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ENERGY AND WORK What is energy? Discuss A REMINDER! Energy cannot be made or destroyed, only transformed (changed). Energy is measured in Joules. That’s me! James Prescott Joule 1818-89 WHAT IS ENERGY? “The capacity to do work” Not a very good definition! WORK In physics, work has a special meaning, different to “normal” English. WORK In physics, work is the amount of energy transformed (changed) when a force moves (in the direction of the force) WORK For example, if Mr J pushes a table, he is doing work against the friction force of the table against the floor. CALCULATING WORK The amount of work done (measured in Joules) is equal to the force used (Newtons) multiplied by the distance the force has moved (metres). Force (N) Distance travelled (m) WORK DONE = FORCE X DISTANCE Another physics formula to learn! Copy it down!! IMPORTANT! The force has to be in the direction of movement. Carrying your shopping home from (insert supermarket name here) is not work in physics! Not working REMEMBER! There are three key ingredients to work – force displacement cause In order for a force to qualify as having done work on an object, there must be a displacement and the force must cause the displacement. GROUP ACTIVITY: Why am I doing this? READ THE STATEMENTS BELOW WHICH ONES ARE EXAMPLES OF WORK BEING DONE? 1. Mr J applies a force to a wall and becomes exhausted. 2. A book falls off a table and free falls to the ground. 3. A waiter carries a tray full of meals above his head straight across the room at constant speed. 4. A rocket accelerates through space. LIFTING OBJECTS When we lift objects, we are doing work because a force is moving. Force Distance moved LIFTING OBJECTS Our lifting force is equal to the weight of the object. Lifting force weight Let’s look at some examples GROUP CHALLENGE WORK DONE (J) = FORCE (N) X DISTANCE (M) A woman pushes a car with a force of 400 N for a distance of 15m. How much work has she done? WORK DONE (J) = FORCE (N) X DISTANCE (M) A woman pushes a car with a force of 400 N for a distance of 15m. How much work has she done? Work = force x distance = 400 x 15 = 6000 J GROUP CHALLENGE WORK DONE (J) = FORCE (N) X DISTANCE (M) A man lifts a mass of 120 kg to a height of 2.5m. How much work did he do? WORK DONE (J) = FORCE (N) X DISTANCE (M) A man lifts a mass of 120 kg to a height of 2.5m. How much work did he do? Force = weight = 1200N Work = F x d = 1200 x 2.5 Work = 3000 J WORK AGAINST GRAVITY. As energy can not be created or destroyed, when you do work against gravity it must be transferred. If you lift an object you increase its gravitational potential energy (g.p.e). The total amount of g.p.e gained must be equal to the amount of energy transferred, or work done against gravity. Lets derive So work done: W = Fxd What would the Force on the sack of potatoes (mass = m) be????? h m Can you derive an equation for the work done against gravity when I lift the sack of potatoes? So work done: W = Fxd If I do work against gravity I raise an object a certain height (h) increasing its g.p.e. So: W=Fxh In order to lift the object I have to overcome the objects weight (m x g), so the force has to be at least equal to the weight. So: W=mxgxh But since the total amount of g.p.e gained must be equal to the amount of energy transferred, or work done against gravity therefore: Work done against gravity = g.p.e it follows that: Easy Write the equation to calculate work done. Medium What is the work done if we apply a 1.2N force and we move 4 m in the direction of force? Hard Thinking about energy transfers: When you rub your hands together what are the energy transfers? Rearrange this equation for force applied. What is the work done if we apply a 7N force and we move 8 m in the direction of the force? When you boil a kettle what energy transfers are happening? Rearrange this equation for distance What distance is moved if we have moved in the direction of force. a 8N force and the work done is 90 J? Walking up a flight of stairs, what are the energy transfers? What is the unit of force? What is the distance moved if we have a 70N force and work done is 8 J? When you roll a ball down a hill what are the energy transfers? What is the unit of work done? What force is required to move 7 m if the work done is 9 J? What is the work done when a force of 5 N is applied to a ball and it moves 80 m? What is the unit of distance? What work is done when we apply a force of 5N and move in the direction of the force 2 m? What force is required to move 19 m What is the work done to a car if a if the work done is 9 J? force of 9 N is applied and it moves 7 km? What force is required to move 7 m if the work done is 21 J? What is the work done to a person if a force of 1.3N is applied and the person moves 6m? Easy Write the equation to calculate work done. Medium What is the work done if we apply a 1.2N force and we move 4 m in the direction of force? Hard Thinking about energy transfers: When you rub your hands together what are the energy transfers? Rearrange this equation for force applied. What is the work done if we apply