Energy Transformations IDS 101 It is difficult to design experiments that reveal what something is. As a result, scientists often define things in terms of what something does, what something did, or what something can do. These are called operational definitions. Scientists have struggled for centuries do define what light is. On the other hand, throughout all of those centuries they have agreed that it is something that radiates from some objects and can be detected by the human eye. What is it? We don t know. What does it do? We can test that with experiments. What is energy? This question has been famously hard to answer so we define it in terms of what energy can do: Energy is the ability to increase temperature or do work. Now we need definitions of temperature and work. Most people have an idea of what it means for things to be warmer or cooler and that will suffice as a definition for temperature. As for work, it will be easier to learn examples of what work does. This module will explore many (but not all) kinds of work. First we ll introduce some vocabulary and recount a little history. Absorption of energy can do two things to an object (one or the other or both): (1) it can make the object warmer- If you feel energy that makes you warmer, you probably call it heat. When we place a cool object in contact with a warm object, thermal energy flows from the warm object to the cool object. Scientists call that flow of thermal energy by the name heat. 1 (2) it can do work on the object - The simplest form of work to understand is mechanical work. Pushing or pulling an object over some distance is doing mechanical work on that object. Notice that the object has to move in order for mechanical work to be done on it. The object also has to move in the direction of the push or pull (you can replace the phrase push or pull with force if you like). Historical note: Now and then civilization is changed by a scientific discovery. History was changed by the discovery that heat and work can be converted into each other but that the total amount of energy in the universe does not change. This idea became known as conservation of energy. It provided the guiding inspiration for the industrial revolution of the nineteenth century. Widespread use of the steam engine, the electric generator, the coal fired power plant, and the locomotive were some of the results. 1 To some people the exact use of this language is important. Technically speaking, heat is the flow of thermal energy. The energy itself is thermal energy and the flow of thermal energy is heat. In daily language most people refer to the energy itself as heat, and that daily language is good enough for this lesson. Suit yourself.
2 When work is done on an object, where does the energy go? 1. The energy can make something move faster. This is called kinetic energy. If you put your friend in a shopping cart on a smooth level floor and then push the cart over some distance (mechanical work) you can make your friend move faster and faster. 2. The energy can be stored somewhere (and possibly released later). This is called potential energy. If you push your friend in the cart up a hill you will do a lot of work. If you then hold the cart near the top of the hill, you are storing (much of) the energy you put into getting the cart up there. That energy can be released when you let go of the cart. 3. The energy can make something warmer. Again, this is thermal energy or heat. If you release the cart and your friend tries to slow down by dragging his or her feet on the ground, the soles of his or her shoes might get hot. 4. Energy can be carried away by some form of radiation. Two common forms of radiation are sound and light. I suspect that a person in a shopping cart careening down a hill would make a lot of noise. That noise carries away some energy with it. Another way of defining work is to say that work is the process that transforms one of these forms of energy into another. Throughout the classroom you should find several gadgets which demonstrate transformations of energy. Some gadgets demonstrate several transformations of energy of several different forms. Go and observe each gadget listed here and describe at least three energy transformations for each object. (You may observe them in any order you like.) 1. The hand-cranked flashlight and radio. 2. The steam engine. (WARNING: The steam engine involves FIRE and STEAM and it gets REALLY REALLY HOT!!! Don t touch.)
3 3. The Flip-flap toy and light source. (Warning: if you use the flip-flap toy with an incandescent light bulb then the light bulb is likely to get hot. Don t touch the bulb.) 4. The Darda cars and track. 5. The woodpecker and/or Minnie Mouse toys. Did your investigations of the five gadgets above bring to mind forms of energy you hadn t thought of before (or perhaps hadn t thought of in a while)? If so, what were the forms of energy?
4 The ball and the ramp: You should find materials to make an inclined ramp for a ball to roll down and onto your table. You will need: A metal stand with a clamp. A wood block with a metal handle and a notch for holding a v-shaped ramp. A v-shaped metal ramp with a notch in one end (the notch rests on the table). A metal ball to roll down the ramp. A small wood block without a handle or notch (for the ball to bump). Place the metal handle of the notched wood block into the clamp on the metal stand. The clamp should be six to ten inches above the table (the exact height is not important for this exercise). The notch should be facing upward so you can rest one end of the metal ramp in the notch. One end of the metal ramp has a small notch cut out of it and the other does not. Place the end without the notch in the grooved wood block and rest the notched end of the ramp on the table. The other wood block should rest on the table a few inches away from the lower end of the ramp. Find a way to measure or mark the distance between the end of the ramp and the wood block since we want that distance to remain the same no matter what else we change in this experiment. Why would we want to keep one aspect of an experiment (in this case the distance from the ramp to the wood block) constant even though we change other things in an experiment? What do we call one aspect of an experiment that we keep constant while other things change?
