Rubber Bands & Springs

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Rubber Bands & Springs:POTENTIAL ENERGY (CONDITION)CONVERTED TO KINETIC ENERGY

MaterialsFor each group:oo K’NEX pieces to build a Rubber Band Rolleroo #32 Rubber Bandsoo K’NEXprint for Rubber Band Roller

ObjectivesStudents will:

_ infer that an object’s condition can determine its potential energy_ understand that Rubber Bands have potential energy when they

are stretched_ design and perform an experiment with a Rubber Band Roller_ collect, organize, graph and analyze data from the experiment_ define inertia and infer that inertia must be overcome before an

object can move

Introduce the Concept 1. Take out a Rubber Band from the K’NEX kit and hold it up for the

students to see.Ask students if the Rubber Band has any energy. Somestudents might suggest that it has potential energy because of its height.Affirm this answer, and ask how you might increase the potential energyof the Rubber Band without raising it any higher. If the students do notsuggest stretching the Rubber Band, gently pull on the Rubber Band tomake it stretch. Students will soon identify that the Rubber Band haspotential energy when it is stretched.

Ask: “Is there another way to add energy to the Rubber Band besidesstretching it?” (someone may suggest twisting the Rubber Band)

2. Ask students to list objects or toys that use energy which is held instretched or twisted Rubber Bands. Students might suggest Rubber Bandpowered road vehicles, airplanes or slingshots. Help students realize thateach of these items produces motion when the Rubber Band is released.A stretched Rubber Band stores energy, which is converted to kineticenergy as the Rubber Band returns to its original shape.You mightcompare Rubber Bands to springs, which also store energy.

In earlier activities, students studied potential energy due to gravity(gravitational potential energy). Now students are investigating

Activity 5:Build and Testa K’NEXRubber BandRollerIn this activity,students will findthat Rubber Bandshave potential(stored) energywhen they arestretched ortwisted. Studentswill use this storedenergy to move aRubber BandRoller across thefloor. They willexperiment to findout how far theRoller can travelfor each twist ofthe Rubber Band.

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Concept (continued)

potential energy due to the condition of a Rubber Band or SpringMotor (elastic potential energy). Both have energy because of theirconditions (stretched, twisted, wound, etc.).

3. Return to the students’ definitions of energy that they worked on in Activity 1. Guide students to reinforce their definitions of potentialenergy.Their definitions should include references to:oo stored energyoo positionoo condition

4. Show students a K’NEX Rubber Band Roller. Let one student wind itup and run it across a table for the class to see. Explain that thisRubber Band Roller can store potential energy in its twisted RubberBand.Then, the potential energy can be converted to kinetic energyas the Rubber Band Roller scoots along.Tell the class that each groupwill build a Rubber Band Roller, store potential energy in the Roller’sRubber Band, and then measure how far the Roller can travel usingthis energy.

Build and Explorea Build a Roller.

Divide the class into small groups, and have each group build aRubber Band Roller according to the K’NEXprint instructions.

Teacher Note:As a safety measure, check Rubber Bandsbefore they are distributed, or have each group bring their Roller to you for inspectionbefore they use it for experimentation.

a Examine how the Roller works.

First, instruct students to hold the Rubber Band Roller in one hand andturn the blue Rod five times with the other hand.Ask them to noticewhat happens to the Rubber Band as they turn the blue Rod. (itbecomes twisted; it stretches within a small space) Have them hold theblue Rod in place for now.

Ask: “Have you changed the amount of potential energy in theRubber Band?”(yes)

Instruct students to let the blue Rod go (and keep their fingers out ofthe way of the Rod!) and watch what happens. (The blue Rod spins asthe Rubber Band unwinds without flying away and causing safetyconcerns.) Guide students to see that the Rubber Band Roller’s potentialenergy was converted to kinetic energy, the energy of motion.Thatenergy actually makes the blue Rod move.

Activity 5:Continued

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a Increase the Rubber Band Roller’s energy.

Ask students to rewind their Roller to increase the amount of potentialenergy in the Rubber Band Roller.They should wind their Rubber BandRollers more than five times to give the Rollers more potential energythan they had before.

Discuss and write the following safety note on the chalkboard.

Safety Note: BE CAREFUL NOT TO OVERWIND THEROLLER. Overwinding could distort the shape of the Roller,and cause the Rubber Band to snap and cause injury. If you notice any deterioration of your Rubber Band, notify your teacher immediately.

Then, have students hold tightly onto the blue Rod, let go of the RubberBand Roller with the other hand, and turn the Rubber Band Roller to avertical position.The Rubber Band should unwind, causing the Roller tospin. Reinforce the idea that the Rubber Band stores energy, which isconverted to kinetic energy.

a Let the Rubber Band Rollers run!

