Chapter 6: Energy and Machines Instructional Sequence

Section 6.1: Energy and the Conservation of Energy Four 45minute class periods Section 6.2: Work and Power Three 45minute class periods

Section 6.3: Simple Machines Four 45minute class periods

1. Complete Chapter 6 Pretest. 2. Complete Investigation 6A: Energy Transformations on a Roller Coaster.

Learning Goals

Activities and Resources

• Give examples of energy transformations in systems. • Compare and contrast potential and kinetic energy. • Apply the law of conservation of energy.

Laboratory Investigation 6A: Energy Transformations on a Roller Coaster

3. Read Section 6.1, pp. 128 to 133 and complete Section Review on page 134. 1. Read Section 6.2, pp. 135 to 138 an complete Section Review on page 139.

• Explain the meanings of work and power. • Apply equations to determine the amount of work done by a force or the power required to do work. • Use proper units to describe work and power.

Teacher’s Resource CD: • Skill and Practice: Work, Power

1. Complete Investigation 6B: Force, Work, and Machines.

• Identify examples and uses of simple machines. • Determine the mechanical advantage of machines. • Describe the forces that prevent machines from operating at 100% efficiency.

Laboratory Investigation 6B: Force, Work, and Machines Connection: A Mighty Energizing Wind Chapter Activity: Pop Goes the Balloon!

2. Read Section 6.3, pp. 140 to 146 and complete Section Review on page 147.

INQ.1.1 INQ.1.3 INQ.1.4 INQ.1.8 PS.B.2.1 PS.B.3.1

Teaching Illustrations CD: • Converting Energy, Systems and Variables, Some Important Variables in the System, Work

3. Complete Chapter Assessment, pp. 151 to 152.

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Teacher’s Resource CD: • Chapter 6 Pretest • Graphic Organizer: Energy and Work • Skill and Practice: Potential and Kinetic Energy

National Standards

UNIT 3: LAWS OF MOTION AND ENERGY

INQ.1.3 INQ.1.8 PS.B.2.3

Teaching Illustrations CD: • Power Example, Power

Teacher’s Resource CD: • Skill and Practice: Mechanical Advantage, Mechanical Advantage of Simple Machines, Gear Ratios, Efficiency, Using a Spreadsheet, Bicycle Gear Ratios Project Teaching Illustrations CD: • Energy Flow, Mechanical Advantage of a Lever, Mechanical Advantage, The Three Classes of Levers

INQ.1.2 INQ.1.3 INQ.1.7 INQ.1.8 PS.B.2.3 PS.B.3.1

CHAPTER 6 RESOURCES

Language Tools

Differentiated Strategies

Literary Picks

ELL Strategies: Listed in the ELL Ancillary

Vocabulary: joule, system, law of conservation of energy, potential energy, kinetic energy

Learning Strategies: • Discussion - Motivate • Cooperative Learning - Investigation 6A • Guided Discussion - Explain: Energy Cards • Discussion/Active Learning - Extend

Word Origins: energy

Materials Investigation 6A: CPO Roller Coasters, CPO Timers and photogates, Physics stands, steel marbles, meter sticks

Teaching Tip: Energy Transformations, Mass and Energy

Vocabulary: work, power, watt, horsepower

Learning Strategies: • Discussion - Motivate • Cooperative Learning - Explore • Guided Practice - Explain: Work and Power • Active Research and Discussion - Extend Teaching Tip: Work Explained, Power in Daily Activities

Vocabulary: machine, input, output, simple machine, efficiency, mechanical advantage Word Origins: machine Closure Cards

Learning Strategies: • Active Learning - Motivate • Cooperative Learning: Investigation 6B • Guided Discussion - Explain • Cross-Curricular Integration - Extend: Political Machines • Performance Assessment - Assess: Constructing Toys

Motivate: flagpole and flag, or window blinds Investigation 6B: Ropes and pulleys, spring scales, Physics stands, steel weights, meter sticks Chapter Activity: box tops from copy/printer paper boxes, scissors, tape, string, small balloons

Teaching Tip: Using Spring Scales

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6.1 Energy and the Conservation of Energy

Literary Picks... Reading Level

About the Lesson

3-5

In this lesson, students are learn about energy transformations in systems.

