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Re: [Phys-l] Percent KE retained in perfectly inelastic collision



At 02:01 PM 12/17/2007, Andrew Duffy, you wrote:

.... the question about the percent KE retained by using vf/v1i,
this is something
I show my intro. physics students at least twice.

In analyzing a ballistic pendulum, for instance, you get this result. I
derive it by saying:

KE initial = 0.5 m v1i^2 = 0.5 (m v1i) v1i

KE final = 0.5 M vf^2 = 0.5 (M vf) vf

The 0.5's cancel, and then the terms in brackets cancel - conservation of
linear momentum - which quickly leads to exactly what the student got:

KEf/KEi = vf/v1i

I do a similar thing later in the semester in analyzing the rotational KE
of a figure skater who pulls her arms in, to show that the rotational KE
goes up when she does this.

As for where the student got it from, that's a good question. It would be
interesting to ask the student and see what he says.

cheers,

Andrew Duffy
Boston University


I would like to tip my cap at Boston U for cultivating an instructor
who does not appeal to Quantum Dynamics, to the supremacy of
momentum, to how physical behavior must conform to some
theoretical construct, does not discount the 'accident'
of a finding which does not show up in the usual texts,
thereby counting it a lucky mistake :
instead for presenting an easy algebraic construct that has
appeal for the youngster; one that likely provokes an Aha!
moment - though it does demand insight into the continuance
of momentum through collisions.


Brian Whatcott Altus OK Eureka!