Reply
 
Thread Tools Display Modes
  #1  
Old 08-06-2018, 02:07 AM
Kavaj Kavaj is offline
Guest
 
Join Date: Jul 2017
Posts: 9
Why don't scissors follow the law of the lever?

We all know that the farther from the fulcrum a force is applied to a lever, the greater the force. You can hold a lever steady with a tiny weight against a huge weight, provided the tiny weight is far from the fulcrum and the huge weight is close to the fulcrum, and you can lift anything given a long enough lever. It's the point of having a lever at all.

Why does this not apply to scissors? Judging by how easily they cut things, the force is greatest closest to the fulcrum. If you want to cut something tough, you don't use the very points, you open the scissors up as much as possible and put the tough thing all the way in. What is it about scissors that makes them work like that?
  #2  
Old 08-06-2018, 02:14 AM
octopus's Avatar
octopus octopus is offline
Guest
 
Join Date: Apr 2015
Posts: 6,998
Scissors do work like a lever which is why the handle isnít near the pivot point.
  #3  
Old 08-06-2018, 02:23 AM
Richard Pearse Richard Pearse is offline
Member
 
Join Date: Aug 2001
Location: On the outside looking in
Posts: 10,218
Your hands are the input force, a long way from the fulcrum, the cutting edge is the output force, strongest close to the fulcrum. Exactly like a lever.
  #4  
Old 08-06-2018, 04:47 AM
Kavaj Kavaj is offline
Guest
 
Join Date: Jul 2017
Posts: 9
Quote:
Originally Posted by Richard Pearse View Post
Your hands are the input force, a long way from the fulcrum, the cutting edge is the output force, strongest close to the fulcrum. Exactly like a lever.
Good answer, makes sense. But then, and forgive me if this is a stupid question, if I'm swinging a long stick at someone, why do I want to hit them in the head with the end of the stick and not some point closer to my hands?
  #5  
Old 08-06-2018, 05:53 AM
running coach's Avatar
running coach running coach is online now
Arms of Steel, Leg of Jello
Charter Member
 
Join Date: Nov 2000
Location: Riding my handcycle
Posts: 35,029
Quote:
Originally Posted by Kavaj View Post
Good answer, makes sense. But then, and forgive me if this is a stupid question, if I'm swinging a long stick at someone, why do I want to hit them in the head with the end of the stick and not some point closer to my hands?
The end of the stick is moving faster. Delivers more skull-crushing force.
  #6  
Old 08-06-2018, 05:54 AM
Sage Rat's Avatar
Sage Rat Sage Rat is offline
Member
 
Join Date: Mar 2004
Location: Howdy
Posts: 19,790
I believe that the issue is the flexibility of the material. Just to make scissors work, they have to be slightly bent against each other so that the point of contact stays together through the entire stroke. Were they simply made straight, the paper would simply fold over one of the blades and wedge the other blade out around itself, making it ineffective. The bend is why there's the slight snap as you fully close the scissors - the blades are popping around the edge of each other in order to be able to site alongside each other rather that pushing edge to edge.

The closer to the hinge the metal is, the less the material is able to flex outward, and so the contact between the blades is even more secure and able to bite through the material.

The amount of force necessary to cut through the paper is sufficiently minimal that factors of leverage are inconsequential to the question of how well the blades are able to maintain contact while having something wedged between them.

Last edited by Sage Rat; 08-06-2018 at 05:58 AM.
  #7  
Old 08-06-2018, 05:58 AM
Richard Pearse Richard Pearse is offline
Member
 
Join Date: Aug 2001
Location: On the outside looking in
Posts: 10,218
Quote:
Originally Posted by Kavaj View Post
Good answer, makes sense. But then, and forgive me if this is a stupid question, if I'm swinging a long stick at someone, why do I want to hit them in the head with the end of the stick and not some point closer to my hands?
What Running Coach said. On the other hand if you just wanted to push someone with a stick, the shorter the stick the better.
  #8  
Old 08-06-2018, 06:44 AM
Kavaj Kavaj is offline
Guest
 
Join Date: Jul 2017
Posts: 9
Quote:
Originally Posted by running coach View Post
The end of the stick is moving faster. Delivers more skull-crushing force.
I get that, but why isn't it the same deal with scissors then? If I'm closing the scissors, the ends are moving faster than the edged bit closest to the fulcrum.
  #9  
Old 08-06-2018, 06:54 AM
Richard Pearse Richard Pearse is offline
Member
 
Join Date: Aug 2001
Location: On the outside looking in
Posts: 10,218
Forget the stick. You are not trying to hit the paper over the head with the end of the scissors. Your question was answered in post 2.
  #10  
Old 08-06-2018, 07:02 AM
GreenWyvern's Avatar
GreenWyvern GreenWyvern is offline
Guest
 
Join Date: Jan 2008
Location: Cape Town
Posts: 881
Scissors do obey the laws of levers... but there are 3 basic types of lever.