Ask students to describe their favorite amusement park ride. Many students will describe some form of roller coaster. Why do they enjoy roller coasters? Do your students ever think about physics when riding their favorite roller coaster. Ask students to consider what happens during a roller coaster ride. What types of forces are involved? Ask students to identify applications of each of Newton’s laws relative to the motion of a roller coaster.

INVESTIGATION 6A: ENERGY TRANSFORMATIONS ON A ROLLER COASTER Setup 1. One class period is needed to complete the investigation. 2. Students work in small groups. 3. Decide whether the timers will run on batteries or with AC adaptors. Then make sure there are electrical outlets within reach of each lab group or that the batteries in the timers are working.

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Materials • • • • •

CPO Roller Coaster CPO Timer and photogates Physics stand steel marble meter stick

UNIT 3: LAWS OF MOTION AND ENERGY

Vocabulary speed energy potential energy kinetic energy

Simple Machines by Deborah Hodge This well-illustrated book offers lower level readers insight into the world of simple machines. It is part of the Starting with Science series and includes simple experiments students can try with friends to learn more about how simple machines make work easier.

6-8

Then ask students, “How does the design of the roller coaster affect the ride?” For example, is a coaster with lots of loops more exciting than one with fewer? Why? Then have students speculate about how a roller coaster really works. Ask leading questions like, “What causes the cars on the coaster to move?” and “Is the speed of the cars the same at all points along the ride?”

Students complete Investigation 6A before reading section 6.1. While doing this investigation, students study the relationship between the speed and height of a marble rolling up and down a roller coaster track. Students gain a conceptual understanding energy conservation that as the marble loses one kind of energy (potential energy due to height), it gains another kind of energy (kinetic energy due to speed). The total amount of energy stays the same, so energy is conserved.

Title

Exploring the World of Physics: From Simple Machines to Nuclear Energy by John Hudson Tiner Part of the Exploring series, Tiner presents various physics concepts in an easy-to-read format. Advanced concepts, which are discussed in sidebars, provide students an opportunity to extend their learning.

9-12

Robots: Bringing Intelligent Machines to Life by Ruth Aylett Recommended by the NSTA, this book piques the interest of those fascinated by mechanical servants. This book connects many fields of science, including physics, biology, and even psychology. Contains many applications for advanced middle school and high school students.

6.1 ENERGY AND THE CONSERVATION OF ENERGY Teaching Tip . . . Guided Discussion: Energy Cards Have students generate a list of all the different forms of energy they know; and then use the list to create energy cards from white cardstock. Instruct students to draw a picture to represent the form of energy on one side of the card and to write its name on the other side. For example, a flame may be used to represent heat energy. Assign each student a partner. Read scenarios to students that involve energy transformations. Some examples are provided below. Once you have completed the reading, ask teams to display cards on their desks for each form of energy described. Walk around the classroom and observe which cards are displayed. Ask volunteers to hold up their cards and discuss each transformation. 1.

Ian and his friends went on a camping trip. They decided to gather logs and build a fire. What are the energy transformations involved in a burning campfire? (The burning logs represent chemical energy which produces energy in the form of heat, light, and sound from the crackling of the wood.)

2.

The gym teacher assigned students to teams for relay races. Describe the energy transformations involved in a student running in the race. (The student obtains chemical energy from the food she eats, which is converted to mechanical energy as she moves different parts of her body. Heat energy is also given off as she runs.)

3.

Amelie rang the doorbell when she arrived at her grandmother’s house. Does a transformation occur when Amelie rings the doorbell? (Yes, electrical energy is converted into sound.)

1.

Challenge students to work in teams to build a device that demonstrates at least three energy conversions.

