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Physicists needed: how much force ...

-e-

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a posting over on wc got me pondering how to measure the force of a rotating object? how does one measure the force of wood into a face? and can this force be compared with the force of a face into a windshield?

my old synapsis can't recall how to calculate the force of a rotating object -- is it centrifugal force = mass * speed^?

say you have a 15 lb block of wood, rotating at 1000 rpm and this impacts your face 20cm away from the rotational axis ... how do you measure the impact to the face?

just wondering (before my next public turning demonstration) if you can mathematically compare the use of a face shield to the use of a seat belt through the force of impact?
 

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e Sounds like a good test for myth busters. We need a balistics gel dummy that we throw wood at and see what kind of damage is done at different speeds of rotation.
 
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e wrote: "say you have a 15 lb block of wood, rotating at 1000 rpm and this impacts your face 20cm away from the rotational axis ... how do you measure the impact to the face?"
-------------------------

e, that is an interesting question. I think the volume of the scream when the 15 lb. block hit would measure the impact to the face.

Ok, seriously, why do you want to know this? You still couldn't move fast enough to avoid getting smacked in the face if that block lets loose.
 
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I believe it'd be a bit more complex, since the piece would have either broken apart and it'd be the force of the fragments, or the piece would have probably bounced off something before hitting something else. I've never had or seen anything whole fly straight off a lathe in a direction where a person would be if during a demo.
 

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a posting over on wc got me pondering how to measure the force of a rotating object? how does one measure the force of wood into a face? and can this force be compared with the force of a face into a windshield?

my old synapsis can't recall how to calculate the force of a rotating object -- is it centrifugal force = mass * speed^?

say you have a 15 lb block of wood, rotating at 1000 rpm and this impacts your face 20cm away from the rotational axis ... how do you measure the impact to the face?

just wondering (before my next public turning demonstration) if you can mathematically compare the use of a face shield to the use of a seat belt through the force of impact?

Well, while you asked for force, that really isn't what you want because force alone doesn't tell you anything about the destructive capability of a projectile. For instance, a static force might have negligible effect.

Kinetic energy is the physical property of a moving projectile that would determine the amount of energy that would be dissipated in an impact between two objects. The kinetic energy of a projectile is given by the expression:
KE = 1/2 m * v^2
where m is the mass of the projectile and
v is its velocity
One of the important things to note about this relationship is that velocity quickly becomes far more significant than mass, for instance, doubling the velocity will quadruple the kinetic energy whereas doubling the mass only doubles the kinetic energy.

This explains why a lightweight supersonic bullet is far more destructive than a heavy, but slow subsonic bullet.

Bill
 
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I'm not a physicist, only a structural engineer. This problem is similar to calculating the force of vessel impact on a bridge, or the design of fendering systems for ship berthing. I've conducted both types of investigation.

As Bill indicates, the computation involves an energy balance. The kinetic energy of the moving object is transformed into strain energy (force times deformation) in the receiving target. The elastic properties of the target provide the other side of the equation, with some algebra to derive the force as the unknown variable. Skinny people have less cushioning on their bones, and would experience more pain - an unintended benefit of obesity.

Calculating the velocity of a chunk after it's left the lathe is a bit of a crap shoot. In the design problems I cited, we include some conservative fudge factors, and experimental results could easily differ from the calculations by many many percent. One significant figure could even be over-precise.

If the "target" places its face in the path of the chunk while it's still ON the lathe, it might qualify for a Darwin award.

Joe
 
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Force into an object? F=Ma. Centrifugal energy in a piece doesn't translate easily into a meaningful impact value, sadly. http://hyperphysics.phy-astr.gsu.edu/hbase/ke.html has a lot of good stuff after you get past the basic kinetic energy consideration.

Two things, always true, to consider. Don't use your digital-readout computer-feedback continuously-variable speed dial to discover the maximum (unloaded) harmonic-free velocity for turning. Sort of meaningless anyway, as soon as you load the tool into the equation. Instead, turn at as slow a speed as you can tolerate without overfeeding the tool. Take a bit thicker or broader shaving to compensate for the few seconds lost.