a 7N force and we move 8 m in the direction of the force? When you boil a kettle what energy transfers are happening? Rearrange this equation for distance What distance is moved if we have moved in the direction of force. a 8N force and the work done is 90 J? Walking up a flight of stairs, what are the energy transfers? What is the unit of force? What is the distance moved if we have a 70N force and work done is 8 J? When you roll a ball down a hill what are the energy transfers? What is the unit of work done? What force is required to move 7 m if the work done is 9 J? What is the work done when a force of 5 N is applied to a ball and it moves 80 m? What is the unit of distance? What work is done when we apply a force of 5N and move in the direction of the force 2 m? What force is required to move 19 m What is the work done to a car if a if the work done is 9 J? force of 9 N is applied and it moves 7 km? What force is required to move 7 m if the work done is 21 J? What is the work done to a person if a force of 1.3N is applied and the person moves 6m? AIM: TO INVESTIGATE HOW HARD WE CAN WORK AS A CLASS! Name Mass (kg) Force (N) Distance (m) Work of one lift (J) # of lifts in 1 min Total work (J) Mean Total Work for the group : J ARM CURLS Force required = weight of object = mass (kg) x 10 distance CLASS RESULTS FOR EACH GROUP Group Mean Total work (J) Class Mean: 1 2 3 4 5 Conclusion: Are we a hardworking class? J LET’S SUMMARISE! Discuss with the person next to you what you have learned today then as a pair write down what you feel is the most important thing you learned. Different Types of Energy. Heat/Thermal Energy Anything with a temperature above absolute zero (-273°C) has heat energy. That means everything has some heat energy. The hotter something is the more heat it has. Kinetic Energy Anything that moves has kinetic energy. Nuclear Energy Released only from nuclear reactions e.g. The sun and all of the stars. Hydrogen bomb (Fusion) Nuclear power plants and the Atomic bomb. (Fission) Sound Energy Anything noisy gives off sound energy like vocal chords, speakers and instruments. Light Energy Anything luminous gives off light energy, like the sun, light bulbs , candles and glow worms. Chemical Energy Anything with stored energy which can be released by a chemical reaction has chemical energy, things like food fuels and batteries. Electrical Energy Electrical energy is very useful, because its easily converted into other forms – wherever there's a current flowing there's electrical energy. Gravitational Potential Energy (GPE) Anything above the ground has gravitational potential energy i.e. anything that can fall, like ski jumpers, aeroplanes and climbers. Elastic Energy Anything stretched, has elastic energy – things like rubber bands, springs, knickers, elastic etc Different types of energy There are many different types of energy: thermal light sound elastic gravitational kinetic electrical chemical nuclear Can you think of examples of each type of energy? Which type of energy? Type Heat Kinetic (movement) Nuclear Sound Light Chemical Electrical Gravitational potential Elastic potential 3 example sources Type 3 example sources Flows from hot objects to colder objects, e.g radiator, heater and fire. Anything that moves has kinetic energy., e.g football in mid air, formula one car at speed, a cheetah. Released only from nuclear reactions e.g. The sun and all of the stars. Hydrogen bomb. Nuclear power plants and the Atomic bomb Anything noisy gives off sound energy like vocal chords, speakers and instruments Anything luminous gives off light energy, like the sun, light bulbs , candles and glow worms. Anything with stored energy which can be released by a chemical reaction has chemical energy, things like food, fuels and batteries. Electrical energy is very useful, because its easily converted into other forms – wherever there's a current flowing there's electrical energy. Anything above the ground has gravitational potential energy i.e. anything that can fall, like ski jumpers, aeroplanes and climbers. Anything stretched, has elastic energy – things like rubber bands, springs, knickers elastic etc Energy Transfer Learning Objectives: Identify the different types of Energy Transfer Learning Outcomes: All - Identified and recorded the different types of energy Most – Discussed and explained their initial thoughts on energy. Some – Explaining why different objects emit more than one type of energy. Type 3 example sources Heat Flows from hot objects to colder objects, e.g radiator, heater and fire. Kinetic (movement) Anything that moves has kinetic energy., e.g football in mid air, formula one car at speed, a cheetah. Nuclear Released only from nuclear reactions e.g. The sun and all of the stars. Hydrogen bomb. Nuclear power plants and the Atomic bomb Sound Anything noisy gives off sound energy like vocal chords, speakers and instruments Light Anything luminous gives off light energy, like the sun, light bulbs , candles and glow worms. Chemical Anything with stored energy which can be released by a chemical reaction has chemical energy, things like food, fuels and