CHANGE: MECHANICAL ENERGY
Grades 8-12

This physics or physical science lesson is designed to introduce students in grades 9-12 to the concept of mechanical energy; however, if the mathematical equations are omitted, the lesson can be adapted for grade 8. Energy describes the ability to do work. Work applied to an object can increase its ability to do work, thus its energy. Work done by an object can likewise decrease its energy. Students will learn that mechanical energy is the energy possessed by an object due to its position or its speed. Energy due to position is referred to as "potential" energy, while energy due to an object's speed is referred to as "kinetic" energy. An object's energy may change from one form to another, or may be used to do work. The video used in the lesson introduces the conservation of energy to students. The hands-on activity uses a pendulum to illustrate energy transformation. This lesson can be completed in 1-2 days.

Mechanical Universe #113: Conservation of Energy

Students will be able to:
1. describe and calculate an object's potential energy.
2. describe and calculate an object's kinetic energy.
3. identify the types of mechanical energy possessed by an object.
4. describe and utilize the relationship between energy and work.
5. describe transformation from one energy type to another by an object.

Texas Assessment of Academic Skills (TAAS), Grade 8
Science Objectives:
#3: Communicate scientific data and/or information.
#4: Interpret scientific data and/or information.
#5: Make inferences, form generalized statements and/or make predictions.
#7: Draw conclusions about the process(es) and/or outcome(s) of a scientific investigation.
#8: Relate and apply scientific and technological information to daily life.

Math Objectives:
#11: Determine solution strategies and analyze or solve problems.
#12: Express or solve problems using mathematical representation.
#13: Evaluate the reasonableness of a solution to a problem situation.

NCTM Standards for Grades 9-12
Standard 13: Conceptual Underpinnings of Calculus
Standard 14: Mathematical Structure

Per group of 2-3 students:

• ring stand with two attachments extending perpendicular to the vertical rod
• 50 cm of string
• 250 g mass which can be attached to the string
• meter stick
• Transforming Energy procedure sheet

• energy-the ability to do work.
• potential energy-energy of an object due to position. PE = mgh
• kinetic energy-energy of an object due to speed. KE = (1/2)mv2
• mechanical energy-energy of an object due to position or motion.
• work-displacement of an object by a force. W = F*d

Prior to this lesson students should be introduced to the concept of work and be able to calculate the work done on an object by a force. Ask a student to state the definition of work. When properly stated, put in on the board or the overhead. Give two or three examples of work and ask students to calculate in their notebooks the work done. Ask a student to show and explain the calculation on the board. Briefly discuss and clarify the calculation. Repeat for other problems if necessary.
Show the students a hammer, a nail, and a piece of wood. Ask students, "If I drive this nail into the wood, is work done?" Students should recognize that since the nail is moved a distance by the force of the hammer, work is done. Ask students to explain from where the work comes. Students should recognize after some discussion that the hammer is doing the actual work on the nail, while you are applying work to the hammer. Ask students to write in their notebooks if the hammer has any "ability" to do work without you.
Pose this focus question to students: What gives an object the ability to do work?

To give students specific responsibilities while viewing, ask students to watch the video and, based on what they see and hear, be prepared to do the following:
a) identify illustrations of work;
b) identify the role of work; and
c) define potential energy.

