[TheForge] heavy hammers (Was Hammer Handles)
Peter Hirst
saltydog335 at aol.com
Sun Jan 11 21:29:27 EST 2009
"I think before you were arguing kinetic energy was most important. Zero
k.e. as well. Perhaps this is an entirely different case."
Precisely one of my points. I was arguing k.e as between hammers of a few
pounds. Your extreme example is a very different case.
"I really don't see the nail slowing the truck down any. The truck is going
to drive the nail in flat."
You sure? At 1/10 mph? I'm betting it wouldnt. But put that aside and
assume that it would. What would the hammer do? that 200 ips was a good
guess at arm hammer speed, so a 1 lb hammer is a little light for a 16d
nail, but it would certainly move it what 1/2"? 1/4"? 1/8"? Pick one and
then consider this: if momentum is the controlling factor here, and your
heavy hammer has 100 times the momentum of the light hammer, the truck in
this scenario would drive 100 nails the same distance as the hammer would
drive one nail. Now tell me if you think that truck would drive 100 nails
the same distance that you picked for the nail driving the one nail.
" It seems reasonable. It takes six or more blows to drive a nail."
OK six blows to drive a 3 1/2 inch nail. Little over 1/2 inch per blow.
Call it 1/2. The truck moving at 1/10 mph would drive 100 nails that
deep? I don't think so, but there's one way to find out.
OK one last example. Firing a rifle. Say an M-16. High velocity 114 grain
5.5 mm. Kind of a 22 magnum. One shot. Hardly any recoil. At the moment
of discharge, equal and opposite forces are exerted on the bullet on the
one hand and on the rear of the chamber, the stock of the weapon and your
shoulder and hence your body on the other, if you are holding the weapon
correctly. At the moment the bullet leaves the muzzle, its momentum and
yours are exactly equal, in opposite directions. They have to be: equal
force applied in opposite directions for the same time. You: big mass,
little velocity. the bullet, little mass big velocity: Mv=mV. The
kinetic energy of the bullet, however mV^2, is immensely greater, since the
same force has acted on it over a longer distance and it has achieved a
hugely greater velocity. Or better still, say you are not holding the rifle
correctly: say you allow a little kick -- an inch or so -- before it hits
your shoulder, then all that momentum is in the weapon at first, then it
hits your shoulder. In this case, the momentum of the rifle and of the
bullet are still exactly equal. But which would you rather hit your
shoulder: the rifle stock or the bullet?
Say you fired the 114 gr. bullet dead center at your 1" cube of iron at
forging temperature at oh, 2600 fps. What do you think would happen to the
cube ? What if the the rifle butt (made of 4140, of course) impacted
another such 1" cube at whatever the recoil velocity is? what would happen
to THAT 1" cube? Same momentum, remember, in the rifle butt and the
bullet.
SO, like I said, think you are assuming, rather than proving, that greater
momentum does more work. Physiscs says over and over that work is a
function of energy, not momentum.
----- Original Message -----
From: <sos at frii.com>
To: "Blacksmithing List Sponsored by ABANA" <theforge at mailman.qth.net>
Sent: Sunday, January 11, 2009 6:13 PM
Subject: Re: [TheForge] heavy hammers (Was Hammer Handles)
>> 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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>>>>>> ___________
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
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>>>>> ___________
>>>>>
>>>>>
>>>>>
>>>>
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>>>>
>>>
>>>
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>
>
> _______________________________________________
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