batteries. Electrical Electrical energy is very useful, because its easily converted into other forms – wherever there's a current flowing there's electrical energy. Gravitational potential Anything above the ground has gravitational potential energy i.e. anything that can fall, like ski jumpers, aeroplanes and climbers. Elastic potential Anything stretched, has elastic energy – things like rubber bands, springs, knickers elastic etc Which type of energy? Energy transfer Energy can be changed from one form to another. For example: Chemical energy in food is converted to thermal energy and kinetic energy by our bodies. Gravitational energy in a ball is converted to kinetic energy when it falls to the ground. What other energy transfers can you think of? What is the energy transfer? What energy transfer takes place in each device? burning match portable torch microphone radio television catapult mobile phone car chemical to heat and light chemical to heat and light sound to electrical electrical to sound and heat electrical to sound and light and heat elastic to kinetic and heat chemical to sound and microwaves (EM radiation) and heat chemical to kinetic and sound and heat In all these transfers the energy is not lost, it is conserved. Energy cannot be destroyed or created. Draw an energy transfer for... YOU!! Sound Chemical Heat Kinetic (movement) Chemical Elastic potential Heat Kinetic (movement) Gravitational potential Nuclear Electrical Sound Light Kinetic Energy Learning Objectives: Recall that a moving object has kinetic energy; State the relationship between kinetic energy and gravitational potential energy; Apply and rearrange the kinetic energy equation DO NOW 1. Define resultant force 2. Describe what happens to an object when the resultant force is zero 3. If a car is moving forwards at a force of 10N but friction acts on the car at 4N, what is the resultant force? Extension What does kinetic mean? What does kinetic energy mean? What would the units be? Kinetic Energy Learning Objectives: Recall that a moving object has kinetic energy; State the relationship between kinetic energy and gravitational potential energy; Apply and rearrange the kinetic energy equation DO NOW 1. The sum of all the forces acting on an object 2. Stationary or constant speed 3. 10 - 4 = 6N Extension movement movement energy Joules (J) Kinetic Energy Learning Objectives: Recall that a moving object has kinetic energy; State the relationship between kinetic energy and gravitational potential energy; Apply and rearrange the kinetic energy equation DO NOW 1. The sum of all the forces acting on an object 2. Stationary or constant speed 3. 10 - 4 = 6N Kinetic Energy Learning Objectives: Recall that a moving object has kinetic energy; State the relationship between kinetic energy and gravitational potential energy; Apply and rearrange the kinetic energy equation • Kinetic energy is energy an object has due to motion. It depends on two things: • Any ideas??? Kinetic Energy Learning Objectives: Recall that a moving object has kinetic energy; State the relationship between kinetic energy and gravitational potential energy; Apply and rearrange the kinetic energy equation • Kinetic energy is energy an object has due to motion. It depends on two things: • The objects mass • The objects speed • Kinetic energy tells us how much movement energy something has. • It doesn't just depend on how fast something is moving, it also depends on mass. • If a car and a lorry are both travelling at the same speed the lorry will do much more damage if it hits something, than if the car does. • The lorry has more kinetic energy even though they are both travelling at the same speed. Kinetic Energy Learning Objectives: Recall that a moving object has kinetic energy; State the relationship between kinetic energy and gravitational potential energy; Apply and rearrange the kinetic energy equation Formula triangle TASK: • Copy the triangle – can you rearrange the equation? • Add units and name next to symbol V = √(2x KE)÷m M = (2xKE) ÷ v2 RECALL: “Killer man-eating squirrels!” Kinetic Energy Learning Objectives: Recall that a moving object has kinetic energy; State the relationship between kinetic energy and gravitational potential energy; Apply and rearrange the kinetic energy equation • Kinetic energy = ½ x mass x speed2 • Units of KE= joules (J) • Mass= kg • Speed= m/s Kinetic Energy Learning Objectives: Recall that a moving object has kinetic energy; State the relationship between kinetic energy and gravitational potential energy; Apply and rearrange the kinetic energy equation • A car with a mass of 500 kg is moving at a speed of 12 m/s. How much kinetic energy does it have? EXAMPLE Kinetic Energy Learning Objectives: Recall that a moving object has kinetic energy; State the relationship between kinetic energy and gravitational potential energy; Apply and rearrange the kinetic energy equation • A car with a mass of 500 kg is moving at a speed of 12 m/s. How much kinetic energy does it have? • ½ x 500 x (122) = 36,000 J Kinetic Energy Learning Objectives: Recall that a moving object has kinetic energy; State the relationship between kinetic energy and gravitational potential energy; Apply and rearrange the kinetic energy equation Calculate Kinetic Energy! 