Don't get under the wood. I read peoples' complaints about needing gloves to dampen vibration or their justification for criticizing my use of a large gouge when roughing because of "splinters" as just plain bad turning technique. Who on earth would bash a tool into the work rather than swing and trim the high spots to round, letting the toolrest take the whacks? Who would cut into the rotation instead of across it? Makes sure you're getting under the wood on both counts. Now get the cut above center on a convex or below on a concave so you've got air to cut with an errant edge, not wood to hit, dismounting the piece at your exponentially greater velocity and possibly flinging it, if you're way under, back up at you.

Then there are those who complain about debris flying off a piece they're turning. All I can say is DOH! If it hits you when you stand there - don't stand there.
 

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e Sounds like a good test for myth busters. We need a balistics gel dummy that we throw wood at and see what kind of damage is done at different speeds of rotation.

Yeah, and a hollow point bowl - uhhh, I mean a hollowed out bowl would do more damage, because it expands more, right? :p


I believe it'd be a bit more complex, since the piece would have either broken apart and it'd be the force of the fragments, or the piece would have probably bounced off something before hitting something else. I've never had or seen anything whole fly straight off a lathe in a direction where a person would be if during a demo.

I have. At least twice, and up close and personal. Once was when I was a fairly new turner, and forgot to turn the speed down on my lathe when I went from a pen to a gnarly knotty log section. It smack me right in the face, and it not for my air helmet, I probably would not be typing this now. The second time was much more recent, at Capitol Area Woodturners, where a banana bowl blank came off the lathe because of an operator error. It actually whizzed right past where I was standing because I had shifted over just a little and was clamping my face shield down.

As a side note, It made a very impressive smack against the wall behind me. The funny part of it all was, it happened right after lunch, and during lunch some of us were discussing the difficulty of keeping people awake right after a nice big meal and in a warm comfortable room. I never saw a more uniform set of rounder, stretched out eyes on a group of people for the rest of the afternoon! :eek:

The first piece, as near as I can tell, did fly right off and hit me square in the face. It continued off my face and into the floor joists above my head, and bounced around a few more times before settling down. I don't know if my friend was more scared of me being injured or him having to haul me up the stairs. The second piece at CAW ricocheted off the tool rest, but it was still moving at considerable speed, and being squared off, it still had some "angular momentum". :p
 
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I think your seat belt / face shield analogy will fall apart fairly quickly. There are too many differences between the two to be valid.

Something you may want to look at is the following link: http://www.safetyglassesusa.com/ansiz8712003.html

ANSI Z87.1 is the standard to which safety glasses and face shields are manufactured. The information in the above link makes it pretty clear what sort of impact our face shields are meant to absorb. At the plant I work at we did a demonstration one time where we took (2) .177 caliber pellets, pumped the rifle get the Z87.1 velocity, and then shot a pair of glasses in a controlled environment.

The two pellets together equalled the standard's weight for the test projectile.

It was impressive to see the glasses and face shields escape with nothing more than a scratch. Then we repeated the test with Walmart glasses that looked like the station's safety glasses but which were not made to Z87.1 standards.

Lots and lots of plastic shards, and the egg we were using to simulate an eyeball was dripping everywhere.:eek:
 

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Ok, seriously, why do you want to know this? You still couldn't move fast enough to avoid getting smacked in the face if that block lets loose.

in these days of so many videos and turning demonstrations, you continuously see people turning without a face shield. once while teaching, i had a smart kid ask "do I REALLY have to wear one". other than just answering with "YES", i tried to come up with an analogy that he could relate the force to, such as: a baseball bat to the face, or skiing into a tree, or ejecting from a jet without a helmet, racing without seat belts ...but these didn't really describe the potential force, nor do i have the numbers to substantiate my answer!

therefore, i've been pondering what would be a good answer to his question. wouldn't it be more effective if the force of the projectile was quantified?

moreover, wouldn't it be fascinating to have a slow-motion video of hunk of wood smashing into the face of a crash-dummy! other than that, mathematically quantifying the force would be helpful.
 

