Physics Energy Conservation

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1 Physics Energy Conservation Roller coasters are thrilling, hurtling you up and down at high speed. Each one of those dips and rises involves the transformation of energy. And the same sorts of transformation occur in rivers and in the rise and fall of ocean waves, which can be tapped to provide electricity. This is a print version of an interactive online lesson. To sign up for the real thing or for curriculum details about the lesson go to

Introduction: Energy Conservation If you ve ever been swimming at the beach and been bowled over by a huge wave, you know how powerful the ocean can be.

But these all put carbon dioxide into the air, which is changing the climate. And even if we can capture the carbon dioxide and store it safely, fossil fuels will eventually run out.

2 Introduction: Energy Conservation If you ve ever been swimming at the beach and been bowled over by a huge wave, you know how powerful the ocean can be. But did you know that we can actually use that power? For well over a century humans have depended on fossil fuels coal, oil, and gas for energy. But these all put carbon dioxide into the air, which is changing the climate. And even if we can capture the carbon dioxide and store it safely, fossil fuels will eventually run out. So we have started to 뀸nd new sources of energy that we can turn into forms that we are able to use. Unlike coal or oil, these renewable energy sources are naturally replenished and will never run out. As long as the Sun keeps rising and the wind keeps blowing, renewable energy sources will ensure that we have energy for generations to come. To use the sea, some projects harness the tides. The ŀow of seawater in and out with each tide rotates turbines that generate electricity. Other methods use the waves. The recently commissioned CETO plant in Australia uses underwater buoys that generate electricity as the waves move the buoys up and down. The plant is powering a naval base and desalination plant. Scotland's Pelamis Wave Energy Converter uses a di erent technique, with long hinged tubes ŀoating on the water's surface. They generate electricity as the hinges ŀex up and down when the waves pass underneath. But every energy source has its disadvantages as well as advantages. For example, tidal energy systems inevitably a ect natural water ŀows, changing the movement of water in the areas nearby. This can impact sea life there. When we take energy from a natural system we may be a ecting ecosystems or geographical processes, such as coastal erosion or the deposition of silt, that rely on that energy. Unfortunately, the law of conservation of energy says that we cannot create energy we can only remove it from nature, transforming it into a form that is useful to us. Read the full Cosmos Magazine article here. Energy transformations occur all around us. From left to right: a CETO buoy transforms wave energy into electricity, leaves transform sunlight into glucose by the process of photosynthesis, muscles transform chemical energy from food into kinetic energy, and hydroelectric dams transform energy in water into electricity. 1

3 Eŀciency In most energy transformations only a part of the energy source is changed into the "desired" energy form. In CETO generators, for example, about 80% of the kinetic energy available in the waves is transformed into electricity. Below are six objects or processes where energy transformations occur. For each one discuss with your classmates and then guess the eŀciency of the transformation. For example, how much of the chemical energy in the petrol do you think is turned into kinetic energy in a car? When you have 뀸nished your teacher will share the answers with you. Poll 1 Muscles 1% 13% 20% 60% 90% Poll 2 Photosynthesis 1% 13% 20% 60% 90% Poll 3 Hydroelectric dam 1% 13% 20% 60% 90% 2

4 Poll 4 Standard (LED) light globe 1% 13% 20% 60% 90% Poll 5 Petrol car 1% 13% 20% 60% 90% Poll 6 Electric car 1% 13% 20% 60% 90% 3

5 Gather: Energy Conservation 0:00 / 2:02 Credit: Roller Coaster Physics by Lori Bradley (YouTube). 4

6 Question 1 Notes: Use this space to take notes for the video. Note: This is not a question and is optional, but we recommend taking notes they will help you remember the main points of the video and also help if you need to come back to answer a question or review the lesson. Question 2 Select: The amount of gravitational potential energy possessed by an object such as a roller coaster carriage: increases with its speed decreases with its height increases with its height never changes Question 3 Recall: As a roller coaster carriage descends the rst hill: Hint: There may be more than one correct answer. its potential energy is transformed into kinetic energy its kinetic energy increases with its speed the main force acting on it is gravity its potential energy increases Question 4 Give examples: Write down two other examples in which an object's energy is transformed back and forth between gravitational potential and kinetic energy. 5