1. A car that travels at a speed of 20m/s and has a mass of 1200kg 2. A year 11 pupil with a mass of 55kg swinging back on their chair and falling off at 0.6m/s 3. A runner with a mass of 62kg running at a speed of 0.8m/s 4. A tennis ball travelling at 46m/s with a mass of 58kg 5. A dog running across a field at a speed of 1.2m/s with a mass of 3.2kg Kinetic Energy Learning Objectives: Recall that a moving object has kinetic energy; State the relationship between kinetic energy and gravitational potential energy; Apply and rearrange the kinetic energy equation Calculate Kinetic Energy! 1. 600kg x 400m/s = 240,000J or 240KJ 2. 25kg x 0.36m/s = 9J 3. 31kg x 0.64m/s = 19.84J 4. 29kg x 2116m/s = 61,364J or 61.4KJ 5. 1.6kg x 1.44m/s = 2.304J Peerassessment Kinetic Energy Learning Objectives: Recall that a moving object has kinetic energy; State the relationship between kinetic energy and gravitational potential energy; Apply and rearrange the kinetic energy equation Complete the Tarsia GPE=KE relationship You will need to be able to rearrange! Kinetic Energy Learning Objectives: Recall that a moving object has kinetic energy; State the relationship between kinetic energy and gravitational potential energy; Apply and rearrange the kinetic energy equation Answer as many questions as you can but you must end on your TAG Kinetic Energy Learning Objectives: Recall that a moving object has kinetic energy; State the relationship between kinetic energy and gravitational potential energy; Apply and rearrange the kinetic energy equation 1. It’s velocity is zero even though the object has mass. Multiplying the mass by zero will result in zero 2. Car B has a greater velocity. KE =1/2mv2 so if they all have the same mass the velocity will impact the KE. (Calculations of all 3 KEs needed) 3. 0.2 x400 =80J 4. Greater vlelocity = greater kinetic energy of the vehicle which will require more force to stop completely from the brakes 5. 2x43200 = 86400/600 =square root answer = 12m/s Answer as many questions as you can but you must end on your TAG Green = E-D Yellow = C-B Blue = A-A* Why does a book on a What is kinetic energy? When a catapult is shelf have zero kinetic stretched and fired name energy? the energy transfers An object weighs 2kg and A car moves at 360KJ of is fired horizontally at a kinetic energy at a speed speed of 5m/s. What is the of 30m/s. Calculate it’s kinetic energy? mass. Why does a lorry have more kinetic energy than a mini? State the equation for Kinetic Energy and rearrange this for Speed and mass Calculate the kinetic What is the kinetic energy energy of a van of mass when a man lands if the 500kg moving at a speed gravitational potential of 12m/s energy of him falling out of a plane is 5642J? Green = E-D Yellow = C-B Blue = A-A* The energy an object The book has mass has due to it’s motion. but it is not moving so It depends on its mass it has zero velocity and speed. Elastic potential energy to kinetic energy 25J 800kg KE = 1/2mv2 M= (KE ÷ v2) x2 V = √ KE ÷ 1/2m The lorry has more mass so it has more kinetic energy if the mini is travelling at the same velocity 36,000J 5642J Sit in your allocated seat, get out your equipment and write the title, LQ and date, and begin the starter. 07 March 2022 Title: Efficiency Learning Questions What is efficiency? Why is efficiency important? Keywords: efficiency Starter “energy saving bulbs” What does this mean? Success criteria • Define what efficiency means. Grade 4 Extension – Grade 6 • Explain some ways in which energy is transferred wastefully by mechanical processes. • Explain some ways of reducing unwanted energy transfers in mechanical processes. Grade 8 What is efficiency? The efficiency of a device is a way of saying how good it is at transferring energy as a useful output. The efficiency of a device is given as a number between 0 and 1. The higher the number, the more efficient the device. 0 0.5 1.0 Wastes all energy transferred to it. Almost all energy transferred into useful energy (no device 100% efficient). 0% 50% 100% What is efficiency? Efficiency is a measure of how much useful energy we get out compared to what goes in Energy In How do we calculate efficiency? Efficiency is calculated using this formula: efficiency = (useful energy transferred by the device) (total energy supplied to the device) We can write this more briefly as: efficiency = useful energy out total energy in Things to remember when you are using the formula: • the useful energy transferred by the device is always less than the total energy supplied • the efficiency of a device can never be greater than 1. If you calculate an efficiency greater than 1, or calculate an amount of useful energy that is greater than the total energy transferred, you have either substituted numbers into the formula incorrectly or you have made a mistake in your calculation! Progress Questions 1 A device transfers 30 J of useful energy every second. The total energy transferred to the device each second is 50 J. Calculate its efficiency. (useful energy transferred by the device) efficiency = (total energy supplied to the device) = 30 J 50 J = 0.6 Check - is the efficiency less than 1? Progress Questions 2 A light bulb transfers 90 J of energy by heating every second. 