See this:

Lever classes

Last edited by GreenWyvern; 08-06-2018 at 07:05 AM.
  #11  
Old 08-06-2018, 07:13 AM
Joey P Joey P is offline
Charter Member
 
Join Date: Jun 1999
Location: Milwaukee, WI
Posts: 27,640
Quote:
Originally Posted by Kavaj View Post
Why does this not apply to scissors? Judging by how easily they cut things, the force is greatest closest to the fulcrum. If you want to cut something tough, you don't use the very points, you open the scissors up as much as possible and put the tough thing all the way in. What is it about scissors that makes them work like that?
Scissors are a lever, but the reason they work so well isn't so much that, as it is that you have a large area to grab with your hands and a very small wedge at the other side to drive through whatever you're cutting. Ignore the lever part of it for now. A scissors works the same way a knife does. You can cut through a tough steak or an apple without any help from a lever. It's razor sharp on the business end but half an inch wide at the part where you grab it.

Getting back to a scissors as a lever, if the handle where as sharp as the blade, you'd need it to be considerably further away from the pivot so you wouldn't have to use as much effort and cut yourself. Similarly, if the blade where as dull as the handle, the handle would still have to be much further away so you could create much more force.

Quote:
if I'm swinging a long stick at someone, why do I want to hit them in the head with the end of the stick and not some point closer to my hands?
As stated, the end of the stick is moving faster. Having said that I think you're shoehorning 'lever' into places it doesn't really belong.
A scissors is a lever, but only because that's what happens when you attach opposing knives to each other.
A swinging stick is a lever, but this is more about tangential velocity.
  #12  
Old 08-06-2018, 07:26 AM
JRDelirious JRDelirious is offline
Charter Member
 
Join Date: Aug 1999
Location: Displaced
Posts: 15,246
Quote:
Originally Posted by Kavaj View Post
I get that, but why isn't it the same deal with scissors then? If I'm closing the scissors, the ends are moving faster than the edged bit closest to the fulcrum.
Not the same mechanics, as Joey P explains.
  #13  
Old 08-06-2018, 07:32 AM
Machine Elf Machine Elf is offline
Guest
 
Join Date: Feb 2007
Location: Challenger Deep
Posts: 11,128
Quote:
Originally Posted by Kavaj View Post
I get that, but why isn't it the same deal with scissors then? If I'm closing the scissors, the ends are moving faster than the edged bit closest to the fulcrum.
If you're smacking someone in the head with a baseball bat, this is a dynamic event: the force being applied to the stick by your hands at the moment of impact is mostly irrelevant, as it's all about the momentum you've delivered to the bat during the swing just prior to impact. Your goal is to create a high impact force; this requires transferring a lot of momentum from your bat to their head, so you want the part of the bat that hits their head to have a lot of speed. For this application, a bat is better than a block of comparable mass because it lets you build up more speed at the point where your bat will impact its target. If you really want to optimize the situation you concentrate as much of the mass as practically possible on the moving end of your weapon, and you end up with a mace or a war hammer. The extreme opposite situation is a spherical ball of wood of the same mass as your baseball bat or mace, and you throw it at your target's head. You won't be able to get this ball of wood moving nearly as fast as your bat; yeah, it'll probably hurt your target if it hits them in the head, but one good hit with a bat will probably kill them.

If you're cutting material with a pair of scissors, this is a static event: unlike swinging a bat at someone's head, you don't build up speed with the scissor blades and then have the cutting edges slam into the material. The force being applied to the material during the cut is due to the force generated by your fingers at that specific moment, modified by the lever ratio. The distance from your fingers to the fulcrum is pretty much fixed, so for max cutting force, you move your material as close as possible to the fulcrum. This exactly obeys the lever rule.

The one exception I can think of is manual hedge clippers, the operation of which is often dynamic: you apply sharp force to get the blades and handles moving, and the momentum stored up in the heavy handles and blades helps keep the blades moving through small twigs and branches. However, the lever rule is still applicable here: maximum cutting force is achieved when the twigs and branches are as close to the fulcrum as possible. Sure, tiny twigs can be cut easily enough out near the blade tips, but when you encounter that one stubborn 5/8" branch, you move it close to the fulcrum so you can hack through it.
  #14  
Old 08-06-2018, 07:33 AM
am77494 am77494 is offline
Guest
 
Join Date: Mar 2012
Posts: 1,279
My understanding is that the cutting action of scissors has nothing to do with lever action (although it does act like a lever).

The cutting action (or shearing action) has to do with the gap between the sharp edges. Near the pivot (or the fulcrum) the gap is smallest and the gap increases as you go towards the edge. So near the pivot, the shearing or cutting forces are the highest.
  #15  
Old 08-06-2018, 07:42 AM
CookingWithGas's Avatar
CookingWithGas CookingWithGas is offline
Charter Member
 
Join Date: Mar 1999
Location: Tysons Corner, VA, USA
Posts: 12,188
The tiny weight (your hands pushing on the handles) is farther from the fulcrum, and the heavy weight is near the fulcrum. The tips of the blades are far from the fulcrum.

