[TheForge] heavy hammers (Was Hammer Handles)
Marc Godbout
takeahikemarc at gmail.com
Sun Jan 11 13:55:32 EST 2009
I think one thing that's not being taken into account is inertia, which is a
resistance to motion changes, and is directly related to mass. The hammer
blow on hot metal isn't totally inelastic. Hitting that 1" cube with an 8-oz
hammer is going to cause that hamer to bounce back some. The 10,000-lb
hammer not so much. So I think a big hammer is still better in some
situations.
And I like using extreme example, too. It tends to amplify the results so I
can see what's going on without doing the math.
--Marc
On Sun, Jan 11, 2009 at 12:54 PM, <sos at frii.com> wrote:
> > 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'm not assuming anything about momentum. I set up two cases with the same
> kinetic energy but quite different momentum and asked what the result
> would be. You could do it the other way around, but the cases wouldn't be
> as dramatically different without the squared term. It doesn't really
> matter what my arm can handle, this is a thought experiment. The purpose
> is to try and tie the discussion back to the real world (instead of
> arguing about exactly what k.e. and momentum do). It is often instructive
> to consider extreme examples.
>
> Do you think that the 1 pound hammer moving at an ordinary hand hammer
> rate of speed will do as much to a 1" cube of steel? The one pound hammer
> will make some dents in the surface, the 10,000 pound hammer will squish
> the cube like a hydraulic press--maybe down to 2/3 or 3/4" of an inch, in
> one blow. The numbers are different enough that this seems pretty clearly
> to support momentum over k.e.
>
> > 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.
>
> Why not? If you hit faster with a lighter hammer, you should do the same
> amount of deformation. This applies whether k.e. or momentum is the key.
>
> > 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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