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Thomas Tretter is an assistant professor of science education at the University ofLouisville in Louisville, Kentucky.

BUNGEE

Egg

JumpIn the spirit of the National Science Education Standards (NRC 1996), manyteachers attempt to have their students experience science in a constructivist,inquiry-oriented manner. The egg bungee jump activity will certainly supportthat mode of teaching, and has the added benefit of providing a concrete con-text within which students can explore rather abstract concepts of force, mo-tion, and energy transformations.

ActivityTo introduce the egg bungee jump activity to the class, the apparatus (Figure 1)is set up on the demonstration table in the front of the room. Students areinformed that they will be gathering data on a raw egg bungee jump usingrubber bands for a bungee cord. The goal of the lab is to have the egg bungeejump from 2 meters and get as close as possible to the floor without hitting it. Itis the students’ job to determine how many rubber bands should be used tocreate the bungee cord. The teacher should discuss the safe use of rubber bandswith students, and may wish to ask students to wear eye protection, even thoughno projectiles are involved.

To prepare for the 2-meter jump, students will first measure how far the eggwill drop using bungee cords made from one rubber band, two bands, threebands, and four bands. Students will then use the data collected to determinehow many rubber bands they’d like to use for the 2-meter jump.

Students will work in teams of four. One member is responsible for keeping thering stand from toppling off the table when the egg is dropped and holding the meter

by Thomas Tretter

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Explore energy transfer

at www.scilinks.org.Enter code SS020501.

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stick vertically with the zero at the bottomof the hook. A 1-meter stick is usually suf-ficient for data collection because studentsare only going to measure up to 4 rubberbands—it typically takes about 10 or 11rubber bands to reach 2 meters. If studentsare using extra large rubber bands and needmore than 1 meter to measure the jumpwith four rubber bands, I suggest studentstape together two 1-meter sticks to get theneeded length. Another team memberlines up the bottom of the egg with thebottom of the hook (see Figure 2), dropsthe egg straight down, and holds a fingerover the rubber band on the hook so thatit doesn’t jump off during the egg’s rebound.The third member is the “egg protector”and keeps the egg from swinging, pendu-lum-like, into the side of the lab bench af-ter it rebounds from its lowest point on theinitial drop. Protection of the egg is bestdone by cupping both hands and lettingthe egg fall to rest in the cupped hands atthe top of its bounce after rebounding fromthe lowest point or cupping the egg on itsway up after the lowest point; this tech-nique avoids allowing the egg to bouncearound multiple times. If the egg shouldbreak, most of the mess will likely be con-tained in the sack it is in. If any raw eggshould leak out, instruct students to wear

rubber gloves to avoid any possibil-ity of salmonella poisoning and usepaper towels to mop up the egg. It

may be necessary to follow up the initialwiping using wet and/or soapy paper tow-els so that the floor isn’t slippery. The fourthmember of the group crouches down to de-termine the lowest falling point of the eggby referring to the adjacent meter stick.

The teacher can either instruct stu-dents how to use the data to determinethe number of rubber bands needed fora 2-meter jump (as suggested in thesample student lab sheet found onpage 18), or the teacher can allow stu-dents to devise their own methods ofusing the data. The amount of dataanalysis guidance given to studentsshould be based on their abilities andyour pedagogical goals.

Laboratory equipmentFIGURE 1

Notes:1. A mesh bag created from cutting a sports ball carry bag into squares about 20 cm

x 20 cm works well, but cloth bags cut from clothing scraps work too. Alternative

materials are possible, such as mesh bags used to package some fruits or onions.The sack is created by bringing all four edges together at the top of the egg.

2. Be sure to have the wire tightly wound (maybe using pliers) around the top of the

mesh bag to avoid the bag slipping through the wire and spilling the raw eggduring the experiments. A large paper clip works well for the wire, but a long twisttie may also work if it can be securely tightened.

3. Make the length of the wire significantly shorter (approximately half works well)than the length of the unstretched rubber band so that the bottom of the egg canbe lifted to be even with the hook for the first drop.