Calculating energy If you were designing a roller coaster, how would you make sure that when the carriages had been hauled to the top of the rst hill they had just the right amount of energy for a

7 Calculating energy If you were designing a roller coaster, how would you make sure that when the carriages had been hauled to the top of the rst hill they had just the right amount of energy for a thrilling, but not killing, ride? What you need is a couple of equations that allow you to precisely calculate the amount of energy the carriages have. We ve already seen that gravitational potential energy depends on height (h, in metres), so: gravitational potential energy, E p = h How do we ll in the gaps? It probably won t surprise you to learn that energy also depends on mass (m, in kilograms). Intuitively, it takes more energy to move a heavy object than a light one. And potential energy also depends on the strength of the gravitational eld (g): it takes more energy to lift an object in a strong gravitational eld than a weak one. On Earth, g = 9.8 m/s 2. Putting this together we end up with the following equation: gravitational potential energy, E p = mgh What about kinetic energy what does the energy in a moving object depend on? Again, mass is a factor. The only other factor is velocity. It's intuitive that a faster-moving object has more energy than a slower-moving one, but it turns out that you need to square the velocity and multiply by a half we won't prove it here. kinetic energy, E k = ½mv 2 6

8 Question 5 Identify: A simple roller coaster track is shown below, with the high and low points marked A, B, C and D. 1. Draw a blue circle around the letter marking the point at which the carriage has the highest gravitational potential energy. 2. Draw a green circle around the letter marking the point at which it has the highest kinetic energy. Below the track are two areas to show how the gravitational potential energy and kinetic energy of the carriage changes. 1. Draw diagonal blue arrows in the "potential increase or decrease" area to indicate whether the gravitational potential energy in each section of track is increasing or decreasing. 2. Draw diagonal green arrows in the "kinetic increase or decrease" area to indicate whether the kinetic energy is increasing or decreasing. The rst arrow of each type is provided. We can use the height (h) and velocity (v) values provided in the diagram to make energy calculations, given that the mass of the carriage is 500 kg and the gravitational eld strength, g = 9.8 m/s 2. Below we have calculated the potential and kinetic energies of the carriage at point B. Check the calculations and then it's over to you to calculate these values for the other points. Important note: For now, assume that no energy is lost in the transformations between potential and kinetic energy. Calculating the potential and kinetic energy of the carriage at point B: gravitational potential energy, E p = mgh = 500 kg x 9.8 m/s 2 x 30 m kinetic energy, E k = ½mv 2 = ½ x 500 kg x (24.25 m/s) 2 = 147,000 J = 147,000 J 7

9 Question 6 Calculate: Use the equations to calculate, for point A, the carriage's: 1. gravitational potential energy, and 2. kinetic energy. Question 7 Calculate: Use the equations to calculate, for point C, the carriage's: 1. gravitational potential energy, and 2. kinetic energy. Question 8 Tabulate: Enter your calculated energy values into the following table. Given that the height of point D is 0 m, ll in the two energy values for this point of the track. Gravitational Point Height (m) potential energy Speed (m/s) Kinetic energy (J) Total energy (J) (J) A ,000 B , , ,000 C ,000 D 0? 294,000 Question 9 Calculate: It's also important to know the maximum speed the carriage will reach. 1. Use the kinetic energy equation to calculate the speed of the carriage at point D. 2. Convert your answer from m/s into km/h by multiplying by 3.6. Note: Use the equation editor in the Text tool to show your answer, or do the calculation on paper and upload a photo. 8

10 The conservation of energy Did you notice that the roller coaster carriage would have had a thrilling or terrifying! velocity as it reached D? In real life it would be quite a bit slower because of the forces of friction and air resistance, which we've so far been ignoring. These forces act to slow down moving objects in all everyday situations. But slowing something down means decreasing its kinetic energy. Where does the energy go? Well, it can't just disappear. The fundamental law of conservation of energy states that energy can never be created or destroyed but only transformed from one type to another. So what forms has the "lost" kinetic energy been transformed into? Some has been transformed as the metal in the wheels and tracks is worn down, but we will focus on two forms more familiar to you heat, or thermal energy, and sound. Both of these transformations come about as the result of friction. You can demonstrate the principle for yourself by rubbing your hands together the kinetic energy of your moving hands transforms into sound and your hands warm up. Question 10 Rewrite: Write a brief explanation of the law of conservation of energy in your own words. Question 11 Calculate: Suppose that the carriage's actual speed at point D was 15 m/s (or 54 km/h). 1. Calculate its kinetic energy in joules. 2. Calculate the percentage of its starting energy that has been converted into thermal and sound energy. 9

Hydro power is a range of methods for extracting useful energy from the motion of water.

