[TheForge] heavy hammers (Was Hammer Handles)

[Peter Hirst]

I look at it this way.  Energy has been defined quantitatively as the 
ability to do work.  Work is the movement of a force over a distance. 
Moving metal is moving a big force over a little distance.  SO moving more 
metal, or moving it farther, is a function of energy.  But even if it were a 
function of momentum, would you rather lift a heavier hammer,or just move 
the same hammer faster on the downstroke?


----- Original Message ----- 
From: "Mike Spencer" <mspencer at tallships.ca>
To: <theforge at mailman.qth.net>
Sent: Sunday, January 11, 2009 1:59 AM
Subject: [TheForge] Re: heavy hammers (Was Hammer Handles)


>
> "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/                        ^^-^^
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