2.

Divide students into small groups. Have them research to find out about alternative energy sources being studied by scientists. Instruct each group to make a presentation about their findings.

3.

Have students use publishing software to create a brochure or pamphlet about energy conservation. Ask students to include helpful tips, such as how to reduce home heating and cooling costs. Distribute the brochure to students throughout the school.

Energy Transformations Before doing the Energy Cards activity, be sure your students understand the meaning of the word transformation. Use the word in a sentence and have students use context clues to infer its meaning. For example, “There are many transformations in the life cycle of a butterfly.” When thinking about transformations, the main idea is change. It is important for students to know energy is not gained or lost; it simply changes form. One way to avoid this misconception is to explain to students that energy is a property of matter. For instance, rather than saying “energy moves” tell students that atoms and molecules move and carry energy.

Word Origins . . . Energy (from Greek energeia meaning “work”) Energy is often defined as the ability to do work. Physical scientists define work as a measure of the energy transferred as force is applied over a distance. Students learn more about work in the next lesson. As you prepare to introduce work, ask students to make a list of work or chores they do at home. Have students share the items on their lists, and then describe how energy is involved. You may even have students describe specific energy transformations related to the work (chores) they do. For example, have a student who does the laundry describe the energy transformations that occur when operating a washing machine.

Students complete section 6.1 review questions.

6.1 ENERGY AND THE CONSERVATION OF ENERGY

103

Investigation 6A: Energy Transformations on a Roller Coaster Today you will be observing what happens to the speed of a marble as it rolls up and down a roller coaster track. When doing the investigation, it might be helpful to imagine yourself coasting up and down hills on a bicycle. The purpose of the investigation is to get you to think about how energy changes from one kind to another. How do we define energy? Energy is the ability to cause change or the ability to do work. What kind of energy does a moving object have? A moving object has kinetic energy. The faster an object is moving, the greater its kinetic energy.

6A Energy Transformations on a Roller Coaster Where does the marble move the fastest, and why? To pedal your bicycle up a hill, you have to work hard to keep the bicycle moving. However, when you start down the other side of the hill, you can coast! In this investigation, you will see how a marble’s speed changes as it moves up and down hills. It’s all about energy!

A

A

2. Place the marble against the starting peg and let it roll down the track. 3. Watch the marble roll along the track. Where do you think it moves the fastest?

B

A hypothesis

C

Testing your idea

Set up the roller coaster

A hypothesis B Look at the diagram that shows the roller coaster track. There are seven numbered locations on the track. Where do you think the marble is moving the fastest? Write down your answer and the reason why you think it is the fastest place. Students should write down their predictions.

C Connect a photogate to input A on the timer. Set the timer to measure the time through Testing your idea

photogate A. You will use the timer to measure the speed of the marble at the seven different locations. How can you find the speed with the timer? The speed is the distance the marble moves (its diameter) divided by the time through the photogate. The diameter of the marble is 1.9 cm, so this is the distance you will use to find the speed. When you connect the photogate, make sure it is pressed against the bottom of the track. If the car is not against the bottom of the track, the full diameter of the marble will not break the photogate beam, causing inaccurate speed measurements. Place the photogate at each of the seven locations and measure the time. Record your data in Table 1. Then calculate the speeds.

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UNIT 3: LAWS OF MOTION AND ENERGY

CPO Roller Coaster steel marble CPO Timer and photogates meter stick Physics stand

1. Attach the roller coaster to the fifth hole from the bottom of the stand.

a.

Set up the roller coaster by attaching it to the fifth hole from the bottom of the stand. Hold the marble against the starting peg, and then release it. What do you notice about the speed? The marble's speed changes as it goes up and down the hills.

• • • • •

Set up the roller coaster

What kind of energy depends on an object's height? Potential energy depends on height. The higher an object is, the greater its potential energy. Keep these two types of energy in mind as you do the investigation.

Materials

Think about the seven places in the diagram. Where do you think the marble is moving the fastest? Choose one of the seven places and write down why you think that will be the fastest place.

