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Comment by BugsJustFindMe | Hacker News Reader

What you described misunderstands or at least poorly describes the problem.

> Right, because which blade half is on the bottom affects which direction you need to apply force so the blades meet tightly.

That would violate Newton's Third Law of motion, so no.

You don't have to apply force in any different direction if you put scissors with symmetric handles in the wrong hand. You will, however, end up tilting the scissors to the side so that you can see what you're doing. But that's not the same as needing to apply force any differently. It's a required sight issue not a required force issue. Anything else only comes from handle shape.

If the scissors don't have ergonomically sloped handles, an ambidextrous person can comfortably cut with either hand if they don't look at what they're doing.

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>You don't have to apply force in any different direction if you put scissors with symmetric handles in the wrong hand.

Yes! You do!

So scissors hinge on a pivot point, and they hinge in two dimensions. They mostly move in the dimension that they cut in, but they also move a little in the perpendicular dimension. And in order to bring the cutting edges closer, you move the handle ends farther away. If you're using the correct-handed scissors, doing so is easy and natural because your hand grip curls one way easily. If you then switch the same scissors to the other hand, it's hard and uncomfortable because your other hand is a mirror image and your natural hand grip curls to bring the handles closer and the cutting edges farther away.

The fact that you keep mentioning that this violates some law of physics is bonkers.

Your thumb and fingers of both hands are equally capable of producing both separating and joining force while closing. You learned to do it one way, by pushing out with your thumb and pulling in with your other fingers, so that way feels better to you, but that's a property of you not a property of scissors.

It's so weird you are choosing this hill to die on. And you are completely wrong; 7-year-old left-handed me had the empirical experience to know this.

>Your thumb and fingers of both hands are equally capable of producing both separating and joining force while closing.

What makes you so sure? The ergonomics of the thumb, fingers, and hand are not symmetric. The left-handed people who struggled with right-handed scissors as kids suggest otherwise.

It's got nothing to do with hand capability, you've misunderstood. For an ambidextrous person, using right-handed scissors in their right hand will work properly, using right-handed scissors in their left hand will work poorly. And vice versa for left-handed scissors.

When holding scissors, the bottom blade is held fairly steady. The top blade tends to get pushed away from the rest of the hand by the action of the thumb. If the top blade is on the far side of the scissors (away from the hand), the far blade edge will get pushed towards the near blade edge when the thumb handle is pushed away (because the blade is on the other side of the hinge from the handle). If the top blade is on the near side of the scissors (the hand side), by pushing the thumb handle away from you, you are separating the two blades rather than pushing them together.

So on right handed scissors, with the thumb handle facing up, the thumb blade is on the left side of the hinge. That way the natural "push away" force from the thumb will keep the blades close together when held in the right hand. On left handed scissors, the thumb blade is on the right side of the hinge, for the correct push-together force when held in the left hand.

It’s because people aren’t aware the 2nd dimension, the looseness of the connection between the planes of the blades, where the screw isn’t (and can never be) perfectly tight yet still allows the blades to slide. It’s hard to get the idea across in text. Your second comment helped a lot.

> You don't have to apply force in any different direction if you put scissors with symmetric handles in the wrong hand. [...] But that's not the same as needing to apply force any differently.

For precision made scissors with tight tolerances at the pivot joint and sharp blades (premium brands like Kai, Gingher, etc), it will cut with either hand.

But for scissors with a loose rivet (like cheaper scissors very common in kids' schools), the gp (ianferrel) you replied to is correct: the blades will not close tightly if the wrong hand is used to squeeze. The loose rivet scissors in the wrong hand will fold the paper instead of cutting it because the natural finger-closing motion will spread the blades apart creating a tiny gap instead of making them touch.

Here's a 1-minute video that tries to visually explain what a left-handed person struggling with a right-handed scissor experiences: https://www.youtube.com/watch?v=wyGCvSCnkWk

It sounds like you and they have decided that a human hand scissoring may only push inward or outward or neither using one side of the hand but not the other. That's not true about hands or about scissors though, even poorly constructed scissors. Your thumb and fingers of both hands are equally capable of producing both separating and joining force while closing. You and they have just decided that curling your fingers and pushing your thumb outward is right and the opposite is wrong, but the opposite works too. The scissors don't care and hands support both.

The grip strength of curling your fingers into the base of your thumb is stronger than the pinch strength of pressing your thumb against your fingers.

Scissor ergonomics aligns the handles so applying the finger-to-base-of-thumb grip to the handles (with the upper handle pressed against the outside of your thumb’s knuckle) pulls the blades together. In order to do that, scissors have to be designed to fit either a left or right hand.

To use scissors wrong handed you have to catch the outside of your finger knuckles against the lower handle and pinch the upper handle down with your thumb.

TIL: I've been using scissors wrong my entire life. I didn't realize that you need to push the handles to apply lateral force to the blades to make them slide against each other, the more the better.

Sorry for asking, but isn't that... obvious? So obvious that I bet most people just don't think about it consciously at all. If you don't do that, they don't cut properly, but it's likely to be something that the brain sorts out for you without you realizing the forces you apply.

You probably haven't been. You probably learned it with muscle memory as a kid and just weren't aware of it. I wasn't aware of it until I was in my late 30s and was helping my kids learn to use scissors.

And for the average scissors I use, it won't cut properly in either hand. Interestingly haircut scissors are better at cutting paper than regular paper scissors! I tried disassembling a pair of haircut scissors, and found the screw is interestingly rotationally locked to the blade next to the nut (rather than screw head), and the nut has a spring-loaded locking mechanism (a planar version of mouse wheel notches) so it doesn't rotate unless you turn the nut with a large amount of torque.

The sight issue is true (but in my opinion not the major difference).

Have you watched little kids learn to use scissors? You really do need to apply force to bring the blades together or they won't cut well. You likely do this unconsciously. I didn't think about it until I was teaching my children how to cut with scissors and noticed that they would generally end up folding the paper were I could cut it (toddler scissors aren't very sharp, so it's even more important to push the blades together from the side).

> You really do need to apply force to bring the blades together or they won't cut well.

It depends on the scissors. With the pair I'm holding right now, even if I'm actively pushing so the blades would move apart, they stay tight enough to cut fine.

Notably, the blades are curved into each other, which is capable of compensating for a moderate amount of hinge slack and narrowing the cutting to a single point. And the hinge has very little slack in the first place because it has a big flat mating surface.

It definitely has also to do with the force applied. If the blades are setup for the other hand your default grip will puah the blades apart and you can get the paper between the blades Thats why as a lefty I had major issues with the cheap scissors (which had a ton of play on the hinge) we had in school, bit if you have a decently made scissor the issue is much less severe.

Sorry, what you're describing violates Newton's Third Law. The force applied by one side is equal and opposite to the force from the opposing side. Scissor blades, paper, and force don't care which side is up. Anything other than line of sight is 100% handle ergonomics, and scissors aren't required to have wrong-side-incompatible handles.

You're thinking in the wrong axis.

If you open and close scissors the main force is up/down but based on the structure of the hand you also apply a sideways force which twists the blades left/right. Depending if the top blade (where your thumb goes in) this pushes the blades closer together, or a bit apart. And if the hinge has to much play, this is enough that the paper gets twisted between the scissors.

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