When you hold the handles of scissors and close them, you are applying torque. Torque is the twisting force of each blade of the scissors rotating about the pivot pin. The torque shows up on the other side of the pin in the form of the two blades pushing together. Torque is force perpendicular to the blade times the distance away from the pin. So for a fixed torque, near the pin the force perpendicular to the blade is higher; closer to the tip of the blade, the force perpendicular to the blade is lower. This makes sense if you think about the fact that if your hands close the handles a fixed amount of space, the blades near the pin just move a little bit but the blades near the tip have to move a lot more, so they are pushing more weakly. It's easier to cut cardboard near the pin vs. near the tips.

The same is true of swinging a stick, but if you are trying to hit something with a stick, you want the end of the stick to move as fast as it can, giving it more kinetic energy. You are trying to generate force with scissors, not kinetic energy; moving them faster doesn't make them work any better. So when swinging a stick (like a baseball bat; let's not resort to violence) you want to keep adding energy throughout the swing until the point of contact. Once the bat makes contact, most of the force is delivered by the momentum of the bat, not by you continuing to push on the bat past the point of contact.
__________________
Making the world a better place one fret at a time.
| | |∑| |∑| |∑| |∑| | |:| | |∑| |∑|
  #16  
Old 08-06-2018, 07:57 AM
Joey P Joey P is offline
Charter Member
 
Join Date: Jun 1999
Location: Milwaukee, WI
Posts: 27,640
Quote:
Originally Posted by Machine Elf View Post
If you're smacking someone in the head with a baseball bat, this is a dynamic event: the force being applied to the stick by your hands at the moment of impact is mostly irrelevant, as it's all about the momentum you've delivered to the bat during the swing just prior to impact. Your goal is to create a high impact force; this requires transferring a lot of momentum from your bat to their head, so you want the part of the bat that hits their head to have a lot of speed.
It should be noted that you could deliver the same amount of energy with a bat moving very slowly, but you would need to push [equally] harder. To do that, you'd likely hold the bat in the same area that it's going to contact the person and push it through their skull. Or better, imagine trying to brain someone with a brick. You wouldn't swing it at them, you'd "punch" them with it.

I think the OP even bringing up the swinging stick idea, while an interesting conversation, suggests a confusion between levers (or wedges) and momentum.
One multiples force, the other is about delivering a given mass more effectively. Actually, I think it's more like a lever multiples force, momentum is about adding a velocity component to it. There's some crossover, sure, but they're two separate things.
  #17  
Old 08-06-2018, 08:14 AM
Machine Elf Machine Elf is offline
Guest
 
Join Date: Feb 2007
Location: Challenger Deep
Posts: 11,128
Quote:
Originally Posted by Joey P View Post
It should be noted that you could deliver the same amount of energy with a bat moving very slowly, but you would need to push [equally] harder. To do that, you'd likely hold the bat in the same area that it's going to contact the person and push it through their skull.
This won't work unless the victim's head is braced against a fixed object; without that, you'll just push their head out of the way without inflicting any localized damage. Basically, you'd need to put the victim's head in a hydraulic press. Swinging a bat at an unbraced head works because the victim's head has its own substantial mass that resists movement during a dynamic impact event, resulting in high impact forces.

Last edited by Machine Elf; 08-06-2018 at 08:16 AM.
  #18  
Old 08-06-2018, 08:33 AM
Machine Elf Machine Elf is offline
Guest
 
Join Date: Feb 2007
Location: Challenger Deep
Posts: 11,128
Quote:
Originally Posted by Machine Elf View Post
Quote:
Originally Posted by Joey P View Post
It should be noted that you could deliver the same amount of energy with a bat moving very slowly, but you would need to push [equally] harder. To do that, you'd likely hold the bat in the same area that it's going to contact the person and push it through their skull.
This won't work unless the victim's head is braced against a fixed object; without that, you'll just push their head out of the way without inflicting any localized damage. Basically, you'd need to put the victim's head in a hydraulic press. Swinging a bat at an unbraced head works because the victim's head has its own substantial mass that resists movement during a dynamic impact event, resulting in high impact forces.
And of course I'm wrong about this.