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In industry, impact force is usually measured in energy - Joules or such.
The kinetic energy (K.E.) is what is measured.
However the task of figuring out how hard you might be hit is a crap shoot because it is difficult to guess how much rotational energy will be translated into translational energy.
If it is a smooth cylinder rotational kinetic energy is pretty safe. If it is a square blank or a stubby tree trunk the rotational component of the energy can hit as hard as the translational component.
Many of us have parted off a finished spindle at speed with no problems from the rotational KE.
It wouldn't be accurate, but for your purposes, I would simply calculate the rotational K.E. of a typical project, and assume the same object at a translational velocity which had the same K.E.
The formula for rotational KE is (the Mass moment of Inertia about the axis of rotation) times (the angular velocity squared).
The formula for rectilinear or translational KE is 1/2 times (the mass) times (the velocity squared).
 
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i tried to come up with an analogy that he could relate the force to, such as: a baseball bat to the face, or skiing into a tree, or ejecting from a jet without a helmet, racing without seat belts ...but these didn't really describe the potential force, nor do i have the numbers to substantiate my answer!

Try this analogy... It's bottom of the ninth inning, your team is up by one, you're squatted behind home plate, and the tying run is at second. In your excitement you forgot to grab your face mask on the way out to the plate. But that won't matter. No balls ever bounce towards your face.

The batter pegs one into center field. The fielder takes it on one hop and throws it hard to home to get the incoming runner. You're in position, the runner is beat, but the throw is slightly short. It take a bad hop and pegs your right in the nose.:eek:

THAT is a valid analogy of what a chunk of wood would do to your face if you weren't wearing a shield.
 
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I really think the argument shouldn't be about force of impact, but about risk. It doesn't take any force at all for even a splinter to ruin one eye if thrown off like a spear. If a younger learner doesn't want to wear a shield/safety glasses/whatever, the obvious question to ask is; "So you don't mind only seeing out of one eye for the rest of your life?" Follow up with the pellet gun analogy and they'll stick that thing on faster than you'd ever think.
 

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I have some more anecdotal evidence for what it is worth. A friend of mine who is a turner, and who steadfastly refuses to wear face protection of any kind has has reconstructive surgery on his face not once, but twice. Once he had his nose all smashed up and once he had to have a couple of teeth replaced. In both cases there were lots of stitches, blood, pain, and other neat stuff to endure. I don't know about quantifying it, but I do know that the kind of injuries he suffered are painful, embarrassing, and completely preventable.
 
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I think Walt has it about right - the smallest force can do an immense amount of damage if the bit of wood hits you in the right place. I have a sign in my workshop that says 'Remember, you are now using your last pair of eyes'.

I always wear safety glasses and they have saved me from an eyeful of superglue (impact force negligible, but wow, I'm glad I had them on). I also have a scar on my upper lip which is the result of using a blunt gouge which I was too idle/impatient to stop and sharpen, and the lack of a face shield. I sharpen much more frequently now, but still rarely use a face shield.

Bob
 

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I think that all of you guys, especially Curt, Walt, Bill G., Mike, Bob, and Charlie are pointing towards something more fundamental and, in the end, more useful when it comes to trying to convey the seriousness of getting hit in the face with a chunk of wood.

Trying to convert this into a physics problem really amounts to a not very productive math exercise in the sense that it would not give any truly meaningful answer at the visceral level to someone wanting to know what will happen if they stick their face into the projected trajectory of a bowl gone ballistic. Dealing with a mixture of rigid bodies and not very quantifiable biological mass characteristics (your face) can amount to either an extremely complex set of differential equations that, at best, would have to settle for statistical solutions anyway or, on the other hand, grossly oversimplified analytical models that would not yield worthwhile results, anyway. And what would a set of numbers do insofar as giving you a visual image of what to expect.

Because the exact mechanics of how an accident might happen is not predictable in the real world, the results can range from a good scare a slight bruise to serious disabling and life threatening injuries.
 