1. Set the timer in interval mode and plug a photogate into input “A.” 2. Measure the time it takes the marble to roll through the photogate at each of the seven places. Be sure the photogate is pushed up against the bottom of the track. 3. The speed of the marble is its diameter divided by the time it takes to pass through the photogate. Find the speed of the marble at each position by dividing the diameter of the marble (1.9 cm) by the time through photogate A.

26

6.1 INVESTIGATION 6A: ENERGY TRANSFORMATIONS ON A Students should try to place the photogate at the same height for locations 2, 4, and 6. Positions 1 and 5 should be at the top of the hills, and position 7 should be at the very bottom.

Stop and think D Where was the marble moving the fastest? It was the fastest at the lowest points, position 3 and 7. Why do you think the speed is the fastest there? The marble speeds up as it goes downhill. It has had the most time to speed up once it gets to the lowest points. Where does the marble have the most potential energy, at the top or the bottom? It has the most potential energy at the top. As the marble rolls down the hill, it loses potential energy. Where does this energy go? It changes into kinetic energy. It takes energy to make the marble's speed increase. Where does this energy come from? The energy comes from potential energy. The force of gravity makes the potential energy change into kinetic energy.

Energy and change E Now you will use the photogate to measure the marble's speed every 10 cm along the track. You must also measure the height of the track at each position. To measure the height at a certain position, place the marble exactly on the mark. Measure the height straight down from the center of the marble to the surface of the table. Record your data in Table 2. Make sure students measure the height by holding the meter stick perpendicular to the table. At the first locations, the physics stand will be directly below the marble. Show students that they can simply measure the height straight down to the stand. They can then measure the distance from the table to the top of the stand and add to get the total height of the marble above the table

Teaching Tip . . . Mass and Energy One way to extend this activity is to ask students to predict the effect of replacing the steel marble with a lighter plastic marble. Students should make a hypothesis about whether the speed at various locations will be faster, slower, or the same. Students can test their hypotheses and discuss their results. The speeds will come out to almost exactly the same. Both kinetic and potential energies are directly proportional to mass, so the effect of mass cancels out. The marble with more mass has more potential energy at the top of the hill and more kinetic energy at the bottom. However, the speed at the bottom is only dependent on the starting height. Students may predict the heavier marble will move faster. They may have experienced a sled, waterslide, or other ride that supports this idea. In situations where the speed of an object going downhill is affected by mass, it is likely because of the effects of sliding friction and air resistance. The effect of these forces is more noticeable on objects with less mass.

Set up your graph paper like the sample shown in the investigation. You will plot two sets of points on your graph, height vs. position and speed vs. position. Assist students who need help setting up the axes. What does the graph show you about the relationship between speed and height of the marble on the roller coaster track? The graph shows that where the speed is fast, the height is small. Where speed is slow, height is large. Explain the graph using the terms potential energy, kinetic energy, and total energy. As the marble goes downhill, potential energy decreases and kinetic energy increases. As the marble goes uphill, potential energy increases and kinetic energy decreases. The total amount of energy always stays the same.

6.1 ENERGY AND THE CONSERVATION OF ENERGY

105

6.2 Work and Power About the Lesson In this lesson students explore the scientific meanings of work and power. They also learn to apply equations to determine the amount of work done by a force or the power required to do work.

Suppose a driver begins to experience problems with his car just as he approaches his home. The driver pulls over to the side of the road and the car comes to a complete stop. Two of his neighbors realize what has happened and offer to help him push the car home. After pushing on the car, the men are unable to move it. Ask students, “Was work done on the car? Why or why not?” Allow students to discuss their opinions. Listen carefully to determine if students are using physics terms, like force and motion to express their beliefs. Then tell students that no work was done on the car because it did not move. Say to students, “A third neighbor came to assist the three men who were attempting to move the car. With his help, they were able to push the car all the way home.” Was work done? Of course, the answer is that work was done. Tell students that work is done whenever an applied force causes an object to move some distance. Have students identify the applied force (men pushing on the car). Then ask, “How might you determine the distance?” The distance could be measured from the position the car came to a complete stop on the roadside to the driver’s home.