Yes, what you describe would certainly work, i.e. holding the bat against their unbraced head and then just pushing it through their skull. You'd need to push with the same amount of force that the bat delivers during its brief impact (as you said), but the trick is that you'd need to develop that huge force within a very short rise time; if you ramp up your pushing force even a little slowly, their head will move out of the way before you get a chance to achieve your maximum pushing force.
  #19  
Old 08-06-2018, 09:00 AM
DavidwithanR DavidwithanR is online now
Guest
 
Join Date: Feb 2018
Posts: 2,872
I can see a book coming from Machine Elf... "Truly Strange Combat Techniques"
  #20  
Old 08-06-2018, 09:04 AM
Jasmine's Avatar
Jasmine Jasmine is offline
Member
 
Join Date: Jul 1999
Location: Chicagoland
Posts: 1,032
Well, for one thing, scissors aren't a standard lever. Scissors are actually two levers working in concert that apply opposing forces. Also, there is the cutting aspect of scissors, which a standard lever doesn't have.
__________________
"The greatest obstacle to discovery is not ignorance -- it is the illusion of knowledge."
--Daniel J Boorstin
  #21  
Old 08-06-2018, 09:29 AM
kayaker's Avatar
kayaker kayaker is offline
Member
 
Join Date: Jul 2009
Location: Western Pennsylvania
Posts: 29,250
Rock make scissor obey law of lever.
  #22  
Old 08-06-2018, 09:30 AM
Jasmine's Avatar
Jasmine Jasmine is offline
Member
 
Join Date: Jul 1999
Location: Chicagoland
Posts: 1,032
Quote:
Originally Posted by kayaker View Post
Rock make scissor obey law of lever.
Yeah, but you're going to run through a lot of scissors if your methodology is to beat them to death with a rock.
__________________
"The greatest obstacle to discovery is not ignorance -- it is the illusion of knowledge."
--Daniel J Boorstin
  #23  
Old 08-06-2018, 09:47 AM
AHunter3 AHunter3 is offline
Charter Member
 
Join Date: Mar 1999
Location: NY (Manhattan) NY USA
Posts: 19,788
If you look at a pair of open scissors edge-on, you'll see that the two blades are not entirely parallel. The points should cross (visually), as if it would be impossible to close them because the points would collide. What happens as you close them is that the point of contact between the two blades remains under mutual pressure, with the blades sliding against each other. But even with that effect in action, they do their best cutting up close to the hinge. The hinge keeps the two blades tightly together so they slice through the material. As you progress outward to the pointy tips, you're increasingly dependent on that slight bend and it can only partially compensate for the increasing distance from the hinge. Cheaper scissors (or older worn-out ones) may have an air gap between the two blades when they pass each other at the tip, and the material may not cut at all -- it may wad up or even get pulled in between the two blades, intact, instead of slicing.
  #24  
Old 08-06-2018, 09:57 AM
Jasmine's Avatar
Jasmine Jasmine is offline
Member
 
Join Date: Jul 1999
Location: Chicagoland
Posts: 1,032
Quote:
Originally Posted by AHunter3 View Post
If you look at a pair of open scissors edge-on, you'll see that the two blades are not entirely parallel. The points should cross (visually), as if it would be impossible to close them because the points would collide. What happens as you close them is that the point of contact between the two blades remains under mutual pressure, with the blades sliding against each other. But even with that effect in action, they do their best cutting up close to the hinge. The hinge keeps the two blades tightly together so they slice through the material. As you progress outward to the pointy tips, you're increasingly dependent on that slight bend and it can only partially compensate for the increasing distance from the hinge. Cheaper scissors (or older worn-out ones) may have an air gap between the two blades when they pass each other at the tip, and the material may not cut at all -- it may wad up or even get pulled in between the two blades, intact, instead of slicing.
Excellent analysis!
__________________
"The greatest obstacle to discovery is not ignorance -- it is the illusion of knowledge."
--Daniel J Boorstin
  #25  
Old 08-06-2018, 09:59 AM
jjakucyk jjakucyk is offline
Guest
 
Join Date: May 2016
Location: Cincinnati
Posts: 321
How are the mechanics of tin snips different from scissors? With scissors you get better cutting action with paper nearest the fulcrum. With tin snips however (at least the ones I've experienced) if you try to cut a piece of sheet metal like you would a piece of paper, opening the jaws as wide as possible and getting the metal as close to the fulcrum as possible, you just mangle the metal as it bends and mashes around. However if you nip at it with the tips of the snips then it cuts through with a satisfying snap. You don't cut it so much as nibble away at it.
  #26  
Old 08-06-2018, 10:19 AM
Machine Elf Machine Elf is offline
Guest
 
Join Date: Feb 2007
Location: Challenger Deep
Posts: 11,128
Quote:
Originally Posted by Jasmine View Post
Well, for one thing, scissors aren't a standard lever. Scissors are actually two levers working in concert that apply opposing forces.
If it'll help to clarify things, we can bolt one part of the scissors to the earth so that it doesn't move, but this doesn't change anything about the analysis. It also doesn't matter whether the the user and the workpiece are on opposite sides of the fulcrum, or both on the same side of the fulcrum (like this cute pair of scissors). You can even combine those two concepts, i.e. a pair of scissors that's bolted to the earth and has the user and workpiece on the same side of the fulcrum, like this paper cutter. The analysis of forces and motions doesn't change: user applies force F1 at speed V1 at a distance D1 from the fulcrum, and at distance D2 from the fulcrum the lever exerts a force F2 at a speed V2, with the force ratio and speed ratio dependent only on the distances D1 and D2 from the fulcrum at which forces F1 and F2 are being applied:

F2 = F1 * D1/D2
V2 = V1 * D2/D1

Quote:
Originally Posted by Jasmine View Post
Also, there is the cutting aspect of scissors, which a standard lever doesn't have.
A "standard lever" multiplies force and speed, and that's it. What the user does with that force is an entirely separate issue, and doesn't change the analysis of forces and motions. It doesn't matter whether the applied force is used for shearing (scissors), pressing machine parts together/apart, moving a boulder, or whatever.

