Work, Power and You

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Objective

Students apply the concepts of work and power to explain how a roller coaster gets its starting energy.

Big Idea

Work is a measure of the transfer of energy and power a measure of the rate at which energy is transferred.

Context

Students learn about the physics definitions of work (a transfer of energy) and power (the rate at which energy is transferred). These concepts are used to build on what students have learned about conservation of energy in Skate Park Energy and Skate Park Energy Revisited. This lesson prepares students to design their own roller coaster as work must be done on the roller coaster to give its starting energy and power is used to determine how long it takes to give the roller coaster that energy.

As students determine the energy to give a roller coaster to its starting energy, they apply NGSS HS-PS3-1: Create a computational model to calculate the change in the energy of one component in a system when the change in energy of the other component(s) and energy flows in and out of the system are known. The system is the Earth and the roller coaster and it is a motor that gives it its starting energy. Students will also apply NGSS Science Practice 6: Constructing explanations (for science) and designing solutions (for engineering), Science Practice 2: Developing and using models and Science Practice 5: Using mathematics and computational thinking as they calculate the variables for another roller coaster project.

Demonstration of Not Doing Work

5 minutes

Because work is a topic that has many uses in our everyday vernacular, we start with a demonstration of what work is NOT as defined by physics. In physics, an object must change position in order for work to be done on it. Holding a heavy object stationary, though it may take great effort, does no work. 

I ask for a big, strong student volunteer to show off his or her ability to do "work". Usually a few of the male students raise their hands and I call on the biggest of the bunch. I challenge the student to grab some physics books (a bigger student like the one in this non-work demonstration video gets three; depending on the student I have have them do 1 or 2 so that they can do the demo for at least 20 seconds). I have the student hold the physics textbooks out in front of him or her. We, as a class, encourage him or her to hold the books out there as long as he or she can. After the student is done, I ask the question, "Did the student do a lot of work to hold those books?" Most students respond with a yes, the student applied tremendous work to accomplish such a feat. I then transition to the lecture.

Introduction to Work and Power Lecture

20 minutes

I instruct students to take out their notebooks for the lecture. I inform the class that, from a physics perspective, the student who did the demo did no work while holding the textbooks in front on him. I begin the work and power power point and ask the student what they think of when they hear the word work. I explain that in our every day vernacular, work means a lot of different things: homework, a job, construction work, etc. However, from a physics perspective work has a sigular definition; a transfer of energy into or out of a system! 

I give the formula definition of work, the SI Units and some example word problems. We then do the same for power; a definition, formula, units and example problems. While all of this is happening, I monitor students to make sure they are talking notes and I answer any questions they have.

Students Practice with Roller Coaster

20 minutes

After the lecture, I hand out Roller Coaster Worksheet 2. It contains the familiar problem of a roller coaster similar to problems seen in a previous lesson. This assignment provides additional practice on the application of conservation of energy and changes from kinetic to potential and back. However, this sheet adds the concepts of work and power as it applies to a roller coaster. Like before, the numbers used are simple (e.g. total energy of 100 J) so that the focus is on the conceptual understanding of energy transformations and not on plugging large, complex numbers into a calculator.

Students work independently on this assignment as most students have the background knowledge and skill to complete this independently. I do walk around the classroom and provide support where needed. Students fill out the table first to determine the different types of energies at different points on the roller coaster. This is a simple task as the total energy at every point is 100 joules and the kinetic energy and potential energy should always add up to 100 joules. Then students calculate the velocities and heights at each point. The final task is to determine the work needed to give the coaster its starting energy. If the coaster starts with zero energy and has 100 joules at the top of the hill, then the work done is 100 joules. Many students believe they have to perform a calculation, but when I remind them that work is a transfer of energy, they realize that the total work done to get the coaster to its starting position is 100 J.

In the last few minutes of class, we review the worksheet. I display Roller Coaster Worksheet 2 - Solutions so that students can check their own answers. I tell the class that in a few days they will design their own roller coaster and this activity can be a resource. So they should fix any mistakes they have on the sheet and seek to understand what their mistakes are.