100 J of energy is transferred to the light bulb every second by electricity. Calculate its efficiency. The energy transferred by heating is wasted energy in a light bulb. Energy is conserved, so: total energy transferred = useful energy + wasted energy 100 J = ?? + 90 J useful energy transferred = 10 J Continued Progress Questions 2 A light bulb transfers 90 J of energy by heating every second. 100 J of energy is transferred to the light bulb every second by electricity. Calculate its efficiency. (useful energy transferred by the device) efficiency = (total energy supplied to the device) = 10 J 100 J = 0.1 Check - is the efficiency less than 1? Progress Questions 3 Calculate the efficiency of this heater. Continued Efficiency practical • What energy transfers take place when a ball bounces? • Does a ball lose energy when it bounces? Ball efficiency Drop height (cm) 20 40 60 80 100 Bounce height (cm) Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 Average Uncertainty How certain are you of your results? • Was it easy to see the bounce height ACCURATELY? • What will this mean for your results? • Did you have any anomalous results? Drop height (cm) Bounce height (cm) Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 20 11 10 7 9 11 40 17 24 22 19 21 60 30 32 34 29 29 80 55 57 59 54 56 100 80 81 83 81 80 Average Uncertaint y Which of my results do you think are the most accurate and why? • Add error bars to your graphs. – Does you line of best fit pass through all of your error bars? • Calculate your gradient. This will be the efficiency of the ball. Progress Questions 3 Calculate the efficiency of the heater. 100 J is supplied to the heater each second. 1 J is transferred by light, 4 J by sound and 95 J by heating. (useful energy transferred by the device) efficiency = (total energy supplied to the device) = 95 J 100 J = 0.95 Check - is the efficiency less than 1? Progress Questions 4 A device transfers 20 J of useful energy every second. It has an efficiency of 0.8. How much energy is transferred to the device each second? total energy = useful energy efficiency = 20 J 0.8 = 25 J Check - is the total energy greater than the useful energy transferred? Progress Questions 5 A light bulb transfers a total of 20 J of energy every second. Its efficiency is 0.45. How much energy does it transfer by light each second? useful energy = efficiency x total energy = 0.45 x 20 J =9J Check - is the useful energy less than the total energy transferred? Progress Checker 1: Your muscles waste about 75 J of energy for every 25 J they convert into movement. How efficient are your muscles? 2: An electric fan has an efficiency of 80%. If it produces 120 W of useful kinetic energy in the air, how much power is it using? Swap & Mark 1: Total energy transferred = 25 J + 75 J = 100 J. Efficiency = 25 J/100 J = 0.25 (or 25%). 2: Total power in = useful power out/efficiency = 120 W/0.8 = 150 W. Progress– plenary task (6 mark question) Grade 4 2 marks Grade 6 4 marks Grade 8 6 marks • The table gives data about two types of light bulb people may use in their homes. • Both types of light bulb produce the same amount of light. • Evaluate, in terms of cost and energy efficiency, the use of the two types of light bulb. • To gain full marks you must compare both types of light bulb and conclude which light bulb would be the best to use. Level 1 (1-2 marks) • There is a basic comparison of either a cost aspect or an energy efficiency aspect. Level 2 (3-4 marks) • There is a clear comparison of either the cost aspect or energy efficiency aspect OR a basic comparison of both cost and energy efficiency aspects Level 3 (5-6 marks) • There is a detailed comparison of both the cost aspect and the energy efficiency aspect. • For full marks the comparisons made should support a conclusion as to which type of bulb is preferable Cost • halogen are cheaper to buy (simply giving cost figures is insufficient) • 6 halogen lamps cost the same as one LED • LEDs last longer • need to buy 18 / more halogen lamps to last the same time as one LED • 18 halogens cost £35.10 • costs more to run a halogen than LED • LED has lower maintenance cost (where many used, eg large departmental store lighting) SWAP & MARK Energy efficiency • LED works using a smaller current • LED wastes less energy • LEDs are more efficient • LED is 22% more energy efficient • LED produces less heat • LED requires smaller input (power) for same output (power) Progression questions 1. What does efficiency mean? 2. How do we calculate the efficiency of an energy transfer? 3. How can we reduce unwanted energy transfers in machines? Plenary What can you do now? define what efficiency means recall and use the formula for calculating energy efficiency explain some ways in which energy is transferred wastefully by mechanical processes explain some ways of reducing unwanted energy transfers in mechanical processes.