Quote:
Originally Posted by Ahunter3
If you look at a pair of open scissors edge-on, you'll see that the two blades are not entirely parallel. The points should cross (visually), as if it would be impossible to close them because the points would collide. What happens as you close them is that the point of contact between the two blades remains under mutual pressure, with the blades sliding against each other. But even with that effect in action, they do their best cutting up close to the hinge. The hinge keeps the two blades tightly together so they slice through the material. As you progress outward to the pointy tips, you're increasingly dependent on that slight bend and it can only partially compensate for the increasing distance from the hinge. Cheaper scissors (or older worn-out ones) may have an air gap between the two blades when they pass each other at the tip, and the material may not cut at all -- it may wad up or even get pulled in between the two blades, intact, instead of slicing.
So far we've been discussing theoretical issues; you're describing a practical aspect of scissors, and when you're cutting stubborn material, it probably matters as much as being able to generate high cutting force. Placing stubborn material close to the fulcrum simultaneously solves both problems: the user has best mechanical advantage, and the short bending length (measured from the fulcrum) also assures that the blades will maintain contact with each other and provide reliable shearing action, even under high cutting forces.

Since most people are right-handed, scissors are usually made for use in one's right hand; even on cheap ones without nicely molded grips, the lever stacking sequence on the pivot pin is typically chosen so that when closing the scissors, the user's hand forces tend to help assure the blades remain in contact during the cut. This matters more on cheap scissors than expensive ones, as the cheap ones often have loose pivots and little or no lateral curvature to the blades. I'm left-handed, and when I was in elementary school in the late 1970s, it used to frustrate me whenever the art teacher handed me a pair of "lefty" scissors; despite writing with my left hand, I always used scissors in my right hand, and these "lefty" scissors always worked like shit for me. I didn't understand it at the time, but the lever stacking sequence on those lefty scissors was reversed: if I had used them in my left hand, they would have worked as well as a "normal" pair of scissors worked in my right hand.
  #27  
Old 08-06-2018, 10:24 AM
naita naita is offline
Guest
 
Join Date: Jun 2002
Location: Norway
Posts: 5,631
Quote:
Originally Posted by jjakucyk View Post
How are the mechanics of tin snips different from scissors? With scissors you get better cutting action with paper nearest the fulcrum. With tin snips however (at least the ones I've experienced) if you try to cut a piece of sheet metal like you would a piece of paper, opening the jaws as wide as possible and getting the metal as close to the fulcrum as possible, you just mangle the metal as it bends and mashes around. However if you nip at it with the tips of the snips then it cuts through with a satisfying snap. You don't cut it so much as nibble away at it.
Qualified guess. Sheet metal is much thicker than paper, and also more resistant to shear. Close to the fulcrum the force of the blade is angled forward so some of it pushes the sheet "out" of the snips, further from the hinge the force of the blade is closer to vertical, relative to the sheet and so it cuts more easily.

The same applies to regular scissors if you try to cut something like really thick cardboard, I think.
  #28  
Old 08-06-2018, 10:34 AM
Machine Elf Machine Elf is offline
Guest
 
Join Date: Feb 2007
Location: Challenger Deep
Posts: 11,128
Quote:
Originally Posted by jjakucyk View Post
How are the mechanics of tin snips different from scissors? With scissors you get better cutting action with paper nearest the fulcrum. With tin snips however (at least the ones I've experienced) if you try to cut a piece of sheet metal like you would a piece of paper, opening the jaws as wide as possible and getting the metal as close to the fulcrum as possible, you just mangle the metal as it bends and mashes around. However if you nip at it with the tips of the snips then it cuts through with a satisfying snap. You don't cut it so much as nibble away at it.
The issue is that the already-cut portions of the workpiece need to bend to clear the fulcrum, but since the two pieces on opposite sides of the cut need to bend in opposite directions, the bend can only happen in the already-cut area. When you try to jam your workpiece in close to the fulcrum, you are shortening the available distance for bending. This isn't a problem with flimsy standard office paper, but anything more rigid - posterboard, cardboard, or sheet metal - will fight you, and will also take on a permanent bend, which is usually undesirable.

