Let’s
Drop It!
By Joan Berger
The EGG DROP Project
What better way to put excitement into the new school year than with a project for the entire class, grade, or school! The Egg Drop Project is a classic, but the physics of it can be way too difficult for lower grades. The web links below will give you many ideas as to how others approached the topic on a variety of grade levels. Many discuss the physics of motion, gravity, force, velocity, etc., but for lower grade levels, we will approach it through understanding more about spring action, cushioning, and shock absorbers.
Before beginning the actual experiment, the students can brainstorm about where we use materials in every day life to absorb the shock of impacts and keep us protected. Some familiar examples are in the up and down movement of vehicles, auto bumpers, prosthetic legs, our shoes and sneakers, shipping breakable materials, and a bicycle seat. A bit more removed for most students would be in controlled deceleration, vibration isolation in motors, and vibration control of equipment in satellites as some examples.
Materials
each student will need:
1 half-gallon milk or juice carton (no glass)
1 fresh egg (the intact eggs must be broken after they are dropped to
determine that they are fresh)-Extra egg
Packing materials (anything can be used inside the container except a
compressed gas,
Nothing is to
be added to the outside)
Procedure:
add the packing materials and
fresh egg at school.
Questions
for analysis:
What
are the forces acting on the egg as it falls?
How
can you control the forces that cause the egg to break?
What
are the common characteristics of the materials that protected some eggs?
Did
layering of materials play a role in protection?
Students
can combine their ideas on the creation of a graph/chart to most effectively
record results. After students analyze results, bring the discussion back to cushioning
and shock absorbers. Are any of the materials that were successful protectors
found in any of the things we brainstormed? Was it the material, the amount of
it, or its compression factor that was the key?
When
you need background information on a particular topic, there are several Ask-the-Expert
Internet sites where you can get help. I queried, http://www.allexperts.com/, (scienceàphysics) asking about how
shock absorbers would factor into the egg drop experiment, and received this
reply from Chuck Borough, a physicist, who also maintains his own
question/answer web site at http://goaskgrandpa.com./
It will help to guide your discussions and analyses. Try and elicit these
conclusions after the experiment is completed.
“This is
not mostly a "shock absorption" problem, but mostly a
"springing"
problem. A car
without springs is almost impossible to drive, while one without shocks
feels almost
normal until moments when the shocks are needed.
What will break
the egg is to have a force on the egg greater than the shell
can
withstand. The shell is very
strong if the force is well distributed,
and very weak
if the force is all at one point - or on a small area.
You can pierce
the egg with a very small force with a needle, yet you can put
it between your
palms and push with great force without breaking it.
If your
"padding" is too soft, it will work well until the padding has
compressed, and
then the egg will experience a large g-force and break. If
your padding is
too hard, then the egg will break while the padding is being
compressed. What
you want is padding that will compress at a rate
that gives
the egg the longest time to stop.
Experimentation
is best. My guess is that a firm
foam around the egg - with
a softer foam
outside of that - and still softer further outside - will give
you the best
result. Make sure the inner foam
that is in contact with the
egg is shaped
well like the egg - not putting extra force on any one part of
the egg.
If you can get
the egg to fall vertically rather than sideways, it is much
stronger - you
know - hitting on its end rather than its side.
Take advantage
of the largest size allowed in the contest. The larger your
package, the
further the egg travels during its stop.”
And
this is from Lt. Col. Marty France, aeronautical engineer, Pentagon, USA:
Shock absorbers
are just part of the question.
Shock absorbers
for cars, trucks and trains, smooth out the bumps,
but just as
importantly, they do it for certain frequencies of
vibration. That's not really applicable here for
the egg since
you're just
smoothing out one BIG input. All
of us engineers do this
in college!
We started by
getting a lot of eggs. Then, we
tested eggs by
dropping them
onto a hard surface from various heights to see what
was the maximum
height from which an egg could survive a fall without
cracking. If you know that height, then you know
how fast the egg is
going when it
hits. If you know the mass of the
egg, then you can
also figure out
the force exerted on the egg when it hits. The
object of
the cushioning is to spread out the deceleration
of the egg over
such a long period (as the cushioning compresses)
that the egg
never sees a deceleration (and hence, point force)
high enough
to crack the egg. You can do this without equations by
just
testing. After you've tested the
"naked" egg, cushion it with one inch of
uncompressed
foam rubber and do the test again.
From how high can
you drop it and
it still survives? Then go to two
inches, three
inches,
etc. Plot it on a graph and see if
you can extrapolate it to
higher heights.
Obviously, the
egg, wrapped in foam, will eventually achieve a
terminal
velocity due to air drag as you drop it from successively
higher
heights. Can you find a way to
figure out what that speed
is? (the answer is, yes. It's a function of the cross-sectional
area of the
blob and its overall mass, plus the "slickness" of the
object. The less aerodynamic and less dense,
the slower the terminal
velocity. Skydivers have a terminal velocity of
about 120 mph.)
And
here is an additional comment from Mary Lou Ciavarra, a HS physics teacher:
“A shock absorber allows a force acting on an object
to act in a long period of time. This
reduces the force of
impact: the longer the period of time a force acts on an object, the
smaller the force. If you
have ever been in a raw egg tossing contest, you know that in
order for the egg not to
crack when you catch it, you "give" with the egg. Another
example, if you are out of
control with your car, don't drive it into a concrete wall,
instead a stack of hay is
much better (that is, if you had that choice!). Boxers roll with
the punch in order to
reduce the force of impact. The stack of hay and "giving" with the
egg are both shock
absorbers of sorts. They increase the length of time a force acts
thus decreasing the
force.”
And finally, from Paul Konichek, physics teacher:
“In
discussing the egg experiment,
IMPULSE = Force
X change in Time = mass X change in Velocity = change in MOMENTUM
since an egg
dropped from a given height has a given mass and a velocity that
can be calculated
using K.E. = P.E. or .5
mass*velocity*velocity =
mass*g*height
canceling the mass's and solving for velocity gives velocity =
the square root
of (2*32 ft per second [or 9.8 m per sec] * height/mass)
then the
whole experiment is to show that if you increase the time of impact
you will
reduce the force on the egg thus not breaking it.
Thus the key word is
impulse, and the
key idea is that as you increase the time of impact, you decrease the force on
the egg.
URLs
to aid in your design and investigation:
http://www.angelfire.com/md/mccscience/eggdrop.html
http://college.hmco.com/education/pbl/project/project3.html#problem
http://home.neo.rr.com/physicsisphun/egg_drop.htm
http://www.geocities.com/r_dman2000/egg_drop_project.htm
http://www.glenbrook.k12.il.us/gbssci/phys/projects/q2/ecrub.html
http://www.silverfalls.k12.or.us/foxes/staff/read_shari/egg_drop_project.htm
http://garcia.me.berkeley.edu/~mesa/cheer_bak/simulation/eggdrop/edsim.html
http://www.glenbrook.k12.il.us/gbssci/phys/projects/q2/ecover.html
Asst.Prof.
CWPost College, LIU
Dept.
of Educational Technology
jberger5@concentric.net
http://www.concentric.net/~jberger5
http://phoenix.liu.edu/~jberger