Have your students work in small groups to create Power Point presentations about inventions or technology that make it possible to do work faster or more efficiently. For example, students may discuss how the introduction of tractors (in place of horses or mules) changed the agricultural industry forever. Their presentations should explain how the power of horses compares to the power of the first tractors. They should also discuss improvements from the first tractors to modern-day agricultural tools and talk about the implications of these improvements. Encourage students to read books or to visit the websites of manufacturers to learn more about the invention or technology they choose. Allow time for students to share their presentations after completing lesson 6.3.

108

UNIT 3: LAWS OF MOTION AND ENERGY

Teaching Tip . . . Work Explained After revealing the equation for work, as W = F ∞ d, ask students to consider its definition. What does the force times distance really mean? It is important for students to understand that calculating work is really about measuring the transfer of energy. Have students read page 136 of the student text. Focus on the second paragraph, which discusses the work done on a block as it is slid across a level table. Slide a wooden block across a level table in your classroom. Then explain to students how energy is transferred in the process. The person pushing the block uses stored energy obtained from the food he or she ate. The block and table can be viewed as a system; so while pushing the block along the table, the particles that make up this system move faster. The slight temperature increase in the system is a result of these particles moving faster and the force of friction. Consequently, energy is transferred from the person to the system. Ask students, “Do you think we could determine the energy gained by the particles in the wooden block and the table?” This would not be possible. Instead, the energy of the wooden block and table system can be calculated by applying the equation to determine the work done on the block. In other words, the energy, in joules, transferred from the person to the system is equal to the force (in newtons) applied to the block multiplied by the distance (in meters) the block is moved along the table in the direction of the force. Then say to students, “Suppose the person pushing the block applies 20.0 N of force and pushes the block 5.00 meters. How much work is done by the person?” Model this calculation on the board for students. The work done is 100 joules.

6.2 WORK AND POWER Teaching Tip . . . Guided Practice: Work and Power Use the problems provided below to help students apply the appropriate formulas to solve work and power problems. 1.

Joy applies 85.00 N of force for a 15.00 meter distance in order to push a chair across the floor. How much work does Joy do? (1275 J)

2.

Trevor does 3545 J of work as he pushes a box of books 25.0 meters. Find the force, in newtons, needed for Trevor to do this work? (142 N; Be prepared to show students how to change the form of the equation to solve for force.)

3.

What is the power needed to do 365 J of work in 5.7 seconds? (64 watts)

4.

Tamika and Sharon are very competitive friends. One afternoon, Tamika challenged Sharon to a wheelbarrow race. The girls obtained identical wheelbarrows and agreed to push them 85 meters. Tamika applied 135 N of force to push her wheelbarrow, while Sharon applied a force of 140 N. a. How much work did each girl do? (Tamika did 11,475 J of work. Sharon did 11,900 J of work.) b. Tamika reached the finish line in 52 seconds, while Sharon needed only 48 seconds to finish the race. Which girl’s power is greater? (Sharon’s power, 248 watts, is greater than Tamika’s 221 watts of power.) c. Tamika challenged Sharon to a rematch. This time, the race ended in a tie with each girl finishing in 50 seconds. If both girls did the same amount of work as in question a, who has the greater power? (Because Sharon did more work than Tamika in the same amount of time, her power is greater. d. If Sharon’s work and power remains the same as in question c, what could Tamika do to win the race? (Tamika could either increase the force applied to the wheelbarrow to surpass Sharon’s work or she could try to push the wheelbarrow much faster.)

has

See the Power in Daily Activities teaching tip for an idea.

Students complete section 6.2 review questions.