4. If no pendulum holder is available, any clamp that attaches to the ring stand (e.g.,a test tube clamp) will work if a rubber band can be looped over some part of theclamp and a student’s finger can hold the rubber band securely. If no ring stands

are available, this activity could be done by extending any long, narrow, and straightobject (e.g., a broom handle) over the side of the lab bench (or table) andsuspending the rubber bands from the end of the extension, from which the egg

will bungee jump.5. List of materials needed per lab group:

• 1 egg (plus a few backups for the class in case of breakage)

• 1 egg sack• 1 wire (a large paper clip works well, or a long twist tie may work)• access to many rubber bands (approximately two dozen or more per lab

group—I usually have available a large box or two for the whole class). Toprovide students options, a teacher may wish to have available rubber bandsof different widths, thicknesses, and lengths—almost any sizes will work as

long as they aren’t so weak that they break with the force exerted by thefalling egg or they aren’t so strong that they hardly stretch from the forceexerted by the falling egg.

• 1 meter stick (or 2 meter sticks taped together if extra long rubber bands are used)• 1 ring stand (or alternative—see note 4 above)• 1 pendulum holder (or alternative—see note 4 above)

rubber band

wire (paper clipworks well)

mesh bag

raw egg

hold here

ring stand

pendulum holder

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Pedagogical benefits and alternativesThere are a number of science skills, process skills, and socialskills that could be addressed with this activity (Figure 3). Stu-dents spend a lot of time working together in inquiry-orientedscience classes, and an activity such as this can increase theircomfort with working together. This activity is also beneficial forcreating cooperative teamwork because the nature of the activ-ity requires four people to be involved, not permitting one or twoteam members to take a purely passive role.

Although this bungee activity could be done with otherobjects such as a golf ball, the possibility of making a mess withraw eggs increases interest. The open-ended nature of the de-tails of carrying out the activity encourages students to thinkabout lab techniques. Some students comment that becausenot all the rubber bands in the boxes provided are the same,their results could be adversely affected. A teacher could replywith something like, “How will you prevent that from becom-ing a problem?” In this way, students are encouraged to take

the initiative with developing the details of their lab proce-dures. Likewise, some teams may decide to repeat each dropseveral times and average them to get a better value. Somestudents may comment that after a number of drops, the rubberbands may become “tired” and that in future drops the distancewill be greater than what they’ve measured. Others may not becareful to always release the egg from exactly the same height,creating a potential source of error. Some of the students read-ing the drop distance may not have their eye horizontal withthe lowest position of the egg, causing parallax to affect theaccuracy of the distances they measure. These and other possi-bilities for affecting accuracy provide a rich source of post-labdiscussion of sources of error in lab measurements. The teachermay wish to create post-lab worksheets with questions on thisaspect of the lab or any other they’d like to reinforce.

When analyzing the data, the teacher may choose to explic-itly instruct students on how to perform the analysis (as is donein item number 5 in the sample student lab sheet in the appen-dix), or they may allow students free rein to devise ways to ana-lyze the data themselves. Some alternatives to creating a graphand linearly extrapolating to 2 meters (as suggested by item num-ber 5 in the appendix) are: Identifying how many additional cen-timeters each rubber band adds (on average) to the drop andcomputing how many rubber bands are needed to reach 2 meters;after graphing the data, writing the equation of the best-fit line(if students have had algebra), and solving that equation for thedesired y-value; and making a table of number of rubber bandsand fallen distance for rubber bands one through four and ex-tending that table until the distance fallen reaches 2 meters.

Students can retain their data from this activity to return tolater on after they have learned how to write equations in slope-intercept form. These data provide a nice context for such equa-tions, with the large benefit of both the slope and the y-inter-cept being readily interpretable as opposed to merely abstractnumbers. The slope of this line represents the additional fallingdistance per added rubber band, while the y-intercept repre-sents the combined length of the wire and the egg in the meshbag hanging down from the hook with no rubber band.