BEGIN the Mechanical Universe video as the weight lifter in the black shirt drops the barbell and the narrator says, "...and why do weights fall?" PAUSE the video on the weight lifter in the black shirt when the narrator says, "Part of the answer is work." Ask students to define work and the variables that it depends on, force and distance. RESUME the video.
PAUSE the video on the weight lifter in the blue shirt when the narrator says, "Work is force times height." Ask the students who is doing work and what is he doing work against. Ask students to draw a force diagram of the barbell. Ask students how many forces are acting on the barbell. Ask students, "If he lifts the barbell slowly enough, how much force is required?" Students should recognize the upward force should equal the weight of the bar. RESUME the video.
PAUSE the video on the weight lifter in blue when the narrator says, "So work is mass times g times height." Ask students if they agree. Ask students to identify what was required of the weight lifter to raise the bar. Keep asking leading questions until students recognize the energy is required. Then ask students to write a definition of energy in their own words in their notebooks. Although students are familiar with the term "energy," they may have trouble coming up with a definition. Encourage them to do their best. RESUME the video.
PAUSE the video on the weight lifter in red when the narrator says, "In the conservation of energy, the role of work is to transfer energy from one place to another." Ask student to write this in notebooks. Ask students, "In the case of the barbell, where was the energy transferred from and to? Is the energy still there? Could the barbell be used to do work? Could the barbell be used to pound in a nail?" Have students explain how the barbell could do work on a nail. RESUME the video.
PAUSE the video on the weight lifter in the black shirt when the narrator says, "In other words, the potential energy is exactly equal to the work put into it." Ask students why it is called "potential energy". Students should recognize after some discussion that the barbell has the "potential" or ability to do work due to its height. State the definition of energy and potential energy for students. RESUME the video.
PAUSE the video on the weight lifter in the black shirt when the narrator says, "For all its ups and downs, potential energy only depends on height." Ask students about the truth of that statement. Students should recognize that potential energy also depends on the mass of the object and the acceleration due to gravity. Clarify for students that only if the mass does not change, and we are near the surface of the earth, where g is constant, will potential energy depend solely on height. RESUME the video
PAUSE the video on the weight lifter in the black shirt when the narrator says, "Potential energy relates only on height, no matter how work is done." Clarify for students that simple machines do not alter the net work done. Simple machines may be explored further in another lesson. RESUME the video.
STOP the video on the weights when the narrator says, "Potential energy by itself is not conserved." Ask students to explain what happens to potential energy when the weight goes down. Goes up? Is potential energy saved or conserved? Emphasize for students that potential energy changes whenever an object moves up or down.

The Transforming Energy activity (see procedure sheet attached to this lesson) explores the transformation from potential energy to kinetic energy. Divide the class into groups of two or three students. Each group needs materials to construct a simple pendulum, a meter stick, a copy of the procedure sheet and sufficient time and space to conduct the activity.
[Special note to teachers: On Procedure #4, it may take a few subtle hints for students to realize the highest point is initially the release point. From then on, it is at the turning points of the pendulum, where the speed is zero. In Analysis Step #2, students may notice that some energy is lost, due primarily to friction at the pivot and in the collision. This is an opportunity to discuss how the pendulum can be designed to minimize friction.]
In the discussion following the activity, students should recognize the following points about the pendulum:
a) No outside work is done on the pendulum after it is initially raised.
b) The potential energy continually rises and falls.
c) The ability of the pendulum to rise from the bottom it due to its speed at the bottom of its swing.

Ask students if the pendulum has the ability to do work at the bottom of its swing. Could it drive a nail? Students should recognize that this ability to do work is a different type of energy. Tell students that this form of energy is "kinetic" energy and state the definition.
Return to the video at this point. Fast forward the video to the point when Galileo appears on the screen just before the narrator says, "Toward the end of the 16th century, ..." Resume the video. As the video plays, point out to students the similarity between Galileo's inclined planes and the pendulum. Stop the video with the girl on the swing when narrator says, "Even though energy transformation is child's play, it all begins with a little work." Ask students to chuckle, if they are not already doing so.

Using their understanding of work and potential energy, students can calculate the work done in climbing stairs. On a stairway, students can time one another and then calculate their power by dividing the work done climbing by the time required to climb the stairs. This will tell them who has the greatest horsepower.
Students can explore the use of work and potential and kinetic energy at an amusement park. A ride on a roller coaster provides a great hands-on demonstration of energy transformation. By finding the height of various peaks and the weight of the roller coaster, students can calculate the work done to start it, the potential energy at the top, and kinetic energy (and thus the speed) at the bottom of the ride.

Social Studies: Students can study the development of hydroelectric dams in the Tennessee Valley and the Northwest used to turn the potential energy and kinetic energy of falling water to produce electrical energy to do work.
English: Students can listen to and read the lyrics of Woodie Guthrie's songs about the Columbia River and the Grand Coulee Dam. Students may also read and discuss Ken Kesey's vivid and physically accurate description of a downhill escape on a railroad handcar, in the novel A Sailor Song.
History/Math/Science: Students can study the role Galileo played in the Renaissance, his difficulties with the church, and his impact on science and mathematics.
Physical Education: Students can calculate the work done and energy consumed over an entire work out. Track and field, skiing, gymnastics, and professional wrestling provide great examples of the use of work and energy transformations.

1995-1996 National Teacher Training Institute / Austin

Master Teacher: Brendan Maxcy