Power in Daily Activities Have students refer to the owner’s manual of their family’s car to find out the amount of horsepower produced by its engine. How many watts does this equal? Then have students maintain a log that contains the following information for two weeks: • • • •

the type of car driven the number of gallons of gasoline put into the car the price per gallon of gasoline the number of miles traveled over the two-week period • the kilowatt hours of electricity used at their home At the end of two weeks, create a large chart for students to record their information. Then ask students these questions: 1.

How does the horsepower of different cars compare?

2.

What was the cost of fuel per mile traveled?

3.

Which car was the most fuel efficient?

4.

How many gallons of gasoline were used by all of the cars? What was the average price of gasoline per gallon?

5.

If the cost of electricity was 8 cents per kilowatt hour and each gallon of gasoline contains about 36 kilowatt hours, which was more costly over the two-week period: fueling the family car or providing electricity to the home? If students have access to an electric bill, have them use the actual cost per kilowatt hour in this calculation.

Be sure to send a letter home to parents which explains the activity and asks for their cooperation. Ask parents to help students with reading the electric meter and odometer; and to pass along gasoline receipts or records to their children.

6.2 WORK AND POWER

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6.3 Simple Machines

Word Origins . . . Machine (from Latin machina meaning “device”)

About the Lesson This lesson focuses on simple machines and how they function in doing work.

Ask students if they have ever observed a person raise and lower a flag on a flagpole. Have a volunteer describe the process. Then ask, “Is there physics involved?” Encourage students to explain how. Then take your students outdoors to observe the lowering and raising of the flag at your school. Get permission from your principal or another administrator in advance. As your raise and lower the flag, point out the parts involved such as the pulleys, the truck, and the halyard (the cable or rope). Then ask students to think about how the ropes and pulleys make the tasks of raising and lowering the flag easier. Can they think of other examples where ropes and pulleys work together to make work easier? If a flagpole is not available, you can demonstrate how ropes and pulleys do work by opening and closing curtains or raising and lowering blinds in your classroom.

Students complete Investigation 6B before reading section 6.3. In this investigation, students use a set of ropes and pulleys to discover the relationship between input and output force and distance.

INVESTIGATION 6B: FORCE, WORK, AND MACHINES Setup 1. Allow one class period for students to complete the investigation. 2. Students work in small groups.

Materials • • • • •

Ropes and pulleys Spring scales Physics stand Four steel weights Meter stick

Vocabulary input force output force input distance output distance mechanical advantage work

Display a number of objects which are made up of simple machines. Lead students in a discussion to help them identify the simple machines working together in each object.

110

UNIT 3: LAWS OF MOTION AND ENERGY

Begin lesson 6.3 by writing the word, MACHINE, on the three to four sheets of chart paper affixed to the walls in your classroom. Position the chart paper so that every student is close to at least one sheet. Then allow groups of students two minutes to write whatever comes to mind when they think of a machine on the sheet of paper that is closest to them. You may opt to divide students into small groups and have one person act as the recorder, while other members generate ideas. Look at the words students recorded. How many of the words or examples of machines listed refer to devices powered by an engine, electricity, or some other means? This is usually students’ first reaction. However, it is important for students to realize that not all machines are powered. Show students everyday objects which are comprised of simple machines, like scissors. Then ask, “Is a pair of scissors an example of a machine?” Have students explain why they believe it is (or is not) a machine. Then ask, “What is a machine?” Steer students toward understanding that a machine is any device that can be used to do work. Ask students to think about the work that scissors allow humans to do. Dismantle the scissors and show students its parts. Scissors are made up of levers and wedges, two types of simple machines. Have students think about how each simple machine contributes to the function of scissors. Are they able to relate these devices to the work scissors can do?

6.3 SIMPLE MACHINES Teaching Tip . . . Learning Across the Curriculum: Political Machines

Try the Political Machines activity as an extension to this lesson.

