MadSci Network: Engineering

Re: When doing an egg drop, is it better to have the egg suspended or held firmly in place

Date: Mon Feb 22 11:50:48 1999
Posted By: David Taylor, Staff, programming, Testronics
Area of science: Engineering
ID: 919127353.Eg

I tried emailing to get a better
description of what is meant by an 'egg drop'. No answer in the better
part of a week. I'll just assume an 'egg drop' is a competition in
building a balsa and string shell around an egg to protect the egg from
a given fall. If I'm wrong, sorry, I tried.

Although you should start by making best design you know how, you
shouldn't count on getting it perfect the first time. The tried and true
method of engineering is to build a model and try it out. Today,
engineers often use computer models where they used to build physical
ones. The underlying process hasn't changed as much as you would think.

Way too often, a model doesn't behave exactly the way the designer
thought it would. The designer then has to try to change his design to
try to improve its observed shortcomings. Then it's time for another
test. It's a learning and adapting process. A finished design that an
engineer would be satisfied with is usually the end result of many
design, test, and evaluation iterations.

That said, in an egg drop, the goal is to spread the force of impact
over as much area and time as possible. A brief, intense impact, or an
impact on a small part of the egg is certain to crack it.

You really have a considerable lattitude about how you achieve the goal.
You could spread the force out on the surface of the egg with a web of
strings woven to resemble several nets holding the egg firmly in place.
You could also spread the force over the surface of the egg with a balsa
cradle. However the egg is supported, there should be as little slop or
looseness between the egg and the holder as is reasonable. Slop results
in the egg being jerked around more.

In spreading the impact force out over time, it is important is that the
finished model have the 'right' amount of give or squishiness. If your
model is too rigid, the shock will be transmitted to the egg in a brief,
intense instant that will crack the egg. If the model is too squishy,
the egg will not decelerate fast enough to avoid smacking your target
and breaking. There is a small plateau in between that works right.

Cyanoacrylic (super) glue allows quick model construction. Use
ventilation. Watch out, it bonds skin really well too. I believe that
acetone (nail polish remover) will dissolve cyanoacrylic glue.

I'm not sure of the rules in your particular contest, but if weight
control of your model is an important consideration, I recommend your
last few models use 'six pound' balsa wood. I think that's per cubic
foot, but don't get upset if that's not quite right. Common balsa sold
in most hobby stores is often about eleven pound grade. In a big city,
there likely to be a specialty hobby shop that can get the different
grades of balsa. They will likely charge a premium price. The lighter
weight balsa isn't as strong as the heavier balsa - but it is stronger
for its weight. If you are switching from heavier balsa to lighter,
don't forget to use a little more balsa to compensate for the somewhat
weaker lightweight balsa.

It would be a good idea to videotape your experiments closely. Lighting
is important for image quality; in general, use lots of light for
better images. If you can borrow an industrial videorecorder rig that
takes more pictures in a second than usual (faster frame rate) it will
help, but that sounds unlikely. Stepping through your videotape in
slow motion will help you see how your shell crushes and what will be
needed in the next experiment.

Further Reading:
  Discover Magazine, November 1998,
    "Light Elements: Precision Package Pounding"

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