[TheForge] heavy hammers (Was Hammer Handles)

[sos at frii.com]

Mike, how about a thought experiment.

I'm taking a wag and saying that during forging your hammer moves at 200
inches per second (~0.1 seconds for the blow from shoulder to anvil). You
probably have a lot better number, the exact value isn't crucial.
Cases:
1. 10,000 pound hammer, falling at 2" per second: mv=20,000 pound-seconds
(don't give me slugs, you know what I mean), mv^2=40,000 pound-second^2
2. 1 pound hammer, falling at 200" per second: mv=2,000 pound-seconds,
mv^2=40,000 pound-second^2
I think it is pretty obvious that #1 is by far preferred, if your arm can
handle it. This makes me think that momentum is the key rather than
kinetic energy.
But I always had trouble with those two.

Steve

>> 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/                        ^^-^^
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