physics difficulties

["Eric Chang" <ericchang@my-deja.com>]

>The engine block will absorb exactly enough energy to
>stop it's momentum.  I think "energy is the square of
>mass times speed" is what the engineering axiom says. 

Physics education, especially the laboratory phase of
it, seems to have really deteriorated these days.  The
purpose of simple experiments is not to display fancy
effects, but to make real the simple impacts of, say,
the equations of mechanics.

The above statement has little to do with either the
principles of conservation of energy or momentum,
although it implicitly "invokes" both.  Conservation
of momentum says that the vector sum of momentum is
exactly the same before and after the collision.  So,
two identical vectors heading in can either head out
in opposite directions or neutralize.  Conservation
of momentum says that either is allowed to happen, or
a host of other possibilities.  It, for example,
disallows both objects to recoil in the direction
that one of them came from.

It does not imply that "the engine block will absorb
exactly enough energy to stop its momentum."  "Energy
is the square of mass times speed" is also somewhat
incorrect, but the error is not directly relevant to
the discussion.  (It is indirectly relevant, since
simultaneous application of the principles of energy
and momentum conservation to show the allowed
outcomes of the collision depend on the correct form
of the expression for kinetic energy).  The engine
block can absorb all of the energy, or none of the
energy, as long as energy is not created, and the
vector sum of the momentum stays fixed.

>Any and all anergy absorbed by an aluminum 4-banger or
>a beefy '72 454 cubic inch cast iron powerplant is
>energy that is not transferred to you.

How about energy that is not absorbed, and just
transferred?  Kind of like saying that a .45 is less
dangerous than a .22, since more energy is "absorbed"
by the .45.

Of course, mass is important in a collision, but if
you work the equations out, the masses have to be
quite different.  I have a friend who was observing
in a locomotive which hit a vehicle at a crossing.
He said that he cringed and shut his eyes, but did not
feel much disturbance.  He said that he heard the
sound of tin cans crumpling, and, of course, the
engineer laughing.

>Oh, and what a shower of acid there will be if those
>batteries actually absorb energy by...deforming....
>I'm right there in front, you know.
   
This is a tougher one.  You can't just look this one
up in an undergraduate physics textbook.  You will
have to do the crash test, or understand something
about hydrodynamics.  For all those who believed that
oil is stored in tanks underground, and all that the
big oil companies have to do is turn up the valves,
this is kind of hard, but fluid does not move through
interstices with infinite ease.  I know a guy who
was rear ended at freeway speeds.  The other fellow
tried to plow through 900 lbs of "deforming" batteries
to get to him.  Those batteries were soft, but it
seems that the fellow's head was softer.  He died to
pop a few batteries, but did not splash much (acid)
on the EV driver.




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