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thank you all for your replies...i still would like to come up with numbers.

if you've ever worked with young minds, you've experienced how difficult it is to help them UNDERSTAND something that they have never experienced. they are inclined to follow what they see. putting facts to an experience will help some grasp -- but not others.

many of you will remember back in the 60's when the concept of seat belts was introduced -- the public screamed bloody-murder in resistance. it wasn't until the NTA started airing videos of crash-dummies did the public start to understand.

and yes, wearing a face shield is all about reducing the risk of likely injury -- just like not smoking or wearing ear protection....

eh, what'da say?
 

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......... it wasn't until the NTA started airing videos of crash-dummies did the public start to understand.........

I think that you just pointed out the futility of looking at obscure numbers (which I think that many of us have been trying to say doesn't exist in any meaningful sense) as opposed to seeing a picture that you can wrap your mind around. The public wasn't particularly interested in numerical data because it doesn't get to them at the gut level. On the other hand, they probably felt that they had a lot in common with crash test dummies.

If you want to give them a number, then tell them #37. That was a real hair raising story.
 
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thank you all for your replies...i still would like to come up with numbers.

if you've ever worked with young minds, you've experienced how difficult it is to help them UNDERSTAND something that they have never experienced. they are inclined to follow what they see. putting facts to an experience will help some grasp -- but not others.

Look at the site I referenced. Not only to they go through Newton's three biggies, they have pictures, formulae for rotational and kinetic and all kinds of things. Lesson plans at http://www.glenbrook.k12.il.us/gbssci/Phys/Class/momentum/momtoc.html might give you some ideas for experimentation.

The problem is that you have to differentiate for them between what is potential and what kinetic. The discussion has sideslipped rapidly, but the real answer there is that there is a maximum available, and then there is what actually happens. You'll have to tell them such things as "a piece of bark from the periphery of a 10" piece rotating at 350 RPM is detatched and thrown at the10:00 o'clock position." Then, and only then, will you have a worthwhile data set to teach beyond.

For turners, the answer is still the same. No matter the dimension or the position of the piece, turning slow is the best answer to the problem of kinetic energy. With less available you'll be safer, your piece won't shake the lathe as hard, and next time someone asks what speed to turn at we won't end up with some magic formula which has nothing to do with how to shave wood, but a lot to do about increasing the potential for harm.

Or, you could armor up and fling away. I used to show kids how much farther I could shoot shavings at the lathe when it was at 800 than at 400, but they always harbored a suspicion that I knew some sort of magic to make it happen. Power of television. Used to win sodas from the new crop every year by showing them how I could twist shavings clockwise or counter at the lathe.
 
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... many of you will remember back in the 60's when the concept of seat belts was introduced -- the public screamed bloody-murder in resistance.

Yes, a lot of people pee'd and moaned about restrictions on their liberties. Same thing in Florida now with motorcycle helmets. But, back then there were a few of us who welcomed seat belts and helmets because they were COOL. Just like the big boys in NASCAR. Persuade folks, especially young folks, that face shields are COOL, and supplies will run short right quick. Maybe add some racing stripes to speed up the process.

Joe
 
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-e-,
The tangential velocity of a rotating thing is (2)(Pi)(r)(N), where r=radius and N=rotational speed, like rpm. Suppose you had a 12" diameter chunk of something (r=6") on the lathe, turning at 800 rpm, and a piece of it broke off the very outside diameter. Let's just say it was a 12 ounce piece, same weight as can of the beverage of your choice. The tangential velocity of this piece is about 28 mph, and it is headed straight for your nose. So visualize a can of beer hitting you in the nose at almost 30 miles per hour. Ouch!

(2)(pi)(1/2 foot)(800/min.)=2,513ft./min
(2,513ft./min.)(60 min./hr.)(mile/5,280 ft.)=28.6mph
 

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-e-,
The tangential velocity of a rotating thing is (2)(Pi)(r)(N), where r=radius and N=rotational speed, like rpm. Suppose you had a 12" diameter chunk of something (r=6") on the lathe, turning at 800 rpm, and a piece of it broke off the very outside diameter. Let's just say it was a 12 ounce piece, same weight as can of the beverage of your choice. The tangential velocity of this piece is about 28 mph, and it is headed straight for your nose. So visualize a can of beer hitting you in the nose at almost 30 miles per hour. Ouch!