11 Process: Energy Conservation Energy and water A vast amount of energy is stored in water. Just think about thundering rivers and waterfalls, heaving tides and powerful waves crashing against the coasts. Hydro power is a range of methods for extracting useful energy from the motion of water. One of the ways that water comes to have energy is simply by being at a high altitude, at the place where it fell as rain or snow. This water has gravitational potential energy. As it runs down to sea level, just like a roller coaster running down into a dip, the potential energy changes into kinetic and other forms of energy. Consider the Yangtze River in China. It starts 5000 m above sea level in the Tibetan Plateau and then travels 6000 km down to the East China Sea. On the way, it meets the world's biggest dam the Three Gorges Dam in Hubei province (pictured above). The Three Gorges Dam 0:00 / 1:52 Credit: The Largest Dam in The World by Largest Dams (YouTube). 10

12 Question 1 Notes: Use this space to take notes for the video. Note: This is not a question and is optional. Question 2 Consider: If the Three Gorges Dam's reservoir was not constantly replenished from upstream its level would fall as power was generated. A low water level in the reservoir indicates: increased potential energy decreased potential energy increased kinetic energy decreased kinetic energy Question 3 Deduce: Water near the dam's intake pipes has the weight of all the water above pressing down on it, so the water is forced into the intake pipes. Which of the following are true? The higher the water level in the reservoir the greater the kinetic energy of the water ២ owing down the intake pipes The higher the water level in the reservoir the greater the speed of the water ២ owing in the intake pipes The reservoir water level makes no di erence to the kinetic energy of the water in the intake pipes The higher the water level in the reservoir the lower the kinetic energy of the water ២ owing in the intake pipes 11

13 Question 4 Label: Label the main types of energy involved in hydroelectric power generation in the numbered white boxes below. Leave the pink boxes blank they relate to the next question. Question 5 Infer: Hydroelectric dams are 90% or more e៨ cient, meaning 90% or more of the original energy is converted into the ២ nal, usable form. But that still leaves up to 10% that isn't transformed. In the diagram above the locations of the two main sources of waste energy are marked with pink labels. Can you guess the forms of the waste energy from these locations? Write them in. Hint 1: One kind is created by friction in a machine. Hint 2: If you can't think of the second one, go to the next question it might help. Question 6 Explain: Does the water coming out from the turbines move at the same speed as the water going into them? Explain your answer with reference to the energy transformation occurring in the turbines. 12

are becoming increasingly important. The Cosmos Magazine article describes one example. The principle is described in this video.

14 Question 7 Reason: Do the turbines use all of the energy in the water? What would happen if they did? Wave power Although most hydro power has traditionally come from the damming of rivers, other ways of getting usable energy from water are becoming increasingly important. The Cosmos Magazine article describes one example. The principle is described in this video. 0:00 / 1:50 Credit: Wave Power by Vega Science Trust Archive (YouTube). Question 8 Notes: Use this space to take notes for the video. Note: This is not a question and is optional. 13

15 Question 9 Describe: In your own words, describe the energy transformations involved in the "duck gen" electricity generator. Question 10 Propose: Systems of this type typically have an e៨ ciency of around 80%. Suggest where the remaining 20% might go. Question 11 Research: The law of conservation of energy means that we can never create energy out of nothing. To supply our needs we need to be able to extract energy e៨ ciently from the sources we already ២ nd around us. But all of these methods have some negative impact on the environment. Search for information about one of the types of hydro power traditional dams, tidal power or wave power. What impacts can it have on the environment? How much risk do you think these pose, and what measures can be taken to minimize them? Present the results of your search in the project space below in text, tables, graphs, videos, infographics, etc. 14

Activity: Energy Conservation Energy changes in a virtual skate park Introduction 1. Open the Energy Skate Park: Basics simulation here, and select Intro. 2. Select Bar Graph from the menu.

