🚀 go-pugleaf

Several things still bother me about the instructional value of the egg
drop in a physics class.

On Wed, 3 May 1995, Richard Bowman wrote:

> Dropping an egg so as not to break it is in fact a good demonstration of
> Newton's Second Law.  Recall
>
>    F = m a
> but a = dv/dt, i.e. the change in velocity divided by the time this
>                     change in velocity takes place in
>

Of course.

> In dropping an egg from some height, we have the mass of the egg remaining
> constant and the change in velocity remaining constant.  (The ending
> velocity is zero, and the "beginning" velocity is the velocity of the
> egg just before it hits the floor.)  Whether we drop a "naked" egg or one
> enclosed in some cushioning material, these quantities are constant when
> an egg is dropped from a given height repeatedly.

No problem.

> So to reduce the force on the egg, and therefore hopefully to keep it intact,
> we have only one quantity we can adjust.  Only the time over which the
> egg goes from its "beginning" velocity to zero.  The longer we can make
> this time, the less force the egg will experience.

Also the distance over which the deceleration occurs. Is there a limit on
the size of the container? Is one allowed to use a parachute to lower the
landing velocity of the container?

> Any viscous fluid (e.g. oil, mud) will make this time longer.  So will
> suspending the egg with rubber bands (somewhat similar to the use of
> seatbelts in a car) or surrounding it with styrofoam beads.  Whether the
> egg breaks or not depends on how well these materials do the job of giving
> the egg a long time before coming to rest.

Now can students be expected to quantitatively compare the rubber band
suspension with the viscous fluid? How about encasing the egg in a
plaster block? This problem has ramifications far beyond simply Newton's
second law.

And, in the case of rubber band suspensions and viscous media, *the
decelerating force on the egg is not constant* during deceleration.
Therefore there's the time derivative of acceleration (the 'jerk') to be
taken into account. Have the students done this? Does it affect the
outcome? How much? Oh, then there's the little matter of the container
bouncing on impact, and conservation of momentum and energy in the
inelastic collision enters the picture. Are the students able to deal
with this? The whole exercise seems too messy and complicated for high
school, or even college freshman, students to deal with, unless they make
a whole host of untested assumptions.

Can't we find a somewhat more *clean* demonstration of Newton's second law?

> (I have placed a raw egg in a plastic bag filled with the expanded mica
> used to pack chemicals when shipped and have kicked it like a football,
> and the egg survived.)

Have you analyzed this with elementary physics?

What bothers me is whether the students know enough physics to make
quantitative comparisons of the various methods, make predictions of which
methods are best, and then have the predictions meet the test of
experiment. Or do they just tinker and test and try various things, until
something works pretty well. It seems that most of the time spent in this
activity must necessarily be on things *other* than Newton's second law,
which some claim is the *object* of the exercise.

Do any of these methods result in a scrambled yolk, even if the shell is
intact? Can we analyze this with elementary physics? Does it matter
whether the egg is blunt side down or point down when it impacts? How can
we analyze and predict that, for the various methods?

It seems to me that the nature of, and shape of, the *outside* of
the container might be of comparable importance to the nature
of the suspension of the egg *within* the container. Or do you
standardize the outer container in these contests? From your comment
about the plastic bag container, I assume you don't. I certainly wouldn't
feel competent to analyze all these possibilities for relative merit? Has
anyone done it and published it.

        -- Donald