For cutting sheet metal, the Beverly shear makes life easier. The blades don't continue all the way to the fulcrum, and the fulcrum is moved up and out of the plane of the workpiece. The result is that the already-cut parts of the workpiece hardly need flex at all anymore, and in fact there's a lot of room available for making curved cuts. Here's one in action. Not as cheap or portable as handheld snips, but if you do a lot of sheet metal work, it's a good investment.
  #29  
Old 08-06-2018, 11:02 AM
Riemann Riemann is online now
Guest
 
Join Date: Nov 2015
Location: Santa Fe, NM, USA
Posts: 4,912
Quote:
Originally Posted by Kavaj View Post
Good answer, makes sense. But then, and forgive me if this is a stupid question, if I'm swinging a long stick at someone, why do I want to hit them in the head with the end of the stick and not some point closer to my hands?
A clearer comparison might be an axe, with a heavy sharp head some distance from the grip. When you wield the axe to get the head moving, the lever effect applies - it's going to take more strength to get a long axe moving through the air than a short axe. But an axe doesn't cut like a pair of scissors, you're not applying much force when it strikes the target, you're just guiding it onto the target. It's the momentum that you previously imparted to the axehead that makes it cut.
  #30  
Old 08-06-2018, 01:45 PM
Joey P Joey P is offline
Charter Member
 
Join Date: Jun 1999
Location: Milwaukee, WI
Posts: 27,640
Quote:
Originally Posted by Machine Elf View Post
This won't work unless the victim's head is braced against a fixed object; without that, you'll just push their head out of the way without inflicting any localized damage. Basically, you'd need to put the victim's head in a hydraulic press. Swinging a bat at an unbraced head works because the victim's head has its own substantial mass that resists movement during a dynamic impact event, resulting in high impact forces.
Couldn't tell you why, but as I went through this in my head to type it in, the person getting hit was on the ground. But yes that, and your next post.

Quote:
Originally Posted by jjakucyk View Post
How are the mechanics of tin snips different from scissors? With scissors you get better cutting action with paper nearest the fulcrum. With tin snips however (at least the ones I've experienced) if you try to cut a piece of sheet metal like you would a piece of paper, opening the jaws as wide as possible and getting the metal as close to the fulcrum as possible, you just mangle the metal as it bends and mashes around. However if you nip at it with the tips of the snips then it cuts through with a satisfying snap. You don't cut it so much as nibble away at it.
Cutting metal with a tin snips is differant than using a scissors on paper.
  #31  
Old 08-07-2018, 11:05 AM
md2000 md2000 is offline
Guest
 
Join Date: Feb 2009
Posts: 13,921
I think Machine Elf has the better explanation.

For cutting a sheet of paper, you don't need mechanical advantage. It's pretty weak. If you try to cut thicker more resistant material, close to the pivot point (the hinge or screw or whatever you call it) works better. The ultimate example is wire cutters or toenail snips where the lever force is extreme.

You do see the same with hedge clippers. When snipping thin twigs, you can cut anywhere along the clipper blades. For big thick branches, the closer to the pivot point the better. Sometimes momentum helps, but for pure leverage, the closer to the pivot point the better.

Scissors and tin snips work on the same principle - an edge coming down against and parallel to another edge shears the material. For metal, this works better the flatter the shear is, so the further form the pivot point near the outside of the snips- but notice the snips do seriously take advantage of leverage.

Also, the problem with lever principle and scissors is that the thumb and fingers can only get so far apart, so scissors can only open so wide. Longer scissor handles mean the blade opens less wide. But since cutting paper requires very little force, there are very long scissors with normal short handles available. The purpose of scissors is to provide a cutting edge (or, a pair), not to heavily exploit leverage.
  #32  
Old 08-07-2018, 11:18 AM
Joey P Joey P is offline
Charter Member
 
Join Date: Jun 1999
Location: Milwaukee, WI
Posts: 27,640
Quote:
Originally Posted by md2000 View Post
Also, the problem with lever principle and scissors is that the thumb and fingers can only get so far apart, so scissors can only open so wide. Longer scissor handles mean the blade opens less wide. But since cutting paper requires very little force, there are very long scissors with normal short handles available. The purpose of scissors is to provide a cutting edge (or, a pair), not to heavily exploit leverage.
I'm not sure if you were responding to me, but if you were, this is what I was replying to:
Quote:
if you try to cut a piece of sheet metal like you would a piece of paper, opening the jaws as wide as possible and getting the metal as close to the fulcrum as possible, you just mangle the metal as it bends and mashes around. However if you nip at it with the tips of the snips then it cuts through with a satisfying snap.
Tin snips are designed to push the sheet metal out of the way as you cut it. Since sheet metal is so stiff, if you tried to cut straight through it, the cut the snips makes wouldn't be large enough for the snips to pass through it.
When you use a scissors, the side of the paper you're not holding (typically) falls below the scissors and you can keep cutting. With tin snips, it pushes the metal up/down to make a space. Further, some of them are meant for curving to the left or the right.
Having said all that, if you don't know this and haven't had any practice it's pretty easy to make a mess of it.
Find a tin knocker and watch them. They'll cut through it just fine, no mangled mess.