(2)(pi)(1/2 foot)(800/min.)=2,513ft./min
(2,513ft./min.)(60 min./hr.)(mile/5,280 ft.)=28.6mph

FWIW, a 12 oz. can doesn't weigh 12 oz. The liquid VOLUME is 12 fluid ounces which has no connection to weight other than a liquid ounce of water weighs 1.0625 ounces avoirdupois.

I haven't ever been hit by a FULL beer can -- folks around here are smarter than throwing away full cans, so I can't quite relate to being hit at 28.6 MPH by a full can. Just for the sake of argument, suppose our turner had cranked the lathe up to 1600 RPM (double the speed). If he hadn't consumed too many of aforementioned beverages, he could probably quickly determine that the speed of the BCE (beer can equivalent) would be doubled to 57.2 MPH. However, the more significant part of the story is that the kinetic energy (which is the term that would tell you how much energy you would absorb in the impact) of the BCE has quadrupled. I think that I gave the equation in a previous post. Anyway, the kinetic energy of the 28.6 MPH BCE is 27.8 Joules and the kinetic energy of the 57.2 MPH BCE is 111.2 Joules. The Joule is sometimes called a Watt-second.

The hardness, size, and shape of the object would influence the extent of a potential injury more than anything else. If it were a 12 oz. Nerf ball the size of a large beach ball, the risk would be far less than something very small, but of the same weight with sharp projections.

Do any of the numbers given for either velocity, mass, or energy convey any meaningful information about the extent of an injury if struck by a piece of wood while turning? It doesn't really give me any insight. What does however impress me are some pictures that I have seen of facial injuries. I suspect that if you were teaching woodturning to a young beginning student, he might be rather unimpressed by 28 MPH velocity. He might think that a baseball weighs roughly what a BCE weighs and also that a pitcher might throw a baseball at almost 100 MPH and he has no problem catching the ball that has 16 times greater kinetic energy. I still think that you may be painting yourself into a corner using this approach to discussing safety. Bill Grumbine's picture is a better attention getter (and I know that it is a truthful documentation). :D

Bill
 
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Those retired rocket surgeons are so picky. -e- just asked for some numbers for purposes of illustration. Was going to calculate the size of a 12 ounce steel ball, but it was getting late.

Meanwhile several excellent comments and advice about safety above, worth reading again! Years before there was a "seatbelt law" I was trying to get my wife to use her seatbelt. The successful approach was- "Given a choice, would you rather put your seat belt on and off 1,000 times, or have your face and chest smashed in by the steering wheel and dashboard?".

How about- Would you rather put on your faceshield, respirator, hearing protection, etc. 1,000 times, or ---
 
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Those retired rocket surgeons are so picky.

Haven't spent much time among adolescents lately, have you? Quite the challenge to gain the interest of the sluggish minds, which might react to the beer-can illustration, but the quick minds will rip you apart over one omission or misstep, even if it takes research and a delayed gloat next day.

They'll tell you they wouldn't pick on another teacher that way, because they're easy pickings, but they still wanted the Pepsi that I occasionally awarded when they gave the correct answer to a "stump the chump" question. Mark of pride to be drinking the old man's beverage.

Newton's first isn't the real problem. Takes no talent to stand away from a possible throw. It's that second law that needs discussion. The beer can itself, unless embedded in the wood, would be released in accordance with two and three, which are also under our control.
 

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Those retired rocket surgeons are so picky. -e- just asked for some numbers for purposes of illustration. Was going to calculate the size of a 12 ounce steel ball, but it was getting late.

Meanwhile several excellent comments and advice about safety above, worth reading again! Years before there was a "seatbelt law" I was trying to get my wife to use her seatbelt. The successful approach was- "Given a choice, would you rather put your seat belt on and off 1,000 times, or have your face and chest smashed in by the steering wheel and dashboard?".

