You wrote that discussing an egg drop vehicle experiment in terms of energy
is wrong. I heartily disagree. It is very instructive for students to
design ways to not only increase the time of the impact (as you stated) but
to decrease the required momentum change by decreasing the kinetic energy of
the vehicle (through drag force or rotation). I don't find physics can be
compartmentalized so much as you state. Energy and momentum should be
considered together by students, because they "occur" together.
Jim Birdsong
Monta Vista High School
Cupertino, CA
In a message dated 7/2/99 9:55:51 AM, you wrote:
<<Incidentally, it is one of my pet peeves that many teachers who do the
"egg drop experiment" discuss this from a viewpoint of energy. That is
not the correct approach... it is clearly an impulse problem. We need
to accomplish a change in momentum without exceeding the force that the
eggshell can withstand. If the students understand momentum changes
and impulse, it is clear that all they need is a device to make the
collision with the ground last for a longer period of time. Or better
stated, the egg has to be brought to rest in sufficiently long time
that the force does not exceed the breaking force of the shell. (And
of course we assume the force is spread out over the shell rather than
occurring at one point.) Energy considerations don't add any insight
into this experiment at all. Although you can use the language that we
need the energy change to occur over a long time, that wording doesn't
lead to meaningful equations as quickly as realizing that delta-p is
equal to integral(Fdt).>>