AssessmentFor the activity itself, I recom-mend assessing in a manner thatis easy on the teacher, excitingfor the students, and almost in-variably results in a good labgrade for students. Each studentmust provide the accompanyingdata tables and graphs in orderto earn the grade. The grade iscomputed from 100 minus thenumber of centimeters off the

Drop position of the egg foreach trial

FIGURE 2

Sampling of skills that could be addressed with theegg bungee jump activity

FIGURE 3

Science skills

Data collection

Creating line graphs

Writing andinterpreting equations

Force, motion, andenergy conceptsaddressed

Process skills

Careful lab techniques

Replicability of results

Identifying and minimizingsources of error

Social skills

Active involvement with group

Sharing responsibility

Collaborating on data analysis

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ground for the 2-meter egg bungee jump. Students can beallowed three trials so that they have a chance to correctproblems, without penalty, on the first or second try if needed.An advantage of this policy is that it encourages students togo back to the data to reanalyze the results when determin-ing how many additional rubber bands to add for trials 2 and3. Another possibility is to establish a minimum score of 80no matter how their egg performs, as long as they’ve com-pleted the lab data and graphs as required. With the mul-tiple trials, most teams earn a score of at least 90, and best ofall, this activity is easy for the teacher to grade.

Energy, force, and motion conceptsThis egg bungee jump activity can be used to teach or reinforcea number of concepts revolving around the topics of energy,force, and motion. Once students have completed the initialactivity and come to appreciate the predictability of this sys-tem, they can then delve more deeply into various underlyingscience concepts. Possible concepts to be addressed includetransformations between potential energy and kinetic energy,analysis of balanced and unbalanced forces and sources of thoseforces, and analysis of the acceleration and velocity of the egg

at various points in its fall. A detailed analysis of this motioncan be rather complex for middle school students because ofthe multitude of forces and the non-constancy of the accelera-tion due to stretching rubber bands, but the analysis can besimplified to be accessible to middle school students.

A way to simplify the analysis for middle school studentsand still address some important force, motion, and energyconcepts is to focus attention only on the starting and end-ing points of the egg drop and eliminate the detailed analy-sis of what happens during the fall. Students should be ableto explain that at the moment before beginning the fall, theegg possessed only gravitational potential energy (GPE) andno kinetic energy (KE). They can identify the weight of theegg as the only force acting on it at this point, and that itsvelocity was zero before being dropped.

At the end of the drop, the egg-and-rubber-band systemnow possesses the least GPE, no KE (it is not moving), andlots of elastic potential energy (PE) due to the stretched rub-ber bands. Thus students could express knowledge of theLaw of Conservation of Energy by stating that all of the ini-tial GPE of the egg went into elastic PE at the bottom of itsfall. Additionally, they could identify that at the bottom of

FIGURE 4

0 = The starting point of the bottom of the egg. This establishes a reference point for this experiment.

x = The length of the unstretched rubber band chain plus egg sack without the egg hanging on it. Note that x must still takeinto account the length of the egg sack for consistency even though the egg sack isn’t attached to the rubber bands forthis measurement.

y = The length of the rubber band chain plus egg sack with the egg hanging on it in equilibrium.z = The length of the rubber band chain plus egg sack at the lowest point in the drop.

2 m

0

x

meter stick

floor

(note: eggnot hanging

but shown tolocate x

unstretchedrubber bands(no egg)

0

y

stretched rubberbands in equilibriumwith egg weight

0

z

stretched to lowestpoint in jump

starting point

Important boundary points for the egg drop

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F O R C E A N D M O T I O N

the fall, there are two forces acting on the egg—the upwardpull of the rubber bands and the downward weight of theegg. Because the upward force is greater, the egg acceleratesin an upward direction, beginning its rebound. Students mayalso be able to describe when the forces (weight of egg andstretch of the rubber bands) are again balanced by recogniz-ing that after the egg settles down from bouncing back andforth and is just hanging there, the acceleration is zero andhence the net force at this equilibrium point (the point wherethe system comes to rest) is zero.

