[TheForge] Re: heavy hammers (Was Hammer Handles)

[Mike Spencer]

"Peter Hirst" <saltydog335 at aol.com> wrote:

> A word of caution about heavy hammers....Used a 5 lb hammer for a
> week and I can still feel it in my elbows.

Yeah.  I have a steel bolt in my right elbow from a teenage hockey
injury and I'm a bit cautious about anything that might cause further
trouble. 

> Movement of metal results from the amount of energy delivered in the
> blow.     [snip longish discussion]

Well, that's something I've been thinking about and haven't been able
to come up with a conclusion or computation that satisfies me.

Kinetic energy (a scalar) is 1/2 m v-squared and momentum (a vector)
is mv.  "Get a bigger hammer" is excellent advice when driving out a
stuck part because what you want is to max out momentum, not energy.

Things are not nearly so clear when talking about forging.  In
particular, the collision is inelastic, i.e. more like dropping a
bearing ball onto wet clay than like dropping one onto an anvil face.

I *think* "get a bigger hammer" is good advice (within your physical
limits or power hammer budget) for forging, too, but I can't quite get
a grip on the physics.

In addition, the "collision" may be thought of as between the hammer
(mass, say, 1 kilo) and the earth (mass around 6x10^24 kilos) with the
hot iron between them. Do I have to carry 10^24 around through the
computations (and keep track of changes in the velocity of the earth on
the order of 10^-24) to get the right result?

So: energy is absolutely conserved.  Momentum is conserved in elastic
collisions but what about inelastic ones?  If you drop, say, a 1
kg. bearing ball into a big block of wet clay,  you can calculate the
momentum of the ball just at impact.  A moment later, though, nothing
is moving. [1] No v, no momentum.  The energy is conserved by conversion
to heat but what happened to the momentum?  

I actually buttonholed a physics profs [2] during one of my gigs at
MIT and asked him to explain this. He could never get beyond the
textbook examples where momentum, as well as energy, is conserved and
even became quite heated about conservation of momentum rather than
explaining why it *appeared* to me not to be conserved.  At that
point I gave up.  Who am I to disagree with Isaac Newton?  But the
textbook examples don't seem to apply to calculations about hammering
hot, soft iron on an anvil solidly fixed to the earth.

Starting from the engineering end, there are a lot of data tables
about forging, drawing, rolling and the like but they're empirical,
i.e., engineers measure what happens under certain industrial
conditions.

Ho hum. :-)  Anyhow, think about momentum, too.


- Mike


[1] Okay, the earth with ball attaches is theoretically now moving,
    oh, say, 10^-24 m/s faster in the direction the ball was
    moving. That doesn't seem like a useful piece of new knowledge. :-)

[2] One of the more junior ones, who was heard to pronounce that he
    was more interested in education than research.  This is a
    career-limiting attitude at MIT but I thought it would be just
    right for my questions.  Oh well.

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