BTW, IME, you can find a tin knocker by shaking hands with people. They're the ones that'll shake your hand with such a string grip you'll wonder if they broke it.
  #33  
Old 08-07-2018, 11:20 AM
ftg's Avatar
ftg ftg is offline
Guest
 
Join Date: Feb 2001
Location: Not the PNW :-(
Posts: 17,150
Another example of scissors designed to use leverage are many kitchen scissors. The handle is often longer than the blades. Esp. if they are (properly) designed for bone cutting as well.
  #34  
Old 08-07-2018, 12:10 PM
Chronos's Avatar
Chronos Chronos is online now
Charter Member
Moderator
 
Join Date: Jan 2000
Location: The Land of Cleves
Posts: 78,104
Tin snips will also often have compound levers: Instead of just one pivot point, you'll effectively have one lever pushing against the input of a second lever.
  #35  
Old 08-07-2018, 12:52 PM
iamthewalrus(:3= iamthewalrus(:3= is offline
Guest
 
Join Date: Jul 2000
Location: Santa Barbara, CA
Posts: 11,051
Quote:
Originally Posted by Kavaj View Post
I get that, but why isn't it the same deal with scissors then? If I'm closing the scissors, the ends are moving faster than the edged bit closest to the fulcrum.
This has mostly been answered, but not framed quite like this, and sometimes more answers framed differently helps explain: it's because scissors don't cut by building up momentum before impacting the paper.

It's the difference between a chef's knife and a cleaver. They're both fairly large knives, but the chef's knife doesn't have to be particularly heavy, because what's providing the cutting force is the person pushing on it. A cleaver on the other hand, has to gain enough momentum to slice through bone, so it's heavy, and you take big old whacks with it.
  #36  
Old 08-08-2018, 05:45 AM
bob++ bob++ is offline
Guest
 
Join Date: Jan 2013
Location: Worcestershire UK
Posts: 5,997
Quote:
Originally Posted by Kavaj View Post
Good answer, makes sense. But then, and forgive me if this is a stupid question, if I'm swinging a long stick at someone, why do I want to hit them in the head with the end of the stick and not some point closer to my hands?
This question has been well answered but can someone explain the physics of striking a ball with a cricket or baseball bat when the point of maximum impact is nowhere near the end.
  #37  
Old 08-08-2018, 06:50 AM
Joey P Joey P is offline
Charter Member
 
Join Date: Jun 1999
Location: Milwaukee, WI
Posts: 27,640
Quote:
Originally Posted by bob++ View Post
This question has been well answered but can someone explain the physics of striking a ball with a cricket or baseball bat when the point of maximum impact is nowhere near the end.
How do you figure? The bat is:
A)largest at the end, therefore having more physical contact with the ball
B)Moving the fastest at the end so it has the most momentum.
C)Heaviest at the end, also contributing to it having the most momentum.
D)The circle/path it creates is the shallowest at the end...that is, the bat and the ball will stay in contact for the longest time when the ball hit the bat near the end, giving the bat more time to transfer it's energy to the bat, it also gives the ball the best chance of going straight.

I'm not quite sure I understand the question. It could be that you and I aren't on the same page about the term 'maximum impact'.
  #38  
Old 08-08-2018, 07:05 AM
scr4 scr4 is offline
Member
 
Join Date: Aug 1999
Location: Alabama
Posts: 14,658
Quote:
Originally Posted by bob++ View Post
This question has been well answered but can someone explain the physics of striking a ball with a cricket or baseball bat when the point of maximum impact is nowhere near the end.
Here, the batter isn't acting as a fixed fulcrum. You should think of the bat as a freely moving & rotating object. To transfer the maximum amount of momentum from the bat to the ball, you want to hit near the center of mass of the bat. If you hit exactly at the center of mass, though, you won't transfer any of the angular momentum, so I think the optimal point is a bit further out than the center of mass.
  #39  
Old 08-08-2018, 07:21 AM
RaffArundel RaffArundel is offline
Guest
 
Join Date: Aug 2014
Posts: 46
Quote:
Originally Posted by scr4 View Post
I think the optimal point is a bit further out than the center of mass.
Generally, it is the center of percussion:
https://en.wikipedia.org/wiki/Center_of_percussion
  #40  
Old 08-08-2018, 07:22 AM
bob++ bob++ is offline
Guest
 
Join Date: Jan 2013
Location: Worcestershire UK
Posts: 5,997
Quote:
Originally Posted by Joey P View Post
How do you figure? The bat is:
A)largest at the end, therefore having more physical contact with the ball
B)Moving the fastest at the end so it has the most momentum.
C)Heaviest at the end, also contributing to it having the most momentum.
D)The circle/path it creates is the shallowest at the end...that is, the bat and the ball will stay in contact for the longest time when the ball hit the bat near the end, giving the bat more time to transfer it's energy to the bat, it also gives the ball the best chance of going straight.