Energy transformations and conservationA teacher could focus only on analysis of the beginning andending points or could expand the analysis to include sig-nificant intermediate points during the fall. Initially, the egghas only GPE as it is held at the top of the 2-meter fall. As itfalls down to distance x shown in Figure 4, this GPE is beingreduced and there is a corresponding increase in KE as theegg gains speed (Figure 5). After the egg passes point x, therubber bands begin to stretch, thus storing elastic potentialenergy in the rubber bands. Once the egg reaches its lowestpoint at z (see Figure 5), it is no longer moving and so all ofthe initial GPE at the top has now been transformed intoelastic PE of the rubber bands. Figure 5 includes statementsabout energy transformations at various points throughoutthe fall, but a teacher may choose to explore these ideas withstudents (described below) or restrict the analysis to onlythe beginning and ending points.

Forces and motionThe forces acting upon the egg can be ana-lyzed next, with connections to its motionsimultaneously made. The analysis describedbelow will include significant points through-out the egg’s fall, but teachers may choose torestrict discussion with their students to onlythe beginning and ending points and not ex-plore the details in the middle of the egg’sfall. For the discussion of motion, studentsneed to know the relationship (encapsulatedin Newton’s Second Law of Motion) that anet force results in an acceleration of an ob-ject (F = ma). Additionally, they should befamiliar with the concept that an accelera-tion is a change in velocity. For simplicity,the following discussion will ignore relativelyminor forces such as air resistance.

When the egg is dropped initially, theonly force acting on it is its own weight.Thus, the egg begins to free fall until pointx (see Figure 4). Since there is a net forceacting downward, the egg will accelerate

downward, which the students might also describe as gain-ing speed in a downward direction. Since the egg is in freefall, this downward acceleration is simply the accelerationdue to gravity, 9.8m/s2. This corresponds nicely with theenergy graphic that shows the egg gaining KE (gaining speed)in the portion of the fall before point x (Figure 5).

At point x, the rubber bands begin to stretch, providing anupward force that begins to counteract the egg’s weight. How-ever, that upward rubber band force doesn’t balance out theweight until the egg reaches point y because that is the point atwhich the weight of the egg hangs in equilibrium with thestretched rubber bands (Figure 4). Thus, between points x andy, the downward weight of the egg is still more than the upwardforce of the rubber bands, resulting in a net downward force inthis section of the drop. This net downward force is smallerthan the full weight of the egg. Although there is still a down-ward acceleration of the egg between points x and y, that accel-eration continually grows smaller and smaller as the egg ap-proaches point y. In terms of speed, this means that the eggcontinues to gain downward speed between x and y, but it gainsthat speed at a slower and slower rate. This analysis permitsstudents to answer the energy question (posed in Figure 5 as“losing KE somewhere in here”) about when the KE of the eggbegins to decrease. Between points x and y, the egg continuesto gain speed (but at a slower rate), thus the egg continues togain KE (but at a slower rate than before).

At point y, the downward weight of the egg is balanced bythe upward force of the rubber bands because that is the hang-

FIGURE 5 Keeping track of energy transformationsthroughout the fall

time to reach 0 (all GPE)

time

for

egg

to fa

ll to

poi

nts

x, y

, z

time to reach y

time to reach x

time to reach z (all elastic PE)

losing GPEgaining KE

losing GPE, gainingKE more slowly, gaining elastic PE

losing GPE, losingKE somewhere in here,gaining elastic PE

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ing equilibrium point. Thus the net force at this point is zero,which means the acceleration is zero. Alternatively, studentscould describe point y as the place at which the egg ceases togain speed. After point y, the upward force of the rubber bandsis greater than the downward weight of the egg, resulting ina net upward force. This net force causes an upward-directedacceleration that opposes the ongoing motion of the egg,resulting in a slowing of the egg’s downward speed. This meansthat the maximum speed of the egg was at point y (see Figure6), hence the KE begins to be reduced after point y but notbefore, a result that can be used to update the energy graphicin Figure 5 to answer the question of when the loss of KEbegins to happen. The upward force of the rubber bands con-tinues to strengthen as the egg is falling past y, resulting inan ever-increasing acceleration upward that slows the egg’sdownward fall more and more quickly until the egg comes toa momentary stop at the bottom of its drop at point z. Ver-bally describing or reading about these various changes in forcesand motion in text form can be a bit confusing, so I recom-mend teachers provide their students with a graphic similar toFigure 6 (and also an energy graphic similar to Figure 5) sothat students can record the various transformations through-out the fall as the class discussion progresses. These graphics(Figures 5 and 6) provide a concise, understandable form toarrange all of these relationships.