Next, have students wind their Rubber Bands 5 or more times and thenplace their Rubber Band Rollers on a smooth floor in an open area.Theyshould not place the Rubber Band Rollers too close together.Whenreleased, the Rubber Band Rollers will race across the floor.

Students might notice that the Rubber Band does not always unwindcompletely. Guide them to see that the Rubber Band Roller still hassome potential energy, but it does not have enough potential energy tomove the Rubber Band Roller any further, due to the friction betweenthe Roller and the floor.

Ask: “Does the Roller have enough energy to spin the blue Rodwhen the roller is lifted off the floor?” (yes) ”Did the RubberBand unwind completely after the Roller was lifted off thefloor? Why? Why not?” Allow the students to offer suggestions toanswer this question but do not confirm any at this time.

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a Design an experiment.

Tell the students that they are going to design and perform anexperiment to determine how far the Rubber Band Roller travels foreach twist of the Rubber Band.

a Determine how to make your measurements.

Tell students that they need to devise an accurate way to measure thedistance the K’NEX Rubber Band Roller travels. Remind them that theyhave observed how the Rubber Band Roller works, so they should havesome idea how to begin.They should consider:

oowhat measuring tools to use;oo on what surface to run the experiment;oo how to mark the starting and ending positions each time they let the

Roller run; andoowhether to consider the front, back, or center of the Rubber Band

Roller as the point that is placed on the starting line and is judged asthe stopping position.

Once they have cooperated to plan their measuring method, have themwrite a short description of the method. Remind them to maintain aconsistent method throughout all the trials. They should make surethat the Rubber Band is completely unwound before they start towind it up for each subsequent trial.

Prepare a Data ChartHave each group prepare a Data Chart like the one shown here.Youmight sketch a chart on the chalkboard for them to copy.They shouldalso complete the Student Worksheet for Activity 5.


Data CHART

Size of Rubber Band used

Trial #1

Trial #2

Trial #3

Average Distance Traveled

Distance Traveled by Roller(centimeters):

Number of Twistsof Rubber Band

10 15 20

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a Try a few practice runs.

Have students try a few practice runs before they start measuring.Sometimes, the Rubber Band Roller may run in a curved path.Askstudents to consider how they could measure the distance traveled bythe Rubber Band Roller if its path is curved. Help them to see that ifthey measure from the starting point to the ending point of a curvedpath, the resulting measurement will be shorter than the real path.

Suggest that students either repeat such trials and use onlymeasurements of straight paths or develop a strategy to accuratelymeasure curved paths. (e.g. If students are using a surface which can be inked, students can measure the curved distance traveled by inking the wheels and using a string to measure the actual curved path traveled. Students would then measure the length of the string with ameter stick.)

Remind students to periodically check their Rubber Band Rollers tomake sure they are in good working condition.They should also checkthe Rubber Band for signs of cracking, wear or deterioration.

a Run repeated trials.

Students should run three trials for each experimental condition (threetrials at 10 twists, three trials at 15 twists, etc.). Explain that thedistance traveled by the Rubber Band Roller for a set number of turnsmay vary. Reasons for this might include variations in the way theRubber Band was twisted, the way the Rubber Band Roller wasreleased, and the surface that the Roller traveled across. Students should review the datafor each set of three trials before they proceed. If one of themeasurements seems out of place, the students should review theirprocedures and consider an additional trial.



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a Test to find out the Rubber Band Roller’s inertia.

Have the students twist the Rubber Band three timesand let the Roller go.What happens? (They should noticethat the Roller does not move.)

Explain to students, using examples, how objects at restare difficult to move and easier to keep moving. Have oneor two students push their desks until they move anddirect them to keep it sliding after it begins to move.Have the students describe their efforts and which waseasier– getting the desk to move or keeping it moving.(keeping it moving)

Have students suggest other examples where it requiresmore effort to start something moving that to keep itmoving. (pushing a full wheelbarrow, the pit crew pushinga racing car)

Relate these discussions to Newton’s first Law of Motionand Inertia. Inertia is the tendency of something at restto remain at rest and of something in motion to stay inmotion. For our study at this point, we are concernedabout the first portion of this simplified definition.TheRoller will remain at rest until the Rubber Band hasstored enough potential energy to overcome its inertiawhen it is released on the floor.

In their experiments, the force to make the Rubber BandRoller move was provided by a Rubber Band. Enoughforce must be applied to overcome inertia, the tendencyof an object to remain at rest.The amount of inertia an

object has is related to its mass. For example, a brickhas more inertia than an empty shoe box.

Help students apply these ideas to the motion oftheir Rubber Band Rollers.Tell them that some ofthe potential energy of the Rubber Band is usedto overcome the inertia of the Rubber BandRoller and some is used to keep it moving.Askthe students to use their Rubber Band Rollerto find out how many turns of the RubberBand are required to overcome the inertiaof the Roller– to get the Roller to begin

to move.This number should be recorded on the Data Chart (page 40) as the quantity I:

I = Number of twists to overcome inertia

It usually takes about two to six turns of the RubberBand to overcome the inertia of the K’NEX RubberBand Roller, depending upon the strength of the RubberBand and the surface the Roller moves across.