While you don't release her the total energy consists only of potential energy. See how it rises and falls with her height above the ground. 4.

16 Activity: Energy Conservation Energy changes in a virtual skate park Introduction 1. Open the Energy Skate Park: Basics simulation here, and select Intro. 2. Select Bar Graph from the menu. The graph shows the skater's levels of gravitational potential, kinetic and thermal energy, as well as the total of them all. 3. Drag the skater up and down. While you don't release her the total energy consists only of potential energy. See how it rises and falls with her height above the ground. 4. Move the skater to the top of the screen between the ramp and menus, as shown on the right. Release and watch how the energy levels change when she falls to the ground. Question 1 Observe: Select Slow Motion and then Restart Skater. The same fall is repeated so you can observe the changes in energy levels more carefully. Describe these changes. 15

Question 2 Explain: 1. Press the blue Pause button. 2. Drag the skater to the top left rim of the ramp.

Press the Play button and watch the energy changes in the bar graph as she goes back and forth.

Explain why the skater rises to the same height on each side of the ramp.

17 Question 2 Explain: 1. Press the blue Pause button. 2. Drag the skater to the top left rim of the ramp. Notice that all of her energy is potential. 3. Press the Play button and watch the energy changes in the bar graph as she goes back and forth. This section of the simulator has no friction or air resistance. Explain why the skater rises to the same height on each side of the ramp. 0:00 / 1:20 Credit: Roller Coaster Physics by Lori Bradley (YouTube). Question 3 Test: Test Galileo's hypothesis by changing the shape of the ramp to the double curve and releasing the skater from the top left. Does she reach the same "stop height" as in the single curve ramp? Why do you think the shape of the ramp doesn't matter? 16

Friction 1. Now select Friction from the main menu at the bottom of the screen. This section is more realistic because the forces of friction and air resistance are taken into account. 2.

Question 4 Calculate: Use the grid to measure the initial height of the skater. Assuming that she has a mass of 55 kg and that the gravitational eld strength, g, is 9.

18 Friction 1. Now select Friction from the main menu at the bottom of the screen. This section is more realistic because the forces of friction and air resistance are taken into account. 2. Select Bar Graph and Grid from the top right menu, press the Pause button. 3. Drag the skater to the top left position on the U-shaped ramp. Question 4 Calculate: Use the grid to measure the initial height of the skater. Assuming that she has a mass of 55 kg and that the gravitational eld strength, g, is 9.8 m/s 2, calculate her initial potential energy. Question 5 Compare: Select Slow Motion and then press Play. Pause when the skater reaches the highest point on the opposite side of the ramp. Calculate her potential energy now. How much energy has she lost? Question 6 Explain: Explain the dierence between your results for the previous two questions. Question 7 Deduce: Select the down ramp, shown at right. Make sure the Bar Graph is still open. If you send the skater down the ramp she will go o the screen. How do you know when she has stopped moving? Playground Select Playground from the main menu. Drag up sections of ramp and connect them to design an exciting ride. Compare with your classmates. 17

Career: Energy Conservation 0:00 / 1:47 Credit: SKF Careers - Renewable Energy Engineering by SKF Group

Question 1 Reŀect: The video shows that pursuing a career in science and engineering doesn't necessarily mean

Write down a few other science-related jobs that involve working outdoors search the internet if you need to.

19 Career: Energy Conservation 0:00 / 1:47 Credit: SKF Careers - Renewable Energy Engineering by SKF Group (YouTube). Question 1 Reŀect: The video shows that pursuing a career in science and engineering doesn't necessarily mean working in a lab in a white coat. Write down a few other science-related jobs that involve working outdoors search the internet if you need to. Would you prefer to work in a lab or outdoors? Why? Image credits istock photos, Shutterstock, Carnegie Wave Energy, Gallery hip. Other credits PhET Interactive Simulations/University of Colorado ( Impact-dams.html. 18

Test: Energy Conservation Note: There may be more than one correct answer to the multiple-choice questions below. Question 1 (1 mark) What type of potential energy depends on the height of an object?

