[TheForge] heavy hammers (Was Hammer Handles)
sos at frii.com
sos at frii.com
Sun Jan 11 18:13:48 EST 2009
> Ok lets dispense with the qualitative and go to the numbers.
> First, the momentum of the small hammer would be 200, not 2,000. 1 lb x
200
> ips = 200 lb-ips . That's one one-hundredth of the momentum of the big
hammer, not one-tenth.
Good catch, I wasn't paying very good attention.
> Your analysis suggests that the larger hammer would
> then do one hundred times as much work on the little cube as the smaller
> hammer would. But would it? Maybe. But why? because just the act of
> lifting it puts tremendous potential energy into it. Just resting the
> 10,000 pound weight on the workpiece with no movement at all would
squish it
> flat at forging heat. Zero momentum.
I think before you were arguing kinetic energy was most important. Zero
k.e. as well. Perhaps this is an entirely different case.
> In my example, I disregarded the
> potential energy of lifting the different weights becaus at reasonable
> weights say 2.5 to 5 lb hammers, the difference is negligible. This is
> where using an extreme example skews the result. YOur larger hammer is
> delivering its kinetic energy plus its static weight of 10,000 lbs. Its the
> energy required to lift that weight -- which is released in addition to the
> momentum of 2" per second velocity -- that does the extra work on the
piece.
> That's why its easy to intuit a huge difference in result. The difference
> in static weight of two hand held hammers-- or even 2.5 lb hammer and a
100
> lb power hammer, is not much, so I disregard it in my example. But it
makes
> a huge difference in your example. To take that out of your example,and
> reduce the analysis only to kinetic energy versus momentum, you have to
> picture the setup working horizontally. Now you have a 10,000 lb hammer
> moving horizontally at 2" per second -- a little over 1/10 of a mile per
> hour -- versus a 1 pound hammer moving at 200 inches per second, a little
> over 11 miles per hour. Picture say a 16 penny nail driven half way into a
> very sold post. A 10,000 pound truck coasting only, not under any power
but
> momentum, hits the nail -- and only the nail -- square with its front
bumper
> at point 0ne 0ne ( .11 ) mph, 2" per second.
I really don't see the nail slowing the truck down any. The truck is going
to drive the nail in flat.
> Now picture the one pound
> hammer hitting that nail at eleven miles per hour 200 ips. A hundredfold
> difference in momentum, no difference in energy. Under your theory, the
> truck would have to drive the nail 100 times deeper than the hammer blow.
> DO you think it would?
It seems reasonable. It takes six or more blows to drive a nail.
> OK, now lets go back to qualitative. Ever see the
> damage a really slow moving vehicle does to a tree? say at 5 mph? I've
> gotten away with a number of such collisions in my lifetime without a mark.
> Hit the same tree with a hammer, and you'll at least leave a dent.
Put it
> another way: would you rather get hit by the truck moving at one tenth
of a
> mile per hour, or the hammer moving at 11 mph?
If I was tied to a tree, so I couldn't dodge, I'd go for the hammer.
>
>
> ----- Original Message -----
> From: <sos at frii.com>
> To: "Blacksmithing List Sponsored by ABANA" <theforge at mailman.qth.net>
> Sent: Sunday, January 11, 2009 12:54 PM
> Subject: Re: [TheForge] heavy hammers (Was Hammer Handles)
>
>
>>> 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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>>>>> password: anvil
>>>>> ___________
>>>>>
>>>>>
>>>>>
>>>>
>>>>
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>>>>
>>>
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>
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