An analysis of the energy, forces, and motion of the eggduring its bungee jump provides students opportunities toencounter a variety of situations encompassed within oneexperiment they are able to perform relatively easily. When

students have completed this analysis and fully understandthe underlying science behind the egg jump, they will havea reasonably sophisticated conceptual knowledge of sometopics that are often quite abstract and difficult for them toapply in typical situations.

Making a lasting impressionWith this activity, students enhance their understanding thatscience is a process of active exploration involving mutualcollaboration with classmates; that it is full of opportunitiesto try out ideas without risk of penalties; and that it com-bines interesting activities with complex concepts. Studentsare challenged to develop laboratory procedures; to conductexperiments with care; to generate, process, and interpret data;and to do so in an atmosphere of respect for others’ ideas.

An important benefit of this activity is that it combinesvery concrete, hands-on experiences and measurements withan analysis of concepts that are often abstract for students. Theease of implementation of the egg bungee jump permits stu-dents serious mental engagement with the fundamental physi-cal science concepts of energy, force, and motion. In spite of therelative ease of implementation, sophisticated science labora-tory skills and analysis skills can be addressed, and an addedbonus is that students tend to enjoy the challenge of keepingtheir egg safe during its dangerous leap. n

ReferenceNational Research Council (NRC). 1996. National science educa-tion standards. Washington, DC: National Academy Press.

FIGURE 6 Forces and motion throughout the egg’s fall

time to reach 0

time

for

egg

to fa

ll to

poi

nts

x, y

, z

time to reach y

time to reach x

time to reach z

net forces

weight of eggdownward

weight of egg downwardminus upward force of stretched rubber bands(net result is a smallerdownward force)

elastic stronger thanweight gives netupward force

resulting acceleration

free fall = constantaccelerationdownward = 9.8 m/s

2

smaller accelerationdownward

upward acceleration

affect on speed

constantly gainingspeed downward

gaining speed ata slower rate

losing speed

Sample lab sheet for egg bungee jump activity

wire

mesh bag

raw egg

hold here

ring stand

lab bench

diagram 1 diagram 2

dist

ance

fal

len

# rubber bands

Procedure1. Put a raw egg in the nylon mesh square and fold it up into a bag, wrapping the top tightly with wire. Make a loop at the

top of the wire—the rubber bands will be attached to this loop. See diagram 1 below.

2. Put a ring stand with a pendulum holder attached at the edge of the lab bench with the holder facing outward. Oneperson will need to hold down the ring stand so that it will be stable during the egg bungee jump. See diagram 2 above.

3. Put one rubber band on the wire loop (thread it through itself as an easy way to attach it to the loop) and the other endof the rubber band on the holder. Putting a finger over the top of the holder’s hook so that the rubber band does notjump off, drop the egg package from the height of the holder and record how far it falls from the holder.

NOTE: A teammate should catch the egg after it starts back up so that the egg doesn’t hit the side of the lab bench andcrack after its bounce.

4. Loop a second rubber band through the first one and repeat the drop and measure. Continue recording the fall distancefor three and four rubber bands—you may not use more than four rubber bands for this portion of the data collection.

5. Make a graph of distance fallen vs. the number of rubber bands.

From the graph, predict the number of rubber bands to use for a 2-meter drop.

GOAL: You want to get as close as possible to the floor without

the egg hitting it for the 2-meter “jump.” You will get three trials.

6. Lab gradea) You must show your data table and graph properly recorded in your lab book.

b) Your grade will be 100 minus the number of centimeters from the floor on the drop.c) If the egg hits on all trials and your data gathering technique is correct, you will get a score of 80 (but you shouldn’t

abuse your egg like that!).

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