Background Information

Transcription

1 Background Information The lesson we will encounter today is the 7 th lesson in an energy unit that is being developed at the University of Michigan for middle school students. There are four learning goals for the unit: 1. There are different types of energy. 2. All of what goes on in the universe involves some type of energy being transformed into another. 3. Energy cannot be created or destroyed, but changed from one type to another. 4. There are multiple energy sources. Investigations in the unit are intended to be primarily qualitative, focusing on developing students conception of energy based on observations of phenomena. The focus of lesson 7 is to introduce students to a particular type of energy (learning goal #1) and to expose them to the concept of energy transformation (learning goal #2). In the days preceding lesson 7, the students will have investigated various types of energy, including kinetic, thermal, chemical, sound, light, and elastic. Immediately before the lesson, students will have been introduced to elastic energy and will have investigated various phenomena based on what types of energy are present before and after a particular event.

2 Teacher Guide Lesson #7 Introduction to Gravitational Energy and Transformations Continued Learning Goal: LG1: Energy appears in different types. LG2: All of what goes on in the universe involves some type of energy being transformed into another. Learning Performances: 1. Students speculate whether there are additional types of energy beyond those discussed in class. 2. Students use evidence to hypothesize about the types of energy are involved in various phenomena. 3. Students use previous observations as evidence to support their hypothesis concerning what types of energy transformations will take place in various phenomena. 4. Students distinguish between the variables that determine energy type and magnitude. 5. Students explain where energy transformations are occurring in an apparatus, and the direction of that change (increase/decrease in magnitude). 6. Students infer from their observations what types of energy transformations have taken place in various phenomena. 7. Students use their observations of a phenomenon to explain that a specific factor influences the change in amount of energy detect. 8. Students reflect on their own observations and classifications of different types of energy. Objectives: 1. Introduce students to gravitational energy. 2. Identify factors that determine the amount of gravitational energy present in a system. 3. Reinforce the concept that energy can be transformed from one form to another Misconceptions: 1. Stored energy is something that causes energy later. It is not energy until it has been released. (GN1) 2. Energy is a thing. (GN2) 3. The terms "energy" and "force" are interchangeable. (GN3) 4. Energy is confined to some particular origin, such as what we get from food or what the electric company sells. (GN5) 5. Energy is only associated with animate objects. (GN7) 6. Energy is like a fluid, ingredient, or product that comes into an object, gets used up and/or leaves. (GN9) 7. Energy transformations involve only one form of energy at a time. (TC5) 8. An object at rest has no energy. Energy associated with movement. (TH1) 9. Gravitational potential energy depends only on the height of the object. (TH3) Materials: For the teacher: Bean bag Yo-yo Rubber band/inclined plane/marble apparatus Pendulum Piledriver/Play-Doh apparatus Hanging mass on a spring For the students: Notebook Pencil 1

3 Daily Primer: (5 minutes) 1. What must happen to a spring in order for it to transform some of its elastic energy to another form? 2. What happens to the kinetic energy of a ball as it falls from a rooftop? Explain. 3. Describe the energy transformations that take place for a stone that is shot into the air using a slingshot. Section 7.1: Establishing the existence of gravitational energy (10 minutes) Show students a video of a ball thrown into the air that stops at the top and falls again In pairs, ask students to record their ideas about what happens to the kinetic energy of the ball as the video goes on. a. If students are stuck on this step, prompt them to think about how the speed of the ball changes as the video goes on Step the video through individual frames so that students can see the amount the ball moves from one frame to the next, which is proportional to the speed of the ball. At the top of its flight, there should be at least two frames during which the ball moves very little or not at all. a. Students should realize that the ball has more kinetic energy at the beginning and end of its flight than it does when it s at the top In pairs, students should record their ideas about whether the ball has energy at the very top of its flight (when it is not moving) and write down their rationale. a. These ideas should be shared in a whole class format by making a table on the board/overhead. Students rationale should be recorded in two columns, separated based on whether they thought the ball had energy at the top. b. Student rationale on the negative side (as well as incorrect rationale on the positive side) should be scrutinized further under your direction. c. The point of this discussion is to build consensus that the ball has energy at the top, and we know because we saw kinetic energy return to the ball on the way down Tell students that the ball does have even though it is stopped, and we will call this gravitational energy because is it the energy associated with being pulled by gravity. Section 7.2: Investigating objects in freefall (10 minutes) Tell students that we are going to do an investigation to decide what things determine how much gravitational energy an object has Use a motion detector to measure the speed of a small object as it falls from a table. Record the maximum speed of the object when it hits the ground on the board or overhead. It is a good idea to round answers to the nearest tenth Use the motion detector to measure the speed of a larger, more massive, object as it falls from the same table. Record this maximum speed on the board or overhead as well. It should be the same as the small object Now use the motion detector to drop the same two objects from a higher height. Record the maximum speed for these falls as well. The maximum speed should again be the same for both objects, and it should be greater than the previously observed speeds Ask the class what two things can be concluded from the investigation. 1. When two objects with different mass fall from the same height, they hit the ground at the same speed. 2. The higher the height from which an object falls, the faster its speed when it hits the ground. Section 7.3: Isolating the factors for gravitational energy (10 minutes) Choose five objects around the room, and have the students rank them according to which has the most gravitational energy. Have them work in their pairs. a. The objects should be chosen strategically: two objects should have the same height, but different masses, and two objects should be identical, but 2

