does were aon an How energy ball airplane traveling very fast. have sitting in your hand? The ball would have no KE relative to the plane but…. a lot relative to the ground Because speed is relative so it KE!! Some times when energy is added to a substance it gets hotter (temperature goes up) What type of energy does Temperature indicate (KE or PE) Atoms at different temperatures applet Moving atoms have KINETIC ENERGY The higher the Temperature The greater their KE The work done on an object is equal to its change in KE Work = DKE How much KE will an object have if it starts from rest and 150 J of work is done on it? A 100 N force is applied over 5 m to a: 1 kg car at rest & 2 kg car at rest Afterwards which car has more KE Afterwards which car is moving faster? Objects with kinetic energy can do work! When they impact the wagon exerts a force on the crate force distance Objects with kinetic energy can do work! force distance The more KE energy an object has the more work it can do before coming to a stop DKE = Work 2 mv 2 Kinetic Energy of an object =Fd How much work it can do before stopping Or the wagon can do work on a spring storing it in the spring as elastic potential energy distance As the wagon does work it loses energy and the spring gains it. distance IN GENERAL Work is equal to the change in energy of an object Work = DE A 10 kg car is moving at 5 m/s What is the car’s KE How much work would be done to stop it? How far will it travel if the stopping force is 10 N? Compare the stopping distances for cars of equal mass, but different speeds (hint: think KE) 10 m/s 1m 20 m/s 4m 40 m/s 16 m SHOW WORK on underlined Book Problems 119-121 PE/KE 7, 9, 22, 24, 27, 31, 32, 33,34 I.) How high would a 5.0 kg object need to be lifted to have 120 J of PE? II.) How fast would a 5.0 kg object need to be moving to have 120 J of KE? III.) How many Joules of elastic potential energy is stored in a rubber band by stretching it a distance of 10 cm by a force of 2.5 N?` ENERGY is conserved What does it mean in science when something is conserved? Uh Like, quit wasting electricity NO Energy is conserved means: Initial Energy = Final Energy Energy is conserved means: The total amount of energy doesn’t change. But the energy can change form ( PE / KE / Heat) Force x distance Total Energy is Conserved Picture a pendulum (on a string) Where Wheredoes doesitithave havethe themost mostPotential Kinetic Energy? Energy? Picture a pendulum (on a string) KE PE If itthe starts with Jall of PE, how much How dosides, we know that at sides, it At At The the total bottom, amount theit10 energy isof energy Kinetic isboth all isPE. constant Energy kinetic energy does it have at the has the amount of PE? itsame changes form bottom? My “Real” Pendulum behaves a little differently. Does the energy appear to be constant? Gravity doesitwork on Just before hits the the massitand changes ground is all in the the form of the form of kinetic energy energy Gravitational PE Heat Kinetic Energy When it hits the ground, the mass does work on the ground Gravitational PE Heat Kinetic Energy Even though the energy changes we always have the same amount Gravitational PE Heat Kinetic Energy Changes in energy for a bungee jumper Consider a bungee jumper, and find the energy conversions Gravitational PE KE + PE Elastic PE Consider a bungee jumper, and find the energy conversions PE = 50 J KE = ? PE = 40 J Elastic PE = Conversion of energy in bouncy ball Does the ball have potential energy when it is on the floor? Why does a dropped ball have potential energy on the ground but one that is just sitting there does not? If the ball is perfectly elastic 10 J PE 10 J KE 10 J PE 10 J PE 10 J KE Energy is often lost as heat 10 J PE 8 J PE 10 J KE 8 J PE 8 J KE 2 J HEAT Note that even though the ball lost energy, it is not destroyed 10 J PE 8 J PE 10 J KE 8 J PE 8 J KE 2 J HEAT How does an impact convert kinetic energy into “heat”? Picture a small cube of atoms which is dropped. When friction or an inelastic collision occurs, the temperature of the atoms in the objects increases The kinetic energy of the moving object, is converted in to kinetic energy of the vibrations of individual atoms THE TOTAL ENERGY IS CONSTANT THE TOTAL ENERGY IS CONSTANT COASTER APPLET THE TOTAL ENERGY IS CONSTANT PE PE KE PE PE KE KE KE KE If there is no friction it is just an exchange between KE & PE Where will the car be moving the slowest? PE KE PE KE Where will the car be moving the FASTEST? PE KE KE At what spots on the ride will the car have the same speed as it does now? Anytime it is at the same height? PE KE Why might this rollercoaster designer be fired? Let’s go Uh oh PE PE KE COASTER APPLET KE Friction does not destroy the energy, it just converts it to heat so will not go as high PE PE PE KE KE Energy converted to heat in red PE KE KE Because of friction the hills must be lower as the ride goes on. (unless energy is added) Let’s go COASTER APPLET Which ball is moving faster when it hits the ground? A ball is thrown straight up at 5 m/s, how fast is it going when it reaches your hand again? 10 J PE 10 J KE 10 J KE Which hits the ground with a greater velocity. A ball which is thrown up or a ball which is dropped from the same height? Conservation of Energy Initial Energy = Final Energy PE1 + KE1 = PE2 + KE2 A .25 kg ball is rolling at 2.0 m/s towards a ramp. How much PE will it have at its highest point? How high above the ground will it reach? height? A 25 kg rock at rest slides down the frictionless incline pictured below. 5.0 m 6.0 m Howfast much does with? KE does ititatstart end How is itPE moving the with? bottom? Compare the speeds of the two rocks when they reach the bottom? 5.0 m 6.0 m 5.0 m 10.0 m A bow is drawn back with a force of 320 N for 0.50 m. How much work was done on the bow? How much elastic PE is stored in the bow? How fast will a 0.028 kg arrow leave the bow when shot? A typical candy bar has an energy content of about 270 Calories or 1,129,000 J. If you are 60 kg and want to work off the energy by climbing stairs, how high will you have to climb? SHOW WORK WHERE APPLICABLE Book Problems 119-121 Conservation 8, 10, 36, 42, 44, 45 I.) 3.0 kg II.) 12 m 2.0 kg 5.0 m/s h Answers +1: 1.3, 7.5 15 v=? Two magnets are attracted to each other. If they are released and slam into each other, Into what forms was the energy converted PE KE Heat Atoms behave like tiny attracted magnets It takes energy to pull them apart, this is like breaking a chemical bond. Atoms behave like tiny attracted magnets When they come back together (forming a new bond) They release this energy The energy atoms release when they form bonds through chemical reactions is called Chemical Potential Energy You hop in the car and go for a ride and then come to a stop. braking Chemical PE KE Heat Heat Sound electricity Stereo, fan, windshield wipers, headlights If you pick up a physics book. (if you haven’t just pretend) You increased the PE of the book, where did that energy come from? Where did you get it from? Where did the food get it from? Thanks, I was a little chilly 4 H atoms Earth 1 He atom Light Nuclear Heat Mmm that’s good corn Heat Kinetic Energy Chemical Energy Chemical Energy Energy Conversion WS Review sheet, Have students work answers on the board (groups) for bonus next day. After test read pages 111-118 answer questions page 120 #’s 14 - 20