[TheForge] Re: make shift anvil

[Mike Spencer]

> On the issue of force, basic physics dictates that force equals mass
> x acceleration.  So a heavy hammer at X velocity has greater force
> than a lighter hammer at the same speed.

That's not quite right. A mass m at velocity v doesn't "have force".

What it has is both momentum (M) and kinetic energy (KE).

    M = m * v          # Momentun is mass times velocity

   KE = 1/2 * m * v^2  # Kinetic energy is 1/2 of mass times velocity squared

If you swing a small hammer faster, you increase the KE way more than
you do the M.  And you soon reach the point beyond which you can't
swing the hammer any faster.

If you "get a bigger hammer", you increase M without having to swing
the hammer faster.

I'm completely happy with the concept of conservation of energy
because KE can be converted into some other form of energy, say, heat.
F'rgzample, when you hammer a cold bar til it's hot, a bunch of your KE
is turning into heat.

I'm having a bit of trouble with grasping conservation of momentum
because if there's no v, there's no M.  I see that momentum is
conserved with bouncy things (perfectly elastic things like steel
billiard balls or knocking a cold steel pin out of a bushing).  But if
I hit a piece of lead or clay with my hammer, the hammer stops dead in
its tracks, the clay or lead moves (mooshes) a bit and stops.  The KE
is presumably transformed into heat but, since noting is moving,
there's no momentum.  Huh.  I gotta re-read my physics book again
because I don't get it.

On one of my MIT trips I had an opportunity to get a physics prof off
in a corner with paper and pencil and ask him to explain this.  He
gave a nice lucid explanation of how momentum is conserved in bouncy
things but kept changing the subject when I asked about hammering
(nearly) completely in-elastic stuff like clay or lead.  Huh.

On a related note, I think (but can't prove) that the anvil doesn't
just "absorb" the energy of the blow (as charles said).  Some of the
energy is absorbed by the (presmably) hot iron and is converted into
heat.  I think that the rest travels through the anvil as a shockwave
and bounces (echoes) back against the workpiece while it's still in
contact with the hammer, thereby amplifying the blow. Cracks in the
anvil or joints between pieces of junk from which you've made an
anvil-substitute dissapate the energy of that shockwave and give you a
dead, less effective blow.

- Mike

-- 
Michael Spencer                  Nova Scotia, Canada       .~. 
                                                           /V\ 
mspencer@tallships.ca                                     /( )\
http://home.tallships.ca/mspencer/                        ^^-^^

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