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Re: bullet holes



At 05:49 PM 10/15/00 -0400, Robert Carlson wrote:
explain the basic physics of this situation using forces.
Consider the bullet/flesh system.
Hole size is not important in this basic analysis. I'm not after a
numerical answer.

Since a _basic_ answer was repeatedly requested, I will make a number of
approximations.

*) Assume homogeneous flesh, no bone.

*) Assume bullet is heavy so flesh does not greatly resist it; flesh is
essentially a fluid. Use fluid dynamics techniques.

*) Assume bullet slow enough that Mach number is << 1. This implies
changes in pressure cause only small-percentage changes in density.

*) Assume bullet is slow and/or small enough that we can use the laminar
flow and potential flow approximations without too much grief.

*) Assume spherical bullet.

Then:

1) Construct velocity potential using techniques borrowed from sophomore
electrostatics: dipole plus uniform flow. Differentiate velocity
potential to get velocity. Differentiate velocity to get acceleration.
Multiply by mass-density to get pressure gradient, i.e. force per unit volume.

2) Optionally throw in effects of viscosity. Result should match Stokes
formula.

=========================================

For those who might still be tempted by the baseless
"acceleration comes from force not vice versa"
dogma, note that this is yet another example where the most natural
procedure is to calculate the velocities and accelerations first, and then
calculate the forces.