I'm not quite sure I understand the question. It could be that you and I aren't on the same page about the term 'maximum impact'.
a) This is not true at all.
b) Correct, so why is the optimum strike point nearrer the fulcrum
c) Not true.
d) Again wrong - See (b)
  #41  
Old 08-08-2018, 07:27 AM
bob++ bob++ is offline
Guest
 
Join Date: Jan 2013
Location: Worcestershire UK
Posts: 5,997
So - It's the centre of percussion, modified by the bending moment or vibration of the bat or racquet? I had never heard of the centre of percussion before.

From RaffArundel's link.

Last edited by bob++; 08-08-2018 at 07:28 AM.
  #42  
Old 08-08-2018, 07:52 AM
Chronos's Avatar
Chronos Chronos is online now
Charter Member
Moderator
 
Join Date: Jan 2000
Location: The Land of Cleves
Posts: 78,104
I'd never heard the term before. I've done a few problems involving it, but they never reified that point, just asked "where must the rod be hit to ensure no normal force at the pivot point?", or the like. It never occurred to me that such a point would be relevant to baseball, but in retrospect, of course it would.
  #43  
Old 08-08-2018, 08:04 AM
Joey P Joey P is offline
Charter Member
 
Join Date: Jun 1999
Location: Milwaukee, WI
Posts: 27,640
Quote:
Originally Posted by Joey P View Post
How do you figure? The bat is:
A)largest at the end, therefore having more physical contact with the ball
B)Moving the fastest at the end so it has the most momentum.
C)Heaviest at the end, also contributing to it having the most momentum.
D)The circle/path it creates is the shallowest at the end...that is, the bat and the ball will stay in contact for the longest time when the ball hit the bat near the end, giving the bat more time to transfer it's energy to the bat, it also gives the ball the best chance of going straight.

I'm not quite sure I understand the question. It could be that you and I aren't on the same page about the term 'maximum impact'.
Quote:
Originally Posted by bob++ View Post
a) This is not true at all.
b) Correct, so why is the optimum strike point nearrer the fulcrum
c) Not true.
d) Again wrong - See (b)
If the end of your baseball bat isn't the largest and heaviest, you're either holding it upside down or using one that I'm not familiar with.

wait...upon some re-reading...
If you were asking why it's a few inches (don't know how many) down from the end, as opposed to near your hands or in the center etc, then I misread/misunderstood your post.
There's been so many odd questions in this thread...why wouldn't someone ask why they shouldn't use the skinny part of the bat near their hands?

Anyway, as mentioned, I'm sure it just gets down to the exact physics of it. Just because the far end may have the most momentum, doesn't mean it's optimal since their may be other factors at play. Also, if you attempt to hit the ball 6 inches from the end and you're a little off, you've got some room for error. If you try to hit it as close to the end as possible and you're off by a half inch, you're looking at a foul or strike.
  #44  
Old 08-08-2018, 08:15 AM
bob++ bob++ is offline
Guest
 
Join Date: Jan 2013
Location: Worcestershire UK
Posts: 5,997
I now wonder if this isn't relative to a problem I had at work a while ago:

I had to lift a machine that was, essentially, a beam around 20' long and weighing 2 tons but was not very strong (in normal use it was supported by 8 legs).

I needed to attach slings to the correct points along the length to minimise distortion when lifted by a single hook on an overhead crane and, lacking the maths, decided that points around 4' in from each end looked about right. It worked and although there was some flexing, the machine survived undamaged. I was not aware of the SD then or I would have asked here.

Last edited by bob++; 08-08-2018 at 08:15 AM.
  #45  
Old 08-08-2018, 10:13 AM
Isilder Isilder is offline
Guest
 
Join Date: Mar 2013
Posts: 4,325
Quote:
Originally Posted by running coach View Post
The end of the stick is moving faster. Delivers more skull-crushing force.
F=mA.

You need Skull crushing Speed.. you'd not get the required force by moving it slowly.
Its the sudden stop at the end that increases the force to do damage.

you might think that you can increase the force from an impact by making the stop more sudden but the amount of damage done is limited by the kinetic energy in the impact. More kinetic energy means it can do more damage, if force is sufficient, and anyway you get higher kinetic energy from higher speed, which therefore means higher forces, all other things being equal.

Last edited by Isilder; 08-08-2018 at 10:14 AM.
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 10:01 PM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2018, vBulletin Solutions, Inc.

Send questions for Cecil Adams to: cecil@straightdope.com

Send comments about this website to: webmaster@straightdope.com

Terms of Use / Privacy Policy

Advertise on the Straight Dope!
(Your direct line to thousands of the smartest, hippest people on the planet, plus a few total dipsticks.)

Copyright © 2018 STM Reader, LLC.

 
Copyright © 2017