20 Test: Energy Conservation Note: There may be more than one correct answer to the multiple-choice questions below. Question 1 (1 mark) What type of potential energy depends on the height of an object? Kinetic Electrical Chemical Gravitational Question 2 (1 mark) Which of the following increase as a roller coaster makes its rst descent? Its velocity Its kinetic energy Its gravitational potential energy Its height Question 3 (1 mark) What does the gravitational potential energy of an object depend on? Height Velocity Mass Gravitational eld strength Question 4 (1 mark) What does the kinetic energy of an object depend on? Height Velocity Mass Gravitational eld strength 19

21 Question 5 (1 mark) Assuming there is no friction or air resistance, what energy transformation occurs as a roller coaster climbs a hill? Kinetic electrical Kinetic gravitational potential Gravitational potential kinetic Thermal gravitational potential Question 6 (1 mark) Assuming there is no friction or air resistance, a roller coaster's total amount of gravitational potential plus kinetic energy as it goes up and down along the track. decreases remains the same increases Question 7 (1 mark) A roller coaster carriage with a mass of 400 kg is raised to an initial height of 50 m. How much gravitational potential energy does it have at this point? Note: g = 9.8 m/s 2. 20,000 J 98,000 J 196,000 J 9,800,000 J Question 8 (1 mark) The same roller coaster carriage reaches a velocity of 20 m/s (72 km/h) just before entering a corkscrew. How much kinetic energy does it have at this point? 4000 J 80,000 J 160,000 J 16,000,000 J Question 9 (1 mark) Most roller coasters aren't given any help by motors after being raised up to the top of the rst hill. This height determines the maximum gravitational and kinetic energy of the roller coaster on its journey because: the total amount of energy in the universe is slowly decreasing the energy it gains on its journey comes from friction and air resistance energy is never created nor destroyed, just transferred and transformed Question 10 (1 mark) Adding passengers to a roller coaster carriage its gravitational potential energy and its kinetic energy. increases; increases increases; decreases decreases; increases decreases; decreases 20

Question 11 (1 mark) Question 12 (1 mark) Friction and air resistance transform some of a carriage's kinetic energy into: Hydro power is the extraction of useful energy from which of the following?

22 Question 11 (1 mark) Question 12 (1 mark) Friction and air resistance transform some of a carriage's kinetic energy into: Hydro power is the extraction of useful energy from which of the following? thermal energy Rivers and lakes gravitational potential energy Tides electrical energy Waves sound energy Wind The following diagram of a hydroelectric power plant should be used to answer Questions 13 and Question 13 (1 mark) Question 14 (1 mark) In the diagram above, the energy transformation(s) between 1 and 3 that are used to generate electricity are: The pink circles represent energy losses that reduce the e ciency of the system. These include: kinetic gravitational potential electrical heat energy in the generator gravitational potential electrical electrical energy in the generator gravitational potential kinetic electrical sound energy in the generator kinetic electrical kinetic energy in the water out ow

23 Question 15 (1 mark) The amount of energy carried by water waves depends on their and. wavelength; period height; period height; underlying water depth Question 16 (1 mark) An experimental wave power system is found to have an e ciency of 80%. This means that for every 100 J of energy in the waves, J is converted into electricity Question 17 (1 mark) Learning goal 1: Explain how the energy of a roller coaster changes as it descends from one high point and climbs to the next one. You can ignore friction and air resistance. Question 18 (1 mark) Learning goal 2: The total kinetic and gravitational potential energy of a roller coaster carriage decreases over its journey along the track. Explain how this is consistent with the law of conservation of energy, giving a clear statement of the law. Question 19 (1 mark) Learning goal 3: Explain the energy transformations that allow hydroelectric dams to extract useful energy from nature. Total available marks: 19 22

Student Exploration: Roller Coaster Physics

Name: Date: Student Exploration: Roller Coaster Physics Vocabulary: friction, gravitational potential energy, kinetic energy, momentum, velocity Prior Knowledge Questions (Do these BEFORE using the Gizmo.)