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Analyze the data

Once all the measurements have been recorded, guide studentsto analyze their data and use them to make predictions. First,show them how to determine how far the Rubber Band Rollermoves for each twist of the Rubber Band.This value will also becalled R.

To calculate R, advise students to use their average values forthe distance traveled by the Rubber Band Roller.When selectingvalues to use for the number of twists of the Rubber Band,caution students that they cannot just use the number of twistslisted at the headings of their charts (10, 15, 20). First they needto account for inertia. Remind them that if the Rubber Band wastwisted 10 times, some of those twists started the Rubber Band Roller(overcame inertia), and the rest allowed the Roller to cover thedistance.Therefore, they must subtract I, the number of twiststo overcome inertia, from the number of twists recorded on the chart:

where R = distance traveled per twist of the Rubber Band;D = total distance traveled;T = total number of twists; andI = the number of twists used to overcome inertia.

a Predict the distance under new conditions.

Ask: “Now that you know how far the Rubber Band Rollerwill travel for each twist of the Rubber Band, how far will theRubber Band Roller travel if the Rubber Band is twisted 25 times?”

Students should subtract from 25 the number of twists neededto overcome inertia.The result should be multiplied by R, theaverage number of centimeters per twist (see the SampleCalculation on page 44).

a Test your prediction.

Have students measure how far the Rubber Band Roller travelswhen the Rubber Band is twisted 25 times.The distance shouldbe measured three times to calculate an average value.Then, askstudents to compare the calculated value to the measured value.

= Total Distance traveled by Rubber Band Roller (D)

Number of twists of the Rubber Band (T)

Distance traveled per twist of the Rubber Band (R)

then R = D

T – I

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Sample DataCOLLECTED

Trial #1

Trial #2

Trial #3


25

391.1

406.4

408.9

402.1

Distance Traveled past the end of the ramp (cm):

Number of Twists of Rubber Band

Sample Calculation to predict distance traveled by a Rubber Band Roller twisted 25 times

R = D Given:

T – I R = 23.3 cm/twist

I = 6 twists

T = 25 twists

D= unknown

23.3cm/twist = D

25-6 twists

23.3 cm = D twist 19 twists

23.3cm/twist (19 twists) = D

443 cm = D

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Sample Data

Trial #1

Trial #2

Trial #3


10 15 20

99.1 210.8 289.6

99.1 213.4 290.8

106.7 215.9 292.1

101.6 213.4 290.8

Distance Traveled past the end of the ramp (cm):

Number of Twists of Rubber Band

Number of twists to overcome inertia = 6

Sample Calculation to determine R (centimeters/twist)

R = Distance traveled by Rubber Band Roller

= R = DNumber of twists of the Rubber Band minus twists to overcome inertia T – I

For 10 twists:

10 twists - 6 twists to overcome inertia = 4 twists for motion

R1

= 101.6 cm

= 25.4 cm/twist

10 - 6 twists

For 15 twists:


R2

= 213.4 cm

= 23.7 cm/twist

15 – 6 twists

For 20 twists:


R3

= 290.8 cm

= 20.8 cm/twist

20 – 6 twists

Average = R AVG = (25.4 + 23.7 + 20.8) cm/twist

= 23.3 cm/twist

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Teacher’s NoteNotice that the R values decrease the more the RubberBand is twisted.The observationis consistent with the elasticproperties of Rubber Bands.Some elastic materials (springs)react more consistently tostretching or compression.A listof R values for a spring wouldshow very similar values(Hooke’s Law). Rubber Bandsdo not follow Hooke’s Law asthey tend to store more energywhen the twisting begins andwhen the Rubber Band istwisted near its elastic limit(breaking point).

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Math Activity (Upper Level)a Compare your prediction to your results.

You might have the students calculate the percentage difference between the two values by subtracting the higher value from the lower value,dividing by the higher value, and multiplying by 100.

a Progress review.

Ask students to summarize what they have done. (they have collecteddata, organized it in a chart, analyzed the data, and then used it to makeand test a prediction) Ask students if they can think of another methodscientists use to analyze data. (plot the data on a graph)


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y

x

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Graph the data

Explain to the students that an experiment is a way to study cause andeffect. Something happens which in turn causes something else to happen.In this case, a variable in the experiment is changed (number of twists) andthe result of the change is measured as distance traveled.