4 have different heights. The last object should be the highest and most massive. b. As students order the objects, they should write down why each object had more GE than the one before it based on what they see. c. It may be helpful to prompt students to think, based on what they just learned about falling objects, about how much kinetic energy each object would gain as it falls to the ground After students finish, write the correct order of objects on the board/overhead, and give students a couple of minutes to reflect upon/revise their rationale that governed their ordering Come back together as a class and ask the students to brainstorm about what the factors for gravitational energy are. a. It may be necessary to remind students that we can tell an object had gravitational energy based on the kinetic energy we observe when it falls. b. If necessary, prompt students to think about what things could be changed to make an object have more kinetic energy as a result of a fall. c. Students should be led to conclude that gravitational energy depends on the height of an object and its mass. Section 7.4: Investigating gravitational energy transformations (15 minutes) Around the room, there should be several apparatuses, including: a. Pendulum b. Rubber band/ramp/marble apparatus students can pluck a rubber band to launch the marble up the ramp or roll the marble down the ramp and watch it bounce back up. c. Yo-yo should not be tied off at the end so that it keeps spinning at the bottom of the rope. d. Hanging mass on a spring Before releasing the students, model the activity (described in 7.4.4) using a piledriver apparatus, which is a mass dropped through a tube (preferably clear) onto a ball of playdoh) Each apparatus should include instructions regarding what to do. a. Pendulum should be raised to a certain height (angled less than 90 from vertical) and released. The pendulum should be caught after one full swing. b. Rubber band apparatus students should place the ball on the rubber band and pluck it so that it goes up the ramp. The marble should be stopped just before it hits the rubber band again. c. Yo-yo students should release the yo-yo from rest and allow it to descend the string. d. Hanging mass on a spring should be lifted upward and released. (not pulled downward because it may be ejected). The mass should be stopped after one full bounce At each station, students should be prompted to complete the following table: Before During After Each apparatus should have a designated point at which the students record the types of energy present, as well as whether those energies are increasing or decreasing. Students should identify which types of energy are present before they release their apparatus Students should identify which typed of energy are present at some point after the designated phenomena is complete. Section 7.5 Wrap-Up: Discussing the results of the stations (5 minutes) In a whole class format, pick one student to share their during results for each of the apparatuses. Write the results on the board and ask whether anyone disagrees with any of the results. 3

5 Ask students if they notice any patterns in the during results. The goal is to get them to realize that any time one type of energy increases, at least one other type must decrease. If they need prompting, try: a. Covering up the types of energy and only focusing on the words increasing or decreasing. b. Have them focus on the pendulum, piledriver, or yo-yo apparatuses, which should only list two during energies, one increasing and one decreasing Revisit the daily primer having to do with the falling ball. Ask them how KE changes, and prompt them to connect how GE changes during the fall. a. Explicitly state that GE decreases as KE increases because it is being transformed into KE. Homework Students should identify one object that transforms gravitational energy into other form(s) and one object that transforms other form(s) of energy into gravitational energy. They should record what the machines are and do a before, during, and after table for each one. 4