[TheForge] heavy hammers (Was Hammer Handles)

Sun Jan 11 12:39:13 EST 2009

Steve:  two things wrong with this.  First, you are ASSUMING, rather than 
proving, that momentum is the key.  But energy is the key to doing work, by 
definition.  second,  if your arm can handle it is a hypothesis contrary to 
fact:  your arm CAN'T  handle it.

Your analysis demonstrates that  an arm moving very fast can create as much 
momentum as a huge hammer moving very slowly.

I saw a discussion recently between a mechanic and a physist recently on 
this analysis.  The mechanic claimed that everyone from Newton to Einstein 
was wrong about conservation of energy, and no matter how hard he tried, the 
physicist could not get the guy to understand the difference between 
momentum and energy.  If momentum were the key, no lighter power hammer 
could as hard or harder than a heavier hammer, but many do.  Ask any 
ballplayer what the key to slugging is: bat speed or weight.  (ANd dont try 
the loaded bat argument:  that increases the area of the sweet spot, not the 
energy delivered to the ball.  That's why the corked bat is just as illegal 
as the leaded bat, and way more popular: it incrreases speed AND sweet 
spot).
----- Original Message ----- 
From: <sos at frii.com>
To: <mspencer at tallships.ca>; "Blacksmithing List Sponsored by ABANA" 
<theforge at mailman.qth.net>
Sent: Sunday, January 11, 2009 9:16 AM
Subject: Re: [TheForge] heavy hammers (Was Hammer Handles)

> 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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