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[Phys-L] Re: Momentum Again



At the risk of venturing into an area I haven't thought about for a w=
hile ..

Suppose the moving car has speed v. It has kinetic energy of 1/2 mv^2=
.

Consider the frame of reference where the initial (and final) total m=
omentum is zero.
Then the objects each have initial speed v/2. So the total energy pri=
or to the collision is 2 x 1/2 m(v/2)^2 =3D 1/4 mv^2.
After the collision, the speed of each car is zero so the initial ene=
rgy is all converted into non-kinetic energy. So the energy converted=
in the collision is 1/4mv^2 - half the original kinetic energy of th=
e initial system.
=20
It seems that the energy change in an inelastic collision should not =
depend on the frame of reference, since the energy change will increa=
se the temperature of the objects. The temperature change should be =
independent of the frame of reference.
=20
Larry Woolf
General Atomics

________________________________

=46rom: Forum for Physics Educators on behalf of David Abineri
Sent: Tue 12/6/2005 2:31 PM
Subject: Momentum Agina

If a stationary railroad car is struck by a moving railroad car of eq=
ual
mass and coupled to it, why is that regardless of the material of whi=
ch
the cars are made and regardless of their aerodynamics and regardless=
of
the density of the atmosphere in which they are moving and regardless=
of
whether there is an atmosphere or not that half the kinetic energy is
lost (converted to other forms)?

To both my students and to me, this does not seem reasonable and yet,=
if
momentum is conserved and the masses are equal this must happen!
How does one explain to a high school class of bright students why,
regardless of all these factors and some I haven't mentioned, exactly
half the kinetic energy is always lost?
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