5 Now it is time to do the experiment. Place the loose block on the table a few inches from the lower end of the ramp (the block should just rest on the table without being held). Place the ball on the table. Have someone pick up the ball and place it on the ramp (near the upper end) and release it. The ball should roll down the ramp and collide with the block at the bottom. (If the ball starts to roll away you can grab it before it rolls off of the table.) Starting from a moment just before the ball was picked up off of the table and ending with a moment after the collision immediately after the block at the bottom stopped moving, list the energy transformations that you saw. You should name at least four transformations of energy from one form into another, but you may list more. 1.. 2.. 3.. 4.. Replace the block near the bottom of the ramp and repeat the experiment, but this time raise the ball to a point only about halfway up the ramp and release it. Record any differences you see between this version of the experiment and the one above. Try do describe the differences you listed in your previous answer in terms of energy. Now we are going to focus on the collision between the ball and the block. When a ball comes off of the ramp it is likely to bounce a few times. To eliminate that variable you should perform this simpler experiment by simply rolling the ball into the small block. (You might not always roll it with the same speed, but that s okay.)
6 The nature of work: For this experiment we want to focus on work that is done on the ball and work that is done on the block. To be perfectly clear about what work we are talking about, one should always specify the work done on what object and by what object. In this experiment we must recognize four different examples of work: the work done on the ball by your hand (as you start it moving), the work done on the ball by the table (as it rolls), the work done on the block by the ball (during the collision), and the work done on the ball by the block (during the collision). With the ball starting at rest on the table, gently roll the ball along the table so that it travels a short distance and then collides with a flat side of the block. 1. Work done on the ball by your hand. The ball started at rest and then a short time later it was rolling horizontally across the table (on the way to the block). It gained kinetic energy from somewhere. a. Where did that kinetic energy come from? b. As your hand was doing work on the ball, your hand exerted a force on the ball. What was the direction of the force that your hand exerted on the ball? (Agree with your colleagues on a way of specifying direction which might be east or west or it might be toward the window or toward the whiteboard. ) c. As your hand was doing work on the ball, the ball moved. What was the direction of the motion of the ball during the time your hand was exerting a force on it? d. Were your answers to questions #b and #c the same or different? e. Did this work add energy to the ball or subtract energy from the ball or neither?
7 2. Work done on the ball by the table. After the ball left your hand it rolled along the table for a short time (on the way to the block). Think about what you can see with regard to the kinetic energy of the ball during this time. a. Did the kinetic energy of the ball (visibly) change during this time? b. As the ball rolled along it was being pulled down by gravity. The table stopped it from falling, so we conclude that the table exerted a force on the ball. What was the direction of the force that the table exerted on the ball? c. As the table was pushing on the ball, the ball moved. What was the direction of the motion of the ball during the time the table was exerting this force on it? d. Were your answers to questions #b and #c the same or different? e. Did this work add energy to the ball or subtract energy from the ball or neither? 3. Work done on the block by the ball. The block started at rest and then a short time later it was sliding across the table (it quickly stopped but for this exercise just consider what happened starting just before the collision and ending while the ball was pushing on the block and the block was still in motion). The kinetic energy of the block changed. a. Where did that kinetic energy come from? b. As the ball was doing work on the block, the ball exerted a force on the ball. What was the direction of the force that the ball exerted on the block? (Use the same way of specifying direction which you used above.) c. As the ball was doing work on the block, the block moved. What was the direction of the motion of the block during the time the ball was exerting a force on it?
8 d. Were your answers to questions #b and #b the same or different? e. Did this work add energy to the block or subtract energy from the block or neither? 4. Work done on the ball by the block. Again, for this exercise just consider what happened starting just before the collision and ending while the ball was pushing on the block and the block was still in motion. a. Did the kinetic energy of the ball change during this time? b. As the ball was in contact with the block, the block exerted a force on the ball. What was the direction of the force that the block exerted on the ball? c. As the block was pushing on the ball, the ball moved. What was the direction of the motion of the ball during the time the block was exerting this force on it? d. Were your answers to questions #b and #c the same or different? e. Did this work add energy to the ball or subtract energy from the ball or neither? Vocabulary: We say that positive work is done on something if that work adds energy to that something. We say that negative work is done on something if that work takes energy away from that something. We say that zero work is done on something if the force involved neither gives energy to nor takes energy away from that something. Mechanical work always involves some sort of force (part b from the questions above) and some sort of movement (part c in the questions above). We would have to do a lot more experiments to convince ourselves that the results we just saw (in questions 1 through 4) are generally true. If we did so, we would find that the patterns we see here are always repeated. From your answers
9 to the previous four questions (especially to parts d and e) we should be able to answer three questions about mechanical work. 1. If object A exerts a force on object B, and object B moves in the same direction as the force that A exerts on B, then the work that A does on B is (positive, negative, or zero)? i. Just a reminder: does this work add energy to B or take it away? 2. If object A exerts a force on object B, and object B moves in the opposite direction as the force that A exerts on B, then the work that A does on B is (positive, negative, or zero)? i. Just a reminder: does this work add energy to B or take it away? 3. If object A exerts a force on object B, and B moves in a direction perpendicular to the force that A exerts on B, then the work that A does on B is (positive, negative, or zero)? i. Just a reminder: what does this work do to the energy of B? Now in your own words, if you want to do mechanical work on an object and add energy to the system, what do you have to do? In your own words, if you want to do mechanical work on something and take energy away from the system, what do you have to do?