Cause: Number of twists of the Rubber BandEffect: Distance traveled by Roller

When plotting data on a graph, the quantity that is changed is plotted alongthe X-axis, and the quantity that is measured is plotted on the Y-axis.Tell thestudents to draw two axes on a sheet of graph paper.Ask them how to labelthe axes. (X-axis: Number of twists;Y-axis: Distance traveled in centimeters)Have students plot their data and draw a best fit line through the points.Youmay have to explain to them that the line should not be drawn from point topoint (see the Sample Graph on page 48).

A graph often reveals information that is notreadily apparent from simply looking at thedata in a chart. For example, show studentshow the extrapolated section of the line onthe Sample Graph intersects the X-axis near 5.

Ask: “What is the significance of thepoint where the line intersects thex–axis?” (with less than 6 twists, inertia is notyet overcome)

Explain that this number shows that when the Rubber Band is twisted 5 times, theRubber Band Roller will move zerocentimeters, because this is less than thenumber needed to overcome inertia.Students should find that their experimentally measured value andtheir graph value for I are similar.

Teacher Note:The Roller will not move until inertia is overcome.This can usually beaccomplished with 2 to 6 twists of theRubber Band.The choice of Rubber Bandswill influence this value, as will the size ofthe vehicle.

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Sample GraphTeacher Note:It is entirely possible that a student’s graph will show a curved line. Do not force a straight line through the points.500

450

400

350

300

250

200

150

100

50

0

0 5 10 15 20 25Number of Twists of the Rubber Band


48

Dis

tanc

e Tr

avel

ed (

cent

imet

ers)

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Wrap Up

Conclude the lesson by reminding students that a valid scientificexperiment is reproducible, which means that any person performing thesame experiment with the same equipment should get similar results.

Ask each group to report the value they obtained for R, the distancetraveled per twist.Write these numbers on the chalkboard forcomparison. (These numbers can only be fairly compared if each group used the same K’NEX Rubber Band Roller and the same sizeRubber Band.)

Analyze how similar or different the numbers are. Is any resultsignificantly different from the rest? Ask students to consider whydifferences might occur.

Possible suggestions might include:oosome Rubber Bands may be slightly larger than othersoosome Rubber Bands may be more worn than othersoo the experimental techniques may have been slightly different

for each groupoo the K’NEX Rubber Band Rollers may have been built with

slight differences

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Student Worksheet for Activity 5

1. Record the distance measurements on a Data Chart.

2. Calculate the averages and record them on the Chart.

3. Experiment to determine how many twists of the Rubber Band arerequired to overcome the inertia of the Roller. Record the value.

4. Calculate R from the formula, R = Distance traveled/Number oftwists. Remember to use only the number of twists used for motion (total twists minus twists for inertia) for the denominator.

5.(a) Predict how far the Roller will travel if the Rubber Band is wound 25 times; 30 times; 40 times.

(b) Measure this distance and compare it to the predicted value.

6. Graph the data. Remember to label the axes appropriately.

7. Where does the line meet the X-axis when it is extrapolated?

8. What is important about the place where the line begins to moveabove the X-axis? What does the change in the line show?

9. How does the number of twists required to overcome inertiadetermined from the graph compare to the value measuredearlier?

Use the formula: R = D

T – I

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AssessmentACTIVITY 5

1. How is potential energy stored in the Rubber Band Roller?

2. Why does the Rubber Band have to be twisted several timesbefore the Roller starts to move?

3. Student A has collected the following data.

ooNumber of twists to overcome inertia = 6

ooDistance traveled for 15 twists of the Rubber Band = 60 cm

a. Calculate how far the Rubber Band Roller travels foreach twist of the Rubber Band.

b. How many times should the Rubber Band be twisted to move the Rubber Band Roller a distance of 30 cm?Show your calculations.

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EXTENSIONS 1 & 2

Extension 1Have students choose one of the variables listed below anddesign an experiment to test how far a K’NEX Rubber BandRoller travels under those conditions.

oo smooth floor vs. carpet or sidewalk

oo large vs. small Rubber Band

oo wheels (white Connectors) with Tires vs. without Tires

oo flat surface vs. a ramp

oo short vs. long winding Rod

oo cold Rubber Band vs. room temperature Rubber Band

Remind students to make every effort to keep other variablesfrom affecting the results of their experiments. Have students:

oo present complete, clear procedures

oo present data in an organized format

oo analyze the results of their experiments

oo suggest possible sources of error

oo present their findings to the class

Extension 2Conduct a discussion with students regarding the varyingcondition of the Rubber Band due to wear and tear, temperatureand humidity and the implications of these on performance.(Students may hypothesize that in dry, hot conditions, there willbe less air resistance and the band will have more elasticity thanin cold wet conditions.Thus, in dry, hot conditions, it will performbetter and the vehicle will travel a greater distance.)

Since it is impossible to control the weather, consider having students conduct experiments with Rubber Bands subjected tomoisture or placed in the refrigerator or freezer to prove ordisprove their hypotheses.

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Rubber Bands & Springs