--- title: "Where does the energy go?" subtitle: "Year 9 Physics · Conservation of energy and efficiency · Lesson 8" --- # Where does the energy go? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Drop a ball and it bounces back up, but never quite as high. Each bounce is lower until it stops. Has its energy been used up? :::teacher-note **In one sentence:** Students state the law of conservation of energy and use it to account for all the energy in a system, including energy transferred as heat and sound (AC9S9U05, node 1, a hinge node). **Students should be able to say:** I can state the law of conservation of energy. I can account for all the energy in a system, including heat and sound. I can explain where energy goes when it seems to be lost. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **A ball bounces lower each time until it stops. What happened to its energy?** ☐ It became heat and sound in the ball, the floor and the air ☐ It was used up ☐ It disappeared when the ball stopped ☐ It went back into the person who dropped it My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** It became heat and sound. Each bounce, some of the ball's energy is transformed into heat (the ball and floor warm very slightly) and sound (the bounce you hear). None of it disappears: it is spread into the surroundings. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: the law :::keep-together ### The law **What does the law of conservation of energy say?** ☐ Energy is used up when things move ☐ You should save energy by switching things off ☐ Energy can't be made or destroyed, only transferred or transformed; the total stays the same ::: :::teacher-note **Answer:** Energy can't be made or destroyed, only transferred or transformed; the total stays the same In any system, total energy in = total energy out. ::: :::keep-together ### Count every kJ **A coaster car has 150 kJ in total. At one point it has 90 kJ of potential energy and 40 kJ of kinetic energy. How many kJ have become heat and sound?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 20 150 − 90 − 40 = 20 kJ. Every kJ is accounted for. ::: :::keep-together ### Braking **People say a car 'loses' energy when it brakes. What really happens?** ☐ The energy is destroyed by the brakes ☐ Its kinetic energy becomes heat in the brakes, which spreads into the air ☐ The energy goes back into the fuel ::: :::teacher-note **Answer:** Its kinetic energy becomes heat in the brakes, which spreads into the air 'Lost' means 'transferred somewhere we can't use it'. Feel a bike's brakes after a long downhill: they're hot. ::: :::keep-together ### The pendulum **Why can a pendulum never swing higher than where it started?** ☐ It would need more energy than it had: energy can't be made ☐ Pendulums only swing lower ☐ Gravity gets stronger ::: :::teacher-note **Answer:** It would need more energy than it had: energy can't be made With no extra energy put in, the most it can do is reach its starting height, and friction makes it a little lower each swing. ::: :::typical ## Elaborate: a lot of friction :::keep-together ### Where it went **The car ended up stopped in a dip. Where is its starting energy now?** ☐ It is kinetic energy, waiting to be used ☐ It has all gone ☐ Most is heat and sound in the surroundings; the rest is potential energy where it rests ::: :::teacher-note **Answer:** Most is heat and sound in the surroundings; the rest is potential energy where it rests The bars still add up to the total: heat and sound, plus the potential energy of its resting height. ::: ::: :::wider ## Elaborate: perpetual motion :::keep-together **An inventor claims a wheel that keeps spinning forever and also powers a light. Why is this impossible?** ☐ It would spin too fast ☐ The light's energy and the friction heat must come from somewhere; with no energy in, the wheel must slow down ☐ Wheels can't power lights ::: :::teacher-note **Answer:** The light's energy and the friction heat must come from somewhere; with no energy in, the wheel must slow down Energy out (light, heat) with nothing in would mean making energy. Every machine needs an input. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - **Conservation of energy:** energy is never made or destroyed; it is only transferred or transformed. The total stays the same. - Energy that seems "lost" or "used up" has been transferred to the surroundings, usually as heat and sound. - Total energy in = total energy out, so you can account for every joule. ## Exit ticket :::keep-together **A skateboarder has 2000 J at the top of a ramp and 1700 J of kinetic energy at the bottom. Where are the other 300 J?** {{lines-3}} ::: :::teacher-note **Answer:** Transferred as heat and sound because of friction and air resistance. The total is still 2000 J. :::