How about- Would you rather put on your faceshield, respirator, hearing protection, etc. 1,000 times, or ---

I guess that we rocket scientists are picky because it dawned on us that a rocket can't be called back to the pad for maintenance once it has left.

I also put seat belts in my '61 Corvair when the seat belt law was just being discussed as a possibility and, at the time, they were available as a dealer-installed option on new vehicles. Knowing what I do now (and was beginning to suspect back then), it is probably a very good thing that I did have seat belts in that vehicle. Its handling was very squirrelly on wet pavement with its extremely aft cg.
 
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thanks texian ... for the formula and numbers!

a beer can at 28mph to the nose -- now that many can relate too!

:>)


If I have done the calculations correctly (its been a long time, and I may have screwed up the conversion from mph) , that velocity is the impact speed you would get if you dropped an object from 8.6m (about 28 feet). So just ask them who would like to catch a 15 pound bowl between their teeth when dropped from that height.
 
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Yeah Bill, I was going to write something about hating to tell your boss at NASA that we missed the moon (or whatever) by 1,000 miles because your calculations were insufficiently picky. But it was getting late.

Had a '64 Corvair, 180 turbo with a few additional mods. Loved it.

Thanks Seafoam. Was gonna calculate that too but it was getting late. And I have forgotten lots of formulae too, but hate to admit it.
 

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I have turned some bowls approaching 14 inches diameter which is the limit of my lathe and a couple larger ones on a Powermatic, but none of them were even remotely close to 15 pounds. That would be a whopping big bowl.

Here is a woodturning safety tip: If you have something that weighs 15 pounds chucked up -- 800 RPM is way too fast to be turning it until you are approaching the final turning thickness. At that point, the weight will be a lot less than 15 pounds unless it is a real whopper. I think that Dale Nish has a rule of thumb about diameter vs. RPM ... I don't remember what it is at the moment, but my rule of thumb is to turn big stuff slowly.
 
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We need an explanation that can be understood by a kid, or people who have no technical knowledge.

I use the analogy that a flyer off the lathe is the same as being hit by a 90-mph fastball, or a kid chucking a rock at us from 10-feet away. Neither is likely to kill you, but there is a good chance you will end up in the hospital getting a few stitches. At the least it will leave a good welt or a bruise that will hurt for a couple days. At the worst you could lose an eye.

Neither is technically correct, but they are close enough, and they are a picture that people can visualize and understand without a lot of explanation.

Yes, I do understand the physics involved, but I am there to teach people how to turn wood, not to impress them with my scientific knowledge.
 

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I like Russ's approach.
If you are going to be sloppy with the calculations, don't pretend that it is a true statement, there might be a sharp kid in the class that will figure out the calculations were mis-applied.
Better to just throw out a statement without any claim of math or physics behind it.
 

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pbase.com
I like Russ's approach.
If you are going to be sloppy with the calculations, don't pretend that it is a true statement, there might be a sharp kid in the class that will figure out the calculations were mis-applied.
Better to just throw out a statement without any claim of math or physics behind it.

Me too! I think that is at the root of my problem with throwing numbers around, especially when the validity of the way that they are being used is rather suspect. I know that it made me groan when I was learning turning a few years ago and the non-technically oriented instructor would make a statement and claim it to be "fact". I understood his intent, but it made me skeptical about other areas of his turning expertise as well.

Bill
 
Joined
May 29, 2004
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Location
billerica, ma
When I show kids how not to use a bandsaw, I take a 1" dowel and whap the blade with it when running. "this is your finger on bandsaw" This is usually enough to get their attention. If not, I use a bone from a porkchop.

For the lathe, I'll stand them up and ask if they're comfortable with me throwing this chunk of wood ("thock, thock") at their face from 10 feet away. For a larger group, bring a ripe watermelon and a broken off half of a bowl. Do the throwing star thing with it. That'll get their attention (and you'll have snacks for later).

dk
 
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