Students complete section 6.3 review questions. Then have students work in groups to design and construct toys using simple machines. Allow time for students to brainstorm possible design ideas and to plan how they will construct their toys. Once students have built their toys, let them share their creations with the class. Use the rubric provided below to assess students’ performance. Have other students score the “cool factor” of their classmates’ toys. The highest possible score is 20 points. Category

4

3

2

1

Scientific Knowledge

Explanations by all groups members point to a solid understanding of simple machines

Explanations by all groups members point to a relatively solid understanding of simple machines

Explanations by most groups members point to a relatively solid understanding of simple machines

Explanations by most groups members point to a clear lack of understanding of simple machines

Plan

Plan is very Plan is neat, Plan is neat, Plan is somewhat disorganized with organized, and all organized, and disorganized but incorrect or structures are most structures most structures unclearly labeled clearly labeled are clearly labeled are clearly labeled structures

Function

Toy functions Toy does not Toy functions very Toy functions well pretty well but function and falls well and holds up and holds up to does not hold up to apart under normal to normal stresses normal stresses normal stresses stresses

Materials

Suitable materials were used to construct the toy. It is obvious that great care was taken in building the toy.

Mostly suitable materials were used to construct the toy. It is obvious that care was taken in building the toy.

Cool Factor

Way cool

Cool

Mostly suitable The materials materials were selected were used to construct inappropriate and the toy but the the toy is obviously product appears carelessly carelessly built. constructed. Sort of cool

What is a political machine? Ask a social studies teacher to partner with you in presenting a lesson on notable political machines in American history. Then have students discuss the similarities between political machines and physics machines. Have students work in groups. Some leading questions may include the following: 1.

Think of one example of a machine (in physics terms). What are its parts and how do these parts work together to accomplish a specific task?

2.

Consider the political machine your group has chosen to learn more about. What are the parts (or members) of this machine? What were some of the specific tasks they hoped to accomplish?

3.

Describe the input and output of your physics machine.

4.

Who was the boss of the political machine you studied? What were some of the things he did to make the machine work? What did the machine do for the boss?

5.

How efficiently have political machines operated throughout American history?

Help students to see that like machines in the physics sense, political machines are designed to do specific things, and they are based on input and output.

So-so

6.3 SIMPLE MACHINES

111

Investigation 6B: Force, Work, and Machines

A

Building a simple machine

How do simple machines operate?

In this investigation, you will be using a set of ropes and pulleys. You will set up various combinations of ropes and pulleys to lift weights on the bottom block. Attach four weights to the block and use a spring scale to measure the weight of the block with the weights. The output force for the investigation is equal to this weight. Record the output force in Table 1. See the teaching tip on the next page. Attach the top block to the top hole of the physics stand. Run the yellow string over the top pulley and clip it to the bottom block. Clip the other end of the yellow string to the 10 N spring scale. When you pull the spring scale down, the bottom block should move up. Make sure you pull straight down, not sideways. Measure the force it takes for you to slowly lift the bottom block. Record it in Table 1. Use equipment from one of the groups to demonstrate the set-up. Show students what happens if they pull sideways instead of downward (the physics stand tips over). Detach the yellow string from the bottom block and the spring scale. Clip one end of the string to the top block. Run the string so it passes through one of the pulleys on the bottom block and then back up over one of the pulleys on the top block. Clip the other end to the spring scale. You should now be able to lift the bottom block by pulling down on the spring scale. You should notice that there are now two lengths of the yellow string supporting the bottom block. Record the spring scale force. Students may need help figuring out how to run the string over the two pulleys. It is much easier to understand if the set-up is demonstrated. You have already build rope and pulley combinations with one and two strings supporting the bottom block. Now build combinations with 3, 4, 5, and 6 strings. Record the force for each in the table. Students may not initially be able to figure out how to build each combination. Do not immediate offer the solution. Encourage students to work with their group members to find the answer.

Thinking about what you observed B What happened to the input force needed to lift the block as you increased the number of supporting strings? The force decreased as the number of strings increased. Write a rule that relates the number of pulleys, input force, and output force. Give the groups ample time to figure out the rule. Then have them share with the class. The solution is that the number of strings multiplied by the input force is equal to the output force. What is the meaning of the term mechanical advantage? Mechanical advantage is the output force divided by the input force.

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6B Force, Work and Machines

UNIT 3: LAWS OF MOTION AND ENERGY

Machines are things humans invent to make tasks easier. Simple machines use directly applied forces. Simple machines allowed humans to build the great pyramids and other monuments using only muscle power. This Investigation is about how simple machines use force to accomplish a task.

A

Materials • • • • •

Ropes and pulleys Spring scales Physics stand Four steel weights Meter stick

Building a simple machine 1. Attach four weights to the bottom block. Use a spring scale to measure the weight of the bottom block and record it as the output force. 2. Attach the top block near the top of the physics stand. 3. Thread the yellow string over one or more of the pulleys of the top and bottom pulley blocks. The yellow string can be clipped to either the top block or the bottom block. 4. Build combinations with 1, 2, 3, 4, 5, and 6 supporting strings directly supporting the bottom block. (Hint: 1, 3, and 5 have the string clipped to the bottom block. 2, 4, and 6 have the string clipped to the top block) 5. Use a force scale to measure the force needed to slowly lift the bottom block for different combinations of supporting strings.

Safety Tip: Don’t pull sideways, or you can tip the stand over! Table 1: Input and Output Forces Input force Output force (newtons) (newtons)

Number of supporting strings 1 2 3

B

Thinking about what you observed

a.

As you increase the number of supporting strings, what happens to the force needed to lift the bottom block?

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6.3 INVESTIGATION 6B: FORCE, WORK, AND MACHINES Find the mechanical advantage for each combination of ropes and pulleys you made. The mechanical advantage is equal to the number of supporting strings for each combination.

The input and output distance C Now you will be measuring the input and output distance. Table 2 has columns for input force. You do not have to measure the force again; simply copy the values from Table 1. Hold the free end of the string with your hand instead of using the spring scale. Push both of the cord stops so they are touching the upper block. See the diagram in your investigation. Make sure students have the cord stops properly positioned at the top of the free end of the string. Choose an output distance that you will lift the bottom block during each trial. It should be at least 20 cm. Pull the yellow string down to lift the bottom block this distance. Try to be as accurate as possible. As you pull the string down, both of the cord stops will move down. Slide one of them up to where it started. The distance between the two stops is equal to the input distance. Record the input and output distances in meters. Demonstrate how to use the cord stops to measure the input distance. Remind students that they must divide the number of centimeters by 100 to convert to meters.

Teaching Tip . . . Using Spring Scales Before handing out the spring scales, remind students that they will permanently stretch out the springs if they put too much force on them. Pulling a 2.5 N scale with a force of 10 N will stretch the spring beyond repair. When taking measurements throughout the investigation, students should start with the spring scale with the largest range. Once they have a rough idea of the force, they should then choose the appropriate smaller spring scale to get the most accurate measurement.

Repeat for a mechanical advantage of 2, 3, 4, 5, and 6.

Thinking about what you observed D How is the mechanical advantage related to the input distance needed to lift the block your chosen output distance? The greater the mechanical advantage, the greater the input distance needed to lift the lower block. The word work can be used in many different ways. Write a sentence explaining the scientific meaning of the word work. Work is equal to force multiplied by the distance an object moves in the direction of the force. Work results in a transfer of energy as an object moves. You may have heard the saying “nothing is free.” Explain how this is true of the ropes and pulleys. Think about a trade-off that occurs. If a set of ropes and pulleys has a large mechanical advantage, it takes only a small input force to create a large output force. However, a large input distance only causes a small output distance. There is a trade-off between force and distance. Calculate the input and output work for each rope and pulley combinations. Point out that students previously converted the distance from centimeters to meters so they could calculate work in joules. One joule equals one newton multiplied by one meter.

The rules of simple machines E Students answer the questions in